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Title: The Principles of Chemistry. Volume II (of 2)
Author: Mendeléeff, D.
Language: English
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THE
PRINCIPLES OF CHEMISTRY
By D. MENDELÉEFF
TRANSLATED FROM THE RUSSIAN (SIXTH EDITION) BY
GEORGE KAMENSKY, A.R.S.M.
OF THE IMPERIAL MINT, ST PETERSBURG: MEMBER OF THE RUSSIAN
PHYSICO-CHEMICAL SOCIETY
EDITED BY
T. A. LAWSON, B.Sc. PH.D.
EXAMINER IN COAL-TAR PRODUCTS TO THE CITY AND GUILDS OF LONDON
INSTITUTE FELLOW OF THE INSTITUTE OF CHEMISTRY
IN TWO VOLUMES
VOLUME II.
LONGMANS, GREEN, AND CO
39 PATERNOSTER ROW, LONDON
NEW YORK AND BOMBAY
1897
All rights reserved
* * * * *
TABLE III.
_The periodic dependence of the composition of the simplest compounds
and properties of the simple bodies upon the atomic weights of
the elements._
+-------------------------+--------------------------------+
| | |
|Molecular composition of | |
|the higher hydrogen and | Atomic weights of the elements |
|metallo-organic compounds| |
|-------------------------+--------------------------------+
| | |
| | |
|E=CH_{3}, C_{2}H_{5}, &c.| |
| | |
| | |
|[1] [2] [3] [4] | [5] [6] |
| | |
| HH| H 1,005 (mean) |
| | Li 7·02 (Stas) |
| | Be 9·1 (Nilson Pettersson)|
| BE_{3} -- --| B 11·0 (Ramsay Ashton) |
| CH_{4} C_{2}H_{6} | |
| C_{2}H_{4} C_{2}H_{2} | C 12·00 (Roscoe) |
| NH_{3} N_{2}H_{4} --| N 14·04 (Stas) |
| OH_{2} --| O 16 (conventional) |
| FH| F 19·0 (Christiansen) |
| | |
| NaE| Na 23·04 (Stas) |
| MgE_{2} --| Mg 24·3 (Burton) |
| AlE_{3} -- --| Al 27·1 (Mallet) |
|SiH_{4} Si_{2}E_{6} -- --| Si 28·4 (Thorpe Young) |
| PH_{3} P_{2}H_{4} --| P 31·0 (v. d. Plaats) |
| SH_{2} --| S 32·06 (Stas) |
| ClH| Cl 35·45 (Stas) |
| | |
| | K 39·15 (Stas) |
| | Ca 40·0 (Dumas) |
| | Sc 44·0 (Nilson) |
| | Ti 48·1 (Thorpe) |
| | V 51·2 (Roscoe) |
| | Cr 52·1 (Rawson) |
| | Mn 55·1 (Marignac) |
| | Fe 56·0 (Dumas) |
| | Co 58·9 (Zimmermann) |
| | Ni 59·4 (Winkler) |
| | Cu 63.6 (Richards) |
| ZnE_{2} --| Zn 65·3 (Marignac) |
| GaE_{3} -- --| Ga 69·9 (Boisbaudran) |
| GeE_{4} -- -- --| Ge 72·3 (Winkler) |
| AsH_{3} -- --| As 75·0 (Dumas) |
| SeH_{2} --| Se 79·0[A] (Pettersson) |
| BrH| Br 79·95 (Stas) |
| | |
| | Rb 85·5 (Godeffroy) |
| | Sr 87·6 (Dumas) |
| | Y 89 (Clève) |
| | Zr 90·6 (Bailey) |
| | Nb 94 (Marignac) |
| | Mo 96·1 (Maas) |
| | Unknown metal |
| | |
| | Ru 101·7 (Joly) |
| | Rh 102·7 (Seubert) |
| | Pd 106·4 (Keller Smith) |
| | Ag 107·92 (Stas) |
| CdE_{2} --| Cd 112·1 (Lorimer Smith) |
| InE_{3} -- --| In 113·6 (Winkler) |
| SnE_{4} -- -- --| Sn 119·1 (Classen) |
| SbH_{3} -- --| Sb 120·4 (Schneider) |
| TeH_{2} --| Te 125·1 (Brauner) |
| | |
| | Cs 132·7 (Godeffroy) |
| | Ba 137·4 (Richards) |
| | La 138·2 (Brauner) |
| | Ce 140·2 (Brauner) |
| | |
| | Ta 182·7 (Marignac) |
| | W 184·0 (Waddel) |
| | Unknown element. |
| | |
| | Ir 193·3 (Joly) |
| | Pt 196·0 (Dittmar McArthur) |
| | Au 197·5 (Mallet) |
| HgE_{2} --| Hg 200·5 (Erdmann Mar.) |
| TlE_{3} -- --| Tl 204·1 (Crookes) |
| PbE_{4} -- -- --| Pb 206·90 (Stas) |
| BiE_{3} -- --| Bi 208·9 (Classen) |
| | Five unknown elements. |
| | Th 232·4 (Krüss Nilson) |
| | Unknown element. |
| | U 239·3 (Zimmermann) |
+-------------------------+--------------------------------+
+----------------------------------------------------------------------+
| |
| |
| Composition of the saline compounds, X = Cl |
| |
+----------------------------------------------------------------------+
| Br, (NO_{3}), 1/2 O, 1/2 (SO_{4}), OH, (OM) = Z, where M = K, |
| 1/2 Ca, 1/3 Al, &c. |
|Form RX RX_{2} RX_{3} RX_{4} RX_{5} RX_{6} RX_{7} RX_{8}|
|Oxi- R_{2}O RO R_{2}O_{3} RO_{2} R_{2}O_{5} RO_{3} R_{2}O_{7} RO_{4}|
|des |
| [7] [8] [9] [10] [11] [12] [13] [14] |
| |
| X or H_{2}O |
| iX |
| -- BeX_{2} |
| -- -- BX_{3} |
| |
| -- CO -- COZ_{2} |
| N_{2}O NO NOZ NO_2 NO_{2}Z |
| -- OX_{2} |
| FZ |
| |
| NaX |
| -- MgX_{2} |
| -- -- AlX_{3} |
| -- -- -- SiOZ_{2} |
| -- -- PX_{3} -- POZ_{3} |
| -- SX_{2} -- SOZ_{2} -- SO_{2}Z_{2} |
| ClZ -- ClOZ -- ClO_{2}Z -- ClO_{3}Z |
| |
| KX |
| -- CaX_{2} |
| -- -- ScX_{3} |
| -- TiX_{2} TiX_{3} TiX_{4} |
| -- VO VOX -- VOZ_{3} |
| -- CrX_{2} CrX_{3} CrO_{2} -- CrO_{2}Z_{2} |
| -- MnX_{2} MnX_{3} MnO_{2} -- MnO_{2}Z_{2} MnO_{3}Z |
| -- FeX_{2} FeX_{3} -- -- FeO_{2}Z_{2} |
| -- CoX_{2} CoX_{3} CoO_{2} |
| -- NiX_{2} NiX_{3} |
| CuX CuX_{2} |
| -- ZnX_{2} |
| -- -- GaX_{3} |
| -- GeX_{2} -- GeX_{4} |
| -- AsS AsX_{3} AsS_{2} AsO_{2}Z |
| -- -- -- SeOZ_{2} -- SeO_{2}Z_{2} |
| BrZ -- BrOZ -- BrO_{2}Z -- BrO_{3}Z |
| |
| RbX |
| -- SrX_{2} |
| -- -- YX_{3} |
| -- -- -- ZrX_{4} |
| -- -- NbX_{3} -- NbO_{2}Z |
| -- -- MoX_{3} MoX_{4} -- MoO_{2}Z_{2} |
|(eka-manganese, Em = 99). EmO_{3}Z |
| RuO_{4}|
| -- RuX_{2} RuX_{3} RuX_{4} -- RuO_{2}Z_{2} RuO_{3}Z |
| -- RhX_{2} RhX_{3} RhX_{4} -- RhO_{2}Z_{2} |
| PdX PdX_{2} -- PdX_{4} |
| AgX |
| -- CdX_{2} |
| -- InX_{2} InX_{3} |
| -- SnX_{2} -- SnX_{4} |
| -- -- SbX_{3} -- SbO_{2}Z |
| -- -- -- TeOZ_{2} -- TeO_{2}Z_{2} |
| IZ -- IZ_{3} -- IO_{2}Z -- IO_{3}Z |
| |
| CsX |
| -- BaX_{2} |
| -- -- LaX_{3} |
| -- -- CeX_{3} CeX_{4} |
| Little known Di = 142.1 and Yb = 173.2, and over 15 unknown elements.|
| -- -- -- -- TaO_{2}Z |
| -- -- -- WX_{4} -- WO_{2}Z_{2} |
| |
| OsO_{4}|
| -- -- OsX_{3} OsX_{4} -- OsO_{2}Z_{2} -- |
| -- -- IrX_{3} IrX_{4} -- IrO_{2}Z_{2} |
| -- PtX_{2} -- PtX_{4} |
| AuX -- AuX_{3} |
| HgX HgX_{2} |
| TlX -- TlX_{3} |
| -- PbX_{2} -- PbOZ_{2} |
| -- -- BiX_{3} -- BiO_{2}Z |
| |
| -- -- -- ThX_{4} |
| |
| -- -- -- UO_{2} -- UO_{2}X_{2} UO_{4}|
+----------------------------------------------------------------------+
+-------------------------+------------+---------+---------------------+
| | | Lower | Simple bodies |
|Molecular composition of | |hydrogen +-----+-------+-------|
|the higher hydrogen and | Peroxides | com- | Sp. | Sp. |Melting|
|metallo-organic compounds| | pounds | gr | vol. | point |
|-------------------------+------------+---------+-----+-------+-------|
| | | | | | |
| | | | | | |
|E=CH_{3}, C_{2}H_{5}, &c.| | | | | |
| | | | | | |
| | | | | | |
|[1] [2] [3] [4] | [15] | [16] |[17] | [18] | [19] |
| | | | | | |
| HH|H_{2}O_{2} | -- |*0·05| 20 | -250°?|
| | -- | -- | 0·59| 11·9 | 180° |
| | -- | BeH | 1·64| 5·5 | 900°?|
| BE_{3} -- --| -- | -- | 2·5 | 4·4 |1,300°?|
| CH_{4} C_{2}H_{6} | | | | | |
| C_{2}H_{4} C_{2}H_{2} |C_{2}O_{5}* | -- |*1·9 | 6·3 |2,600°?|
| NH_{3} N_{2}H_{4} --|N_{2}O_{6}* | N_{3}H |*0·6 | 23 | -203° |
| OH_{2} --|O_{3} | -- |*0·9 | 18 | -230°?|
| FH| -- | -- |?1·0 | 19 | ? |
| | | | | | |
| NaE|NaO | Na_{2}H | 0·98| 23·5 | 96° |
| MgE_{2} --| -- | MgH | 1·74| 14 | 500° |
| AlE_{3} -- --| -- | -- | 2·6 | 11 | 600° |
|SiH_{4} Si_{2}E_{6} -- --| -- | -- | 2·3 | 12 |1,300°?|
| PH_{3} P_{2}H_{4} --| -- | P_2H | 2·2 | 14 | 44° |
| SH_{2} --|S_{2}O_{7} | -- | 2·07| 15 | 114° |
| ClH| -- | -- |*1·3 | 27 | -75° |
| | | | | | |
| |KO_{2} | K_{2}H | 0·87| 45 | 58° |
| |CaO_{2} | CaH | 1·56| 26 | 800° |
| | -- | -- |?2·5 | ?18 |1,200°?|
| |TiO_{3} | -- | 3·6 | 13 |2,500°?|
| | -- | -- | 5·5 | 9 |3,000°?|
| |Cr_{2}O_{7} | -- | 6·7 | 7·7 |2,000°?|
| | -- | -- | 7·5 | 7·3 |1,500° |
| | -- |Fe_{n}H* | 7·8 | 7·2 |1,450° |
| | -- | -- | 8·6 | 6·8 |1,400° |
| | -- | Ni_{n}H | 8·7 | 6·8 |1,350° |
| |Cu_{2}O_{5}*| CuH | 8·8 | 7·2 |1,054° |
| ZnE_{2} --|ZnO_{2} | -- | 7·1 | 9·2 | 418° |
| GaE_{3} -- --| -- | -- | 5·96| 11·7 | 30° |
| GeE_{4} -- -- --| -- | -- | 5·47| 13·2 | 900° |
| AsH_{3} -- --| -- |As_{4}H* | 5·65| 13·3 | 500° |
| SeH_{2} --| -- | -- | 4·8 | 16 | 217° |
| BrH| -- | -- | 3·1 | 26 | -7° |
| | | | | | |
| |RbO |Rb_{2}H* | 1·5 | 57 | 39° |
| |SrO_{2} | SrH | 2·5 | 35 | 600°?|
| | -- | -- |*3·4 | *26 |1,000°?|
| | -- |Zr_{4n}H*| 4·1 | 22 |1,500°?|
| | -- |Nb_{n}H* | 7·1 | 13 |1,800°?|
| |Mo_{2}O_{7} | -- | 8·6 | 11 |2,200°?|
| | -- | -- | -- | -- | -- |
| | | | | | |
| | -- |Ru_{n}H* |12·2 | 8·4 |2,000°?|
| | -- |Rh_{n}H* |12·1 | 8·6 |1,900°?|
| | -- | Pd_{2}H |11·4 | 8·3 |1,500° |
| |AgO | -- |10·5 | 10·3 | 950° |
| CdE_{2} --|CdO_{2} | -- | 8·6 | 13 | 320° |
| InE_{3} -- --| -- | -- | 7·4 | 14 | 176° |
| SnE_{4} -- -- --|SnO_{3} | -- | 7·2 | 16 | 232° |
| SbH_{3} -- --| -- | -- | 6·7 | 18 | 432° |
| TeH_{2} --| -- | -- | 6·4 | 20 | 455° |
| IH| -- | -- | 4·9 | 26 | 114° |
| | | | | | |
| | -- |Cs_{2}H* | 2·37| 56 | 27° |
| |BaO_{2} | BaH | 3·75| 36 | ? |
| | -- | -- | 6·1 | 23 | ? |
| | -- | -- | 6·6 | 21 | 700°?|
| | | | | | |
| | -- |Ta_{n}H* |10·4 | 18 | ? |
| |W_{2}O_{7} | -- |19·1 | 9·6 |2,600° |
| | | | | | |
| | | | | | |
| | -- | -- |22·5 | 8·5 |2,700°?|
| | -- | Ir_nH* |22·4 | 8·6 |2,000° |
| | -- |Pt_{n}H* |21·4 | 9·2 |1,775° |
| | -- | -- |19·3 | 10 |1,045° |
| HgE_{2} --| -- | -- |13·6 | 15 | -39° |
| TlE_{3} -- --| -- | -- |11·8 | 17 | 294° |
| PbE_{4} -- -- --| -- | -- |11·3 | 18 | 328° |
| BiE_{3} -- --| -- | -- | 9·8 | 21 | 269° |
| | | | | | |
| | -- | -- |11·1 | 21 | ? |
| | | | | | |
| | -- | -- |18·7 | 13 |2,400°?|
+-------------------------+------------+---------+-----+-------+-------+
[A] From analogy there is reason for thinking that the atomic weight
of selenium is really slightly less than 79·0.
Columns 1, 2, 3, and 4 give the molecular composition of the hydrogen
and metallo-organic compounds, exhibiting the most characteristic forms
assumed by the elements. The first column contains only those which
correspond to the form RX_{4}, the second column those of the form
RX_{3}, the third of the form RX_{2}, and the fourth of the form RX, so
that the periodicity stands out clearly (see Column 16).
Column 5 contains the symbols of all the more or less well-known
elements, placed according to the order of the magnitude of their atomic
weights.
Column 6 contains the atomic weights of the elements according to the
most trustworthy determinations. The names of the investigators are given
in parenthesis. The atomic weight of oxygen, taken as 16, forms the basis
upon which these atomic weights were calculated. Some of these have been
recalculated by me on the basis of Stas's most trustworthy data (_see_
Chapter XXIV. and the numbers given by Stas in the table, where they are
taken according to van der Plaats and Thomsen's calculations).
Columns 7-14 contain the composition of the saline compounds of the
elements, placed according to their forms, RX, RX_{2} to RX_{8} (in the
14^{th} column). If the element R has a metallic character like H, Li,
Be, &c., then X represents Cl, NO_{3}, 1/2 SO_{4}, &c., haloid radicles,
or (OH) if a perfect hydrate is formed (alkali, aqueous base), or 1/2 O,
1/2 S, &c. when an anhydrous oxide, sulphide, &c. is formed. For
instance, NaCl, Mg(NO_{3})_{2}, Al_{2}(SO_{4})_{3}, correspond to NaX,
MgX_{2}, and AlX_{3}; so also Na(OH), Mg(OH)_{2}, Al(OH)_{3}, Na_{2}O,
MgO, Al_{2}O_{3}, &c. But if the element, like C or N, be of a metalloid
or acid character, X must be regarded as (OH) in the formation of
hydrates; (OM) in the formation of salts, where M is the equivalent of a
metal, 1/2 O in the formation of an anhydride, and Cl in the formation of
a chloranhydride; and in this case (_i.e._ in the acid compounds) Z is
put in the place of X; for example, the formulæ COZ_{2}, NO_{2}Z,
MNO_{2}Z, FeO_{2}Z_{2}, and IZ_{3} correspond to CO(NaO)_{2} =
Na_{2}CO_{3}, COCl_{2}, CO_{2}, NO_{2}(NaO) = NaNO_{3}, NO_{2}Cl,
NO_{2}(OH) = HNO_{3}; MnO_{3}(OK) = KMnO_{4}, ICl, &c.
The 15th column gives the compositions of the peroxides of the
elements, _taking them as anhydrous_. An asterisk (*) is attached to
those of which the composition has not been well established, and a dash
(--) shows that for a given element no peroxides have yet been obtained.
The peroxides contain more oxygen than the higher saline oxides of the
same elements, are powerfully oxidising, and easily give peroxide of
hydrogen. This latter circumstance necessitates their being referred to
the type of peroxide of hydrogen, if bases and acids are referred to the
type of water (see Chapter XV., Note 7 and 11 bis).
The 16th column gives the composition of the lower hydrogen
compounds like N_{3}H and Na_{2}H. They may often be regarded as alloys
of hydrogen, which is frequently disengaged by them at a comparatively
moderate temperature. They differ greatly in their nature from the
hydrogen compounds given in columns 1-4 (_see_ Note 12).
Column 17 gives the specific gravity of the elements in a solid and
a liquid state. An asterisk (*) is placed by those which can either only
be assumed from analogy (for example, the sp. gr. of fluorine and
hydrogen, which have not been obtained in a liquid state), or which vary
very rapidly with a variation of temperature and pressure (like oxygen
and nitrogen), or physical state (for instance, carbon in passing from
the state of charcoal to graphite and diamond). But as the sp. gr. in
general varies with the temperature, mechanical condition, &c., the
figures given, although chosen from the most trustworthy sources, can
only be regarded as approximate, and not as absolutely true. They clearly
show a certain periodicity; for instance, the sp. gr. diminishes from Al
on both sides (Al, Mg, Na, with decreasing atomic weight; and Al, Si, P,
S, Cl, with increasing atomic weight, it also diminishes on both sides
from Cu, Ru, and Os.)
The same remarks refer to the figures in the 18th column, which
gives the so-called atomic volumes of the simple bodies, or the quotient
of their atomic weight and specific gravity. For Na, K, Rb, and Cs the
atomic volume is greatest among the neighbouring elements. For Ni, Pd,
and Os it is least, and this indicates the periodicity of this property
of the simple bodies.
The last (19th) column gives the melting points of the simple
bodies. Here also a periodicity is seen, i.e. a maximum and minimum value
between which there are intermediate values, as we see, for instance, in
the series Cl, K, Ca, Sc, and Ti, or in the series Cr, Mn, Fe, Co, Ni,
Cu, Zn, Ga, and Ge.
* * * * *
CHAPTER XV
THE GROUPING OF THE ELEMENTS AND THE PERIODIC LAW
It is seen from the examples given in the preceding chapters that the sum
of the data concerning the chemical transformations proper to the
elements (for instance, with respect to the formation of acids, salts,
and other compounds having definite properties) is insufficient for
accurately determining the relationship of the elements, inasmuch as this
may be many-sided. Thus, lithium and barium are in some respects
analogous to sodium and potassium, and in others to magnesium and
calcium. It is evident, therefore, that for a complete judgment it is
necessary to have, not only qualitative, but also quantitative, exact and
measurable, data. When a property can be measured it ceases to be vague,
and becomes quantitative instead of merely qualitative.
Among these measurable properties of the elements, or of their
corresponding compounds, are: (_a_) isomorphism, or the analogy of
crystalline forms; and, connected with it, the power to form crystalline
mixtures which are isomorphous; (_b_) the relation of the volumes of
analogous compounds of the elements; (_c_) the composition of their
saline compounds; and (_d_) the relation of the atomic weights of the
elements. In this chapter we shall briefly consider these four aspects of
the matter, which are exceedingly important for a natural and fruitful
grouping of the elements, facilitating, not only a general acquaintance
with them, but also their detailed study.
Historically the first, and an important and convincing, method for
finding a relationship between the compounds of two different elements is
by _isomorphism_. This conception was introduced into chemistry by
Mitscherlich (in 1820), who demonstrated that the corresponding salts of
arsenic acid, H_{3}AsO_{4}, and phosphoric acid, H_{3}PO_{4}, crystallise
with an equal quantity of water, show an exceedingly close resemblance in
crystalline form (as regards the angles of their faces and axes), and are
able to crystallise together from solutions, forming crystals containing
a mixture of the isomorphous compounds. Isomorphous substances are those
which, with an equal number of atoms in their molecules, present an
analogy in their chemical reactions, a close resemblance in their
properties, and a similar or very nearly similar crystalline form: they
often contain certain elements in common, from which it is to be
concluded that the remaining elements (as in the preceding example of As
and P) are analogous to each other. And inasmuch as crystalline forms are
capable of exact measurement, the external form, or the relation of the
molecules which causes their grouping into a crystalline form, is
evidently as great a help in judging of the internal forces acting
between the atoms as a comparison of reactions, vapour densities, and
other like relations. We have already seen examples of this in the
preceding pages.[1] It will be sufficient to call to mind that the
compounds of the alkali metals with the halogens RX, in a crystalline
form, all belong to the cubic system and crystallise in octahedra or
cubes--for example, sodium chloride, potassium chloride, potassium
iodide, rubidium chloride, &c. The nitrates of rubidium and cæsium appear
in anhydrous crystals of the same form as potassium nitrate. The
carbonates of the metals of the alkaline earths are isomorphous with
calcium carbonate--that is, they either appear in forms like calc spar or
in the rhombic system in crystals analogous to aragonite.[1 bis]
Furthermore, sodium nitrate crystallises in rhombohedra, closely
resembling the rhombohedra of calc spar (calcium carbonate), CaCO_{3},
whilst potassium nitrate appears in the same form as aragonite, CaCO_{3},
and the number of atoms in both kinds of salts is the same: they all
contain one atom of a metal (K, Na, Ca), one atom of a non-metal (C, N),
and three atoms of oxygen. The analogy of form evidently coincides with
an analogy of atomic composition. But, as we have learnt from the
previous description of these salts, there is not any close resemblance
in their properties. It is evident that calcium carbonate approaches more
nearly to magnesium carbonate than to sodium nitrate, although their
crystalline forms are all equally alike. Isomorphous substances which are
perfectly analogous to each other are not only characterised by a close
resemblance of form (homeomorphism), but also by the faculty of entering
into analogous reactions, which is not the case with RNO_{3} and RCO_{3}.
The most important and direct method of recognising perfect
isomorphism--that is, the absolute analogy of two compounds--is given by
that property of analogous compounds of separating from solutions _in
homogeneous crystals, containing the most varied proportions_ of the
analogous substances which enter into their composition. These quantities
do not seem to be in dependence on the molecular or atomic weights, and
if they are governed by any laws they must be analogous to those which
apply to indefinite chemical compounds.[2] This will be clear from the
following examples. Potassium chloride and potassium nitrate are not
isomorphous with each other, and are in an atomic sense composed in a
different manner. If these salts be mixed in a solution and the solution
be evaporated, independent crystals of the two salts will separate, each
in that crystalline form which is proper to it. The crystals will not
contain a mixture of the two salts. But if we mix the solutions of two
isomorphous salts together, then, under certain circumstances, crystals
will be obtained which contain both these substances. However, this
cannot be taken as an absolute rule, for if we take a solution saturated
at a high temperature with a mixture of potassium and sodium chlorides,
then on evaporation sodium chloride only will separate, and on cooling
only potassium chloride. The first will contain very little potassium
chloride, and the latter very little sodium chloride.[3] But if we take,
for example, a mixture of solutions of magnesium sulphate and zinc
sulphate, they cannot be separated from each other by evaporating the
mixture, notwithstanding the rather considerable difference in the
solubility of these salts. Again, the isomorphous salts, magnesium
carbonate, and calcium carbonate are found together--that is, in one
crystal--in nature. The angle of the rhombohedron of these magnesia-lime
spars is intermediate between the angles proper to the two spars
individually (for calcium carbonate, the angle of the rhombohedron is
105° 8´; magnesium carbonate, 107° 30´; CaMg(CO_{3})_{2}, 106° 10´).
Certain of these _isomorphous mixtures_ of calc and magnesia spars appear
in well-formed crystals, and in this case there not unfrequently exists a
simple molecular proportion of strictly definite chemical combination
between the component salts--for instance, CaCO_{3},MgCO_{3}--whilst in
other cases, especially in the absence of distinct crystallisation (in
dolomites), no such simple molecular proportion is observable: this is
also the case in many artificially prepared isomorphous mixtures. The
microscopical and crystallo-optical researches of Professor Inostrantzoff
and others show that in many cases there is really a mechanical, although
microscopically minute, juxtaposition in one whole of the heterogeneous
crystals of calcium carbonate (double refracting) and of the compound
CaMgC_{2}O_{6}. If we suppose the adjacent parts to be microscopically
small (on the basis of the researches of Mallard, Weruboff, and others),
we obtain an idea of isomorphous mixtures. A formula of the following
kind is given to isomorphous mixtures: for instance, for spars, RCO_{3},
where R = Mg, Ca, and where it may be Fe,Mn ..., &c. This means that the
Ca is partially replaced by Mg or another metal. Alums form a common
example of the separation of isomorphous mixtures from solutions. They
are double sulphates (or seleniates) of alumina (or oxides isomorphous
with it) and the alkalis, which crystallise in well-formed crystals. If
aluminium sulphate be mixed with potassium sulphate, an alum separates,
having the composition KAlS_{2}O_{8},12H_{2}O. If sodium sulphate or
ammonium sulphate, or rubidium (or thallium) sulphate be used, we obtain
alums having the composition RAlS_{2}O_{8},12H_{2}O. Not only do they all
crystallise in the cubic system, but they also contain an equal atomic
quantity of water of crystallisation (12H_{2}O). Besides which, if we mix
solutions of the potassium and ammonium (NH_{4}AlS_{2}O_{8},12H_{2}O)
alums together, then the crystals which separate will contain various
proportions of the alkalis taken, and separate crystals of the alums of
one or the other kind will not be obtained, but each separate crystal
will contain both potassium and ammonium. Nor is this all; if we take a
crystal of a potassium alum and immerse it in a solution capable of
yielding ammonia alum, the crystal of the potash alum will continue to
grow and increase in size in this solution--that is, a layer of the
ammonia or other alum will deposit itself upon the planes bounding the
crystal of the potash alum. This is very distinctly seen if a colourless
crystal of a common alum be immersed in a saturated violet solution of
chrome alum, KCrS_{2}O_{8},12H_{2}O, which then deposits itself in a
violet layer over the colourless crystal of the alumina alum, as was
observed even before Mitscherlich noticed it. If this crystal be then
immersed in a solution of an alumina alum, a layer of this salt will form
over the layer of chrome alum, so that one alum is able to incite the
growth of the other. If the deposition proceed simultaneously, the
resultant intermixture may be minute and inseparable, but its nature is
understood from the preceding experiments; the attractive force of
crystallisation of isomorphous substances is so nearly equal that the
attractive power of an isomorphous substance induces a crystalline
superstructure exactly the same as would be produced by the attractive
force of like crystalline particles. From this it is evident that one
isomorphous substance may _induce the crystallisation_[4] of another.
Such a phenomenon explains, on the one hand, the aggregation of different
isomorphous substances in one crystal, whilst, on the other hand, it
serves as a most exact indication of the nearness both of the molecular
composition of isomorphous substances and of those forces which are
proper to the elements which distinguish the isomorphous substances.
Thus, for example, ferrous sulphate or green vitriol crystallises in the
monoclinic system and contains seven molecules of water,
FeSO_{4},7H_{2}O, whilst copper vitriol crystallises with five molecules
of water in the triclinic system, CuSO_{4},5H_{2}O; nevertheless, it may
be easily proved that both salts are perfectly isomorphous; that they are
able to appear in identically the same forms and with an equal molecular
amount of water. For instance, Marignac, by evaporating a mixture of
sulphuric acid and ferrous sulphate under the receiver of an air-pump,
first obtained crystals of the hepta-hydrated salt, and then of the
penta-hydrated salt FeSO_{4},5H_{2}O, which were perfectly similar to the
crystals of copper sulphate. Furthermore, Lecoq de Boisbaudran, by
immersing crystals of FeSO_{4},7H_{2}O in a supersaturated solution of
copper sulphate, caused the latter to deposit in the same form as ferrous
sulphate, in crystals of the monoclinic system, CuSO_{4},7H_{2}O.
[1] For instance the analogy of the sulphates of K, Rb, and Cs (Chapter
XIII., Note 1).
[1 bis] The crystalline forms of aragonite, strontianite, and witherite
belong to the rhombic system; the angle of the prism of CaCO_{3} is
116° 10´, of SrCO_{3} 117° 19´, and of BaCO_{3} 118° 30´. On the
other hand the crystalline forms of calc spar, magnesite, and
calamine, which resemble each other quite as closely, belong to the
rhombohedral system, with the angle of the rhombohedra for CaCO_{3}
105° 8´, MgCO_{3} 107° 10´, and ZnCO_{3} 107° 40´. From this
comparison it is at once evident that zinc is more closely allied
to magnesium than magnesium to calcium.
[2] Solutions furnish the commonest examples of indefinite chemical
compounds. But the isomorphous mixtures which are so common among
the crystalline compounds of silica forming the crust of the earth,
as well as alloys, which are so important in the application of
metals to the arts, are also instances of indefinite compounds. And
if in Chapter I., and in many other portions of this work, it has
been necessary to admit the presence of definite compounds (in a
state of dissociation) in solutions, the same applies with even
greater force to isomorphous mixtures and alloys. For this reason
in many places in this work I refer to facts which compel us to
recognise the existence of definite chemical compounds in all
isomorphous mixtures and alloys. This view of mine (which dates
from the sixties) upon isomorphous mixtures finds a particularly
clear confirmation in B. Roozeboom's researches (1892) upon the
solubility and crystallising capacity of mixtures of the chlorates
of potassium and thallium, KClO_{3} and TlClO_{3}. He showed that
when a solution contains different amounts of these salts, it
deposits crystals containing either an excess of the first salt,
from 98 p.c. to 100 p.c., or an excess of the second salt, from
63·7 to 100 p.c.; that is, in the crystalline form, either the
first salt saturates the second or the second the first, just as in
the solution of ether in water (Chapter I.); moreover, the
solubility of the mixtures containing 36·3 and 98 p.c. KClO_{3} is
similar, just as the vapour tension of a saturated solution of
water in ether is equal to that of a saturated solution of ether in
water (Chapter I., Note 47). But just as there are solutions
miscible in all proportions, so also certain isomorphous bodies can
be present in crystals in all possible proportions of their
component parts. Van 't Hoff calls such systems 'solid solutions.'
These views were subsequently elaborated by Nernst (1892), and Witt
(1891) applied them in explaining the phenomena observed in the
coloration of tissues.
[3] The cause of the difference which is observed in different
compounds of the same type, with respect to their property of
forming isomorphous mixtures, must not be looked for in the
difference of their volumetric composition, as many investigators,
including Kopp, affirm. The molecular volumes (found by dividing
the molecular weight by the density) of those isomorphous
substances which do give intermixtures are not nearer to each other
than the volumes of those which do not give mixtures; for example,
for magnesium carbonate the combining weight is 84, density 3·06,
and volume therefore 27; for calcium carbonate in the form of calc
spar the volume is 37, and in the form of aragonite 33; for
strontium carbonate 41, for barium carbonate 46; that is, the
volume of these closely allied isomorphous substances increases
with the combining weight. The same is observed if we compare
sodium chloride (molecular volume = 27) with potassium chloride
(volume = 37), or sodium sulphate (volume = 55) with potassium
sulphate (volume = 66), or sodium nitrate 39 with potassium nitrate
48, although the latter are less capable of giving isomorphous
mixtures than the former. It is evident that the cause of
isomorphism cannot be explained by an approximation in molecular
volumes. It is more likely that, given a similarity in form and
composition, the faculty to give isomorphous mixtures is connected
with the laws and degree of solubility.
[4] A phenomenon of a similar kind is shown for magnesium sulphate in
Note 27 of the last chapter. In the same example we see what a
complication the phenomena of dimorphism may introduce when the
forms of analogous compounds are compared.
Hence it is evident that isomorphism--that is, the analogy of forms and
the property of inducing crystallisation--may serve as a means for the
discovery of analogies in molecular composition. We will take an example
in order to render this clear. If, instead of aluminium sulphate, we add
magnesium sulphate to potassium sulphate, then, on evaporating the
solution, the double salt K_{2}MgS_{2}O_{8},6H_{2}O (Chapter XIV., Note
28) separates instead of an alum, and the ratio of the component parts
(in alums one atom of potassium per 2SO_{4}, and here two atoms) and the
amount of water of crystallisation (in alums 12, and here 6 equivalents
per 2SO_{4}) are quite different; nor is this double salt in any way
isomorphous with the alums, nor capable of forming an isomorphous
crystalline mixture with them, nor does the one salt provoke the
crystallisation of the other. From this we must conclude that although
alumina and magnesia, or aluminium and magnesium, resemble each other,
they are not isomorphous, and that although they give partially similar
double salts, these salts are not analogous to each other. And this is
expressed in their chemical formulæ by the fact that the number of atoms
in alumina or aluminium oxide, Al_{2}O_{3}, is different from the number
in magnesia, MgO. Aluminium is trivalent and magnesium bivalent. Thus,
having obtained a double salt from a given metal, it is possible to judge
of the analogy of the given metal with aluminium or with magnesium, or of
the absence of such an analogy, from the composition and form of this
salt. Thus zinc, for example, does not form alums, but forms a double
salt with potassium sulphate, which has a composition exactly like that
of the corresponding salt of magnesium. It is often possible to
distinguish the bivalent metals analogous to magnesium or calcium from
the trivalent metals, like aluminium, by such a method. Furthermore, the
specific heat and vapour density serve as guides. There are also indirect
proofs. Thus iron gives ferrous compounds, FeX_{2}, which are isomorphous
with the compounds of magnesium, and ferric compounds, FeX_{3}, which are
isomorphous with the compounds of aluminium; in this instance the
relative composition is directly determined by analysis, because, for a
given amount of iron, FeCl_{2} only contains two-thirds of the amount of
chlorine which occurs in FeCl_{3}, and the composition of the
corresponding oxygen compounds, _i.e._ of ferrous oxide, FeO, and ferric
oxide, Fe_{2}O_{3}, clearly indicates the analogy of the ferrous oxide
with MgO and of the ferric oxide with Al_{2}O_{3}.
Thus in the building up of similar molecules in crystalline forms we
see one of the numerous means for judging of the internal world of
molecules and atoms, and one of the weapons for conquests in the
invisible world of molecular mechanics which forms the main object of
physico-chemical knowledge. This method[5] has more than once been
employed for discovering the analogy of elements and of their compounds;
and as crystals are measurable, and the capacity to form crystalline
mixtures can be experimentally verified, this method is a numerical and
measurable one, and in no sense arbitrary.
[5] The property of solids of occurring in regular crystalline
forms--the occurrence of many substances in the earth's crust in
these forms--and those geometrical and simple laws which govern the
formation of crystals long ago attracted the attention of the
naturalist to crystals. The crystalline form is, without doubt, the
expression of the relation in which the atoms occur in the
molecules, and in which the molecules occur in the mass, of a
substance. Crystallisation is determined by the distribution of the
molecules along the direction of greatest cohesion, and therefore
those forces must take part in the crystalline distribution of
matter which act between the molecules; and, as they depend on the
forces binding the atoms together in the molecules, a very close
connection must exist between the atomic composition and the
distribution of the atoms in the molecule on the one hand, and the
crystalline form of a substance on the other hand; and hence an
insight into the composition may be arrived at from the crystalline
form. Such is the elementary and _a priori_ idea which lies at the
base of all researches into _the connection between composition and
crystalline form_. Haüy in 1811 established the following
fundamental law, which has been worked out by later investigators:
That the fundamental crystalline form for a given chemical compound
is constant (only the combinations vary), and that with a change of
composition the crystalline form also changes, naturally with the
exception of such limiting forms as the cube, regular octahedron,
&c., which may belong to various substances of the regular system.
The fundamental form is determined by the angles of certain
fundamental geometric forms (prisms, pyramids, rhombohedra), or the
ratio of the crystalline axes, and is connected with the optical
and many other properties of crystals. Since the establishment of
this law the description of definite compounds in a solid state is
accompanied by a description (measurement) of its crystals, which
forms an invariable, definite, and measurable character. The most
important epochs in the further history of this question were made
by the following discoveries:--Klaproth, Vauquelin, and others
showed that aragonite has the same composition as calc spar, whilst
the former belongs to the rhombic and the latter to the hexagonal
system. Haüy at first considered that the composition, and after
that the arrangement, of the atoms in the molecules was different.
This is dimorphism (_see_ Chapter XIV., Note 46). Beudant,
Frankenheim, Laurent, and others found that the forms of the two
nitres, KNO_{3} and NaNO_{3}, exactly correspond with the forms of
aragonite and calc spar; that they are able, moreover, to pass from
one form into another; and that the difference of the forms is
accompanied by a small alteration of the angles, for the angle of
the prisms of potassium nitrate and aragonite is 119°, and of
sodium nitrate and calc spar, 120°; and therefore dimorphism, or
the crystallisation of one substance in different forms, does not
necessarily imply a great difference in the distribution of the
molecules, although some difference clearly exists. The researches
of Mitscherlich (1822) on the dimorphism of sulphur confirmed this
conclusion, although it cannot yet be affirmed that in dimorphism
the arrangement of the atoms remains unaltered, and that only the
molecules are distributed differently. Leblanc, Berthier,
Wollaston, and others already knew that many substances of
different composition appear in the same forms, and crystallise
together in one crystal. Gay-Lussac (1816) showed that crystals of
potash alum continue to grow in a solution of ammonia alum. Beudant
(1817) explained this phenomenon as the _assimilation_ of a foreign
substance by a substance having a great force of crystallisation,
which he illustrated by many natural and artificial examples. But
Mitscherlich, and afterwards Berzelius and Henry Rose and others,
showed that such an assimilation only exists with a similarity or
approximate similarity of the forms of the individual substances
and with a certain degree of chemical analogy. Thus was established
the idea of _isomorphism_ as an analogy of forms by reason of a
resemblance of atomic composition, and by it was explained the
variability of the composition of a number of minerals as
isomorphous mixtures. Thus all the garnets are expressed by the
general formula: (RO)_{3}M_{2}O_{3}(SiO_{2})_{3}, where R = Ca, Mg,
Fe, Mn, and M = Fe, Al, and where we may have either R and M
separately, or their equivalent compounds, or their mixtures in all
possible proportions.
But other facts, which render the correlation of form and
composition still more complex, have accumulated side by side with
a mass of data which may be accounted for by admitting the
conceptions of isomorphism and dimorphism. Foremost among the
former stand the phenomena of _homeomorphism_--that is, a nearness
of forms with a difference of composition--and then the cases of
polymorphism and hemimorphism--that is, a nearness of the
fundamental forms or only of certain angles for substances which
are near or analogous in their composition. Instances of
homeomorphism are very numerous. Many of these, however, may be
reduced to a resemblance of atomic composition, although they do
not correspond to an isomorphism of the component elements; for
example, CdS (greenockite) and AgI, CaCO_{3} (aragonite) and
KNO_{3}, CaCO_{3} (calc spar) and NaNO_{3}, BaSO_{4} (heavy spar),
KMnO_{4} (potassium permanganate), and KClO_{4} (potassium
perchlorate), Al_{2}O_{3} (corundum) and FeTiO_{3} (titanic iron
ore), FeS_{2} (marcasite, rhombic system) and FeSAs (arsenical
pyrites), NiS and NiAs, &c. But besides these instances there are
homeomorphous substances with an absolute dissimilarity of
composition. Many such instances were pointed out by Dana.
Cinnabar, HgS, and susannite, PbSO_{4}3PbCO_{3} appear in very
analogous crystalline forms; the acid potassium sulphate
crystallises in the monoclinic system in crystals analogous to
felspar, KAlSi_{3}O_{8}; glauberite, Na_{2}Ca(SO_{4})_{2}, augite,
RSiO_{3} (R = Ca, Mg), sodium carbonate, Na_{2}CO_{3},10H_{2}O,
Glauber's salt, Na_{2}SO_{4},10H_{2}O, and borax,
Na_{2}BrO_{7},10H_{2}O, not only belong to the same system
(monoclinic), but exhibit an analogy of combinations and a nearness
of corresponding angles. These and many other similar cases might
appear to be perfectly arbitrary (especially as a _nearness_ of
angles and fundamental forms is a relative idea) were there not
other cases where a resemblance of properties and a distinct
relation in the variation of composition is connected with a
resemblance of form. Thus, for example, alumina, Al_{2}O_{3}, and
water, H_{2}O, are frequently found in many pyroxenes and
amphiboles which only contain silica and magnesia (MgO, CaO, FeO,
MnO). Scheerer and Hermann, and many others, endeavoured to explain
such instances by _polymetric isomorphism_, stating that MgO may be
replaced by 3H_{2}O (for example, olivine and serpentine), SiO_{2}
by Al_{2}O_{3} (in the amphiboles, talcs), and so on. A certain
number of the instances of this order are subject to doubt, because
many of the natural minerals which served as the basis for the
establishment of polymeric isomorphism in all probability no longer
present their original composition, but one which has been altered
under the influence of solutions which have come into contact with
them; they therefore belong to the class of _pseudomorphs_, or
false crystals. There is, however, no doubt of the existence of a
whole series of natural and artificial homeomorphs, which differ
from each other by atomic amounts of water, silica, and some other
component parts. Thus, Thomsen (1874) showed a very striking
instance. The metallic chlorides, RCl_{2}, often crystallise with
water, and they do not then contain less than one molecule of water
per atom of chlorine. The most familiar representative of the order
RCl_{2},2H_{2}O is BaCl_{2},2H_{2}O, which crystallises in the
rhombic system. Barium bromide, BaBr_{2},2H_{2}O, and copper
chloride, CuCl_{2},2H_{2}O, have nearly the same forms: potassium
iodate, KIO_{4}; potassium chlorate, KClO_{4}; potassium
permanganate, KMnO_{4}; barium sulphate, BaSO_{4}; calcium
sulphate, CaSO_{4}; sodium sulphate, Na_{2}SO_{4}; barium formate,
BaC_{2}H_{2}O_{4}, and others have almost the same crystalline form
(of the rhombic system). Parallel with this series is that of the
metallic chlorides containing RCl_{2},4H_{2}O, of the sulphates of
the composition RSO_{4},2H_{2}O, and the formates
RC_{2}H_{2}O_{4},2H_{2}O. These compounds belong to the monoclinic
system, have a close resemblance of form, and differ from the first
series by containing two more molecules of water. The addition of
two more molecules of water in all the above series also gives
forms of the monoclinic system closely resembling each other; for
example, NiCl_{2},6H_{2}O and MnSO_{4},4H_{2}O. Hence we see that
not only is RCl_{2},2H_{2}O analogous in form to RSO_{4} and
RC_{2}H_{2}O_{4}, but that their compounds with 2H_{2}O and with
4H_{2}O also exhibit closely analogous forms. From these examples
it is evident that the conditions which determine a given form may
be repeated not only in the presence of an isomorphous
exchange--that is, with an equal number of atoms in the
molecule--but also in the presence of an unequal number when there
are peculiar and as yet ungeneralised relations in composition.
Thus ZnO and Al_{2}O_{3} exhibit a close analogy of form. Both
oxides belong to the rhombohedral system, and the angle between the
pyramid and the terminal plane of the first is 118° 7´, and of the
second 118° 49´. Alumina, Al_{2}O_{3}, is also analogous in form to
SiO_{2}, and we shall see that these analogies of form are
conjoined with a certain analogy in properties. It is not
surprising, therefore, that in the complex molecule of a siliceous
compound it is sometimes possible to replace SiO_{2} by means of
Al_{2}O_{3}, as Scheerer admits. The oxides Cu_{2}O, MgO, NiO,
Fe_{3}O_{4}, CeO_{2}, crystallise in the regular system, although
they are of very different atomic structure. Marignac demonstrated
the perfect analogy of the forms of K_{2}ZrF_{6} and CaCO_{3}, and
the former is even dimorphous, like the calcium carbonate. The same
salt is isomorphous with R_{2}NbOF_{5} and R_{2}WO_{2}F_{4}, where
R is an alkali metal. There is an equivalency between CaCO_{3} and
K_{2}ZrF_{6}, because K_{2} is equivalent to Ca, C to Zr, and F_{6}
to O_{3}, and with the isomorphism of the other two salts we find
besides an equal contents of the alkali metal--an equal number of
atoms on the one hand and an analogy to the properties of
K_{2}ZrF_{6} on the other. The long-known isomorphism of the
corresponding compounds of potassium and ammonium, KX and NH_{4}X,
may be taken as the simplest example of the fact that an analogy of
form shows itself with an analogy of chemical reaction even without
an equality in atomic composition. Therefore the ultimate progress
of the entire doctrine of the correlation of composition and
crystalline forms will only be arrived at with the accumulation of
a sufficient number of facts collected on a plan corresponding with
the problems which here present themselves. The first steps have
already been made. The researches of the Geneva _savant_, Marignac,
on the crystalline form and composition of many of the double
fluorides, and the work of Wyruboff on the ferricyanides and other
compounds, are particularly important in this respect. It is
already evident that, with a definite change of composition,
certain angles remain constant, notwithstanding that others are
subject to alteration. Such an instance of the relation of forms
was observed by Laurent, and named by him _hemimorphism_ (an
anomalous term) when the analogy is limited to certain angles, and
_paramorphism_ when the forms in general approach each other, but
belong to different systems. So, for example, the angle of the
planes of a rhombohedron may be greater or less than 90°, and
therefore such acute and obtuse rhombohedra may closely approximate
to the cube. Hausmannite, Mn_{3}O_{4}, belongs to the tetragonal
system, and the planes of its pyramid are inclined at an angle of
about 118°, whilst magnetic iron ore, Fe_{3}O_{4}, which resembles
hausmannite in many respects, appears in regular octahedra--that
is, the pyramidal planes are inclined at an angle of 109° 28´. This
is an example of paramorphism; the systems are different, the
compositions are analogous, and there is a certain resemblance in
form. Hemimorphism has been found in many instances of saline and
other substitutions. Thus, Laurent demonstrated, and Hintze
confirmed (1873), that naphthalene derivatives of analogous
composition are hemimorphous. Nicklès (1849) showed that in
ethylene sulphate the angle of the prism is 125° 26´, and in the
nitrate of the same radicle 126° 95´. The angle of the prism of
methylamine oxalate is 131° 20´, and of fluoride, which is very
different in composition from the former, the angle is 132°. Groth
(1870) endeavoured to indicate in general what kinds of change of
form proceed with the substitution of hydrogen by various other
elements and groups, and he observed a regularity which he termed
_morphotropy_. The following examples show that morphotropy recalls
the hemimorphism of Laurent. Benzene, C_{6}H_{6}, rhombic system,
ratio of the axes 0·891 : 1 : 0·799. Phenol, C_{6}H_{5}(OH), and
resorcinol, C_{6}H_{4}(OH)_{2}, also rhombic system, but the ratio
of one axis is changed--thus, in resorcinol, 0·910 : 1 : 0·540;
that is, a portion of the crystalline structure in one direction is
the same, but in the other direction it is changed, whilst in the
rhombic system dinitrophenol, C_{6}H_{3}(NO_{2})_{2}(OH) =
O·833 : 1 : 0·753; trinitrophenol (picric acid),
C_{6}H_{2}(NO)_{3}(OH) = 0·937 : 1 : 0·974; and the potassium salt
= 0·942 : 1 : 1·354. Here the ratio of the first axis is
preserved--that is, certain angles remain constant, and the
chemical proximity of the composition of these bodies is undoubted.
Laurent compares hemimorphism with architectural style. Thus,
Gothic cathedrals differ in many respects, but there is an analogy
expressed both in the sum total of their common relations and in
certain details--for example, in the windows. It is evident that we
may expect many fruitful results for molecular mechanics (which
forms a problem common to many provinces of natural science) from
the further elaboration of the data concerning those variations
which take place in crystalline form when the composition of a
substance is subjected to a known change, and therefore I consider
it useful to point out to the student of science seeking for matter
for independent scientific research this vast field for work which
is presented by the correlation of form and composition. The
geometrical regularity and varied beauty of crystalline forms offer
no small attraction to research of this kind.
The regularity and simplicity expressed by the exact laws of crystalline
form repeat themselves in the aggregation of the atoms to form molecules.
Here, as there, there are but few forms which are essentially different,
and their apparent diversity reduces itself to a few fundamental
differences of type. There the molecules aggregate themselves into
crystalline forms; here, the atoms aggregate themselves into molecular
forms or into _the types of compounds_. In both cases the fundamental
crystalline or molecular forms are liable to variations, conjunctions,
and combinations. If we know that potassium gives compounds of the
fundamental type KX, where X is a univalent element (which combines with
one atom of hydrogen, and is, according to the law of substitution, able
to replace it), then we know the composition of its compounds: K_{2}O,
KHO, KCl, NH_{2}K, KNO_{3}, K_{2}SO_{4}, KHSO_{4},
K_{2}Mg(SO_{4})_{2},6H_{2}O, &c. All the possible derivative crystalline
forms are not known. So also all the atomic combinations are not known
for every element. Thus in the case of potassium, KCH_{3}, K_{3}P,
K_{2}Pt, and other like compounds which exist for hydrogen or chlorine,
are unknown.
Only a few fundamental types exist for the building up of atoms into
molecules, and the majority of them are already known to us. If X stand
for a univalent element, and R for an element combined with it, then
eight atomic types may be observed:--
RX, RX_{2}, RX_{3}, RX_{4}, RX_{5}, RX_{6}, RX_{7}, RX_{8}.
Let X be chlorine or hydrogen. Then as examples of the first type we
have: H_{2}, Cl_{2}, HCl, KCl, NaCl, &c. The compounds of oxygen or
calcium may serve as examples of the type RX_{2}: OH_{2}, OCl_{2}, OHCl,
CaO, Ca(OH)_{2}, CaCl_{2}, &c. For the third type RX_{3} we know the
representative NH_{3} and the corresponding compounds N_{2}O_{3}, NO(OH),
NO(OK), PCl_{3}, P_{2}O_{3}, PH_{3}, SbH_{3}, Sb_{2}O_{3}, B_{2}O_{3},
BCl_{3}, Al_{2}O_{3}, &c. The type RX_{4} is known among the hydrogen
compounds. Marsh gas, CH_{4}, and its corresponding saturated
hydrocarbons, C_{_n_}H_{2_n_ + 2}, are the best representatives. Also
CH_{3}Cl, CCl_{4}, SiCl_{4}, SnCl_{4}, SnO_{2}, CO_{2}, SiO_{2}, and a
whole series of other compounds come under this class. The type RX_{5} is
also already familiar to us, but there are no purely hydrogen compounds
among its representatives. Sal-ammoniac, NH_{4}Cl, and the corresponding
NH_{4}(OH), NO_{2}(OH), ClO_{2}(OK), as well as PCl_{5}, POCl_{3}, &c.,
are representatives of this type. In the higher types also there are no
hydrogen compounds, but in the type RX_{6} there is the chlorine compound
WCl_{6}. However, there are many oxygen compounds, and among them SO_{3}
is the best known representative. To this class also belong
SO_{2}(OH)_{2}, SO_{2}Cl_{2}, SO_{2}(OH)Cl, CrO_{3}, &c., all of an acid
character. Of the higher types there are in general only oxygen and acid
representatives. The type RX_{7} we know in perchloric acid, ClO_{3}(OH),
and potassium permanganate, MnO_{3}(OK), is also a member. The type
RX_{8} in a free state is very rare; osmic anhydride, OsO_{4}, is the
best known representative of it.[6]
[6] The still more complex combinations--which are so clearly expressed
in the crystallo-hydrates, double salts, and similar
compounds--although they may be regarded as independent, are,
however, most easily understood with our present knowledge as
aggregations of whole molecules to which there are no corresponding
double compounds, containing one atom of an element R and many
atoms of other elements RX_{_n_}. The above types embrace all cases
of direct combinations of atoms, and the formula MgSO_{4},7H_{2}O
cannot, without violating known facts, be directly deduced from the
types MgX_{_n_} or SX_{_n_}, whilst the formula MgSO_{4}
corresponds both with the type of the magnesium compounds MgX_{2}
and with the type of the sulphur compounds SO_{2}X_{2}, or in
general SX_{6}, where X_{2} is replaced by (OH)_{2}, with the
substitution in this case of H_{2} by the atom Mg, which always
replaces H_{2}. However, it must be remarked that the sodium
crystallo-hydrates often contain 10H_{2}O, the magnesium
crystallo-hydrates 6 and 7H_{2}O, and that the type PtM_{2}X_{6} is
proper to the double salts of platinum, &c. With the further
development of our knowledge concerning crystallo-hydrates, double
salts, alloys, solutions, &c., in the _chemical sense_ of feeble
compounds (that is, such as are easily destroyed by feeble chemical
influences) it will probably be possible to arrive at a perfect
generalisation for them. For a long time these subjects were only
studied by the way or by chance; our knowledge of them is
accidental and destitute of system, and therefore it is impossible
to expect as yet any generalisation as to their nature. The days of
Gerhardt are not long past when only three types were recognised:
RX, RX_{2}, and RX_{3}; the type RX_{4} was afterwards added (by
Cooper, Kekulé, Butleroff, and others), mainly for the purpose of
generalising the data respecting the carbon compounds. And indeed
many are still satisfied with these types, and derive the higher
types from them; for instance, RX_{5} from RX_{3}--as, for example,
POCl_{3} from PCl_{3}, considering the oxygen to be bound both to
the chlorine (as in HClO) and to the phosphorus. But the time has
now arrived when it is clearly seen that the forms RX, RX_{2},
RX_{3}, and RX_{4} do not exhaust the whole variety of phenomena.
The revolution became evident when Würtz showed that PCl_{5} is not
a compound of PCl_{3} + Cl_{2} (although it may decompose into
them), but a whole molecule capable of passing into vapour, PCl_{5}
like PF_{5} and SiF_{4}. The time for the recognition of types even
higher than RX_{8} is in my opinion in the future; that it will
come, we can already see in the fact that oxalic acid,
C_{2}H_{2}O_{4}, gives a crystallo-hydrate with 2H_{2}O; but it may
be referred to the type CH_{4}, or rather to the type of ethane,
C_{2}H_{6}, in which all the atoms of hydrogen are replaced by
hydroxyl, C_{2}H_{2}O_{4}2H_{2}O = C_{2}(OH)_{6} (_see_ Chapter
XXII., Note 35).
The four lower types RX, RX_{2}, RX_{3}, and RX_{4} are met with in
compounds of the elements R with chlorine and oxygen, and also in their
compounds with hydrogen, whilst the four higher types only appear for
such acid compounds as are formed by chlorine, oxygen, and similar
elements.
Among the oxygen compounds the _saline oxides_ which are capable of
forming salts either through the function of a base or through the
function of an acid anhydride attract the greatest interest in every
respect. Certain elements, like calcium and magnesium, only give one
saline oxide--for example, MgO, corresponding with the type MgX_{2}. But
the majority of the elements appear in several such forms. Thus copper
gives CuX and CuX_{2}, or Cu_{2}O and CuO. If an element R gives a higher
type RX_{_n_}, then there often also exist, as if by symmetry, lower
types, RX_{_n_-2}, RX_{_n_-4}, and in general such types as differ from
RX_{_n_} by an even number of X. Thus in the case of sulphur the types
SX_{2}, SX_{4}, and SX_{6} are known--for example SH_{2}, SO_{2}, and
SO_{3}. The last type is the highest, SX_{6}. The types SX_{5} and SX_{3}
do not exist. But even and uneven types sometimes appear for one and the
same element. Thus the types RX and RX_{2} are known for copper and
mercury.
Among the _saline_ oxides only the _eight types_ enumerated below are
known to exist. They determine the possible formulæ of the compounds of
the elements, if it be taken into consideration that an element which
gives a certain type of combination may also give lower types. For this
reason the rare type of the _suboxides_ or quaternary oxides R_{4}O (for
instance, Ag_{4}O, Ag_{2}Cl) is not characteristic; it is always
accompanied by one of the higher grades of oxidation, and the compounds
of this type are distinguished by their great chemical instability, and
split up into an element and the higher compound (for instance, Ag_{4}O =
2Ag + Ag_{2}O). Many elements, moreover, form transition oxides whose
composition is intermediate, which are able, like N_{2}O_{4}, to split up
into the lower and higher oxides. Thus iron gives magnetic oxide,
Fe_{3}O_{4}, which is in all respects (by its reactions) a compound of
the suboxide FeO with the oxide Fe_{2}O_{3}. The independent and more or
less stable saline compounds correspond with the following eight
types:--
R_{2}O; salts RX, hydroxides ROH. Generally basic like K_{2}O, Na_{2}O,
Hg_{2}O, Ag_{2}O, Cu_{2}O; if there are acid oxides of this
composition they are very rare, are only formed by distinctly acid
elements, and even then have only feeble acid properties; for
example, Cl_{2}O and N_{2}O.
R_{2}O_{2} or RO; salts RX_{2}, hydroxides R(OH)_{2}. The most simple
basic salts R_{2}OX_{2} or R(OH)X; for instance, the chloride
Zn_{2}OCl_{2}; also an almost exclusively basic type; but the basic
properties are more feebly developed than in the preceding type.
For example, CaO, MgO, BaO, PbO, FeO, MnO, &c.
R_{2}O_{3}; salts RX_{3}, hydroxides R(OH)_{3}, RO(OH), the most simple
basic salts ROX, R(OH)X_{3}. The bases are feeble, like
Al_{2}O_{3}, Fe_{2}O_{3}, Tl_{2}O_{3}, Sb_{2}O_{3}. The acid
properties are also feebly developed; for instance, in B_{2}O_{3};
but with the non-metals the properties of acids are already clear;
for instance, P_{2}O_{3}, P(OH)_{3}.
R_{2}O_{4} or RO_{2}; salts RX_{4} or ROX_{2}, hydroxides R(OH)_{4},
RO(OH)_{2}. Rarely bases (feeble), like ZrO_{2}, PtO_{2}; more
often acid oxides; but the acid properties are in general feeble,
as in CO_{2}, SO_{2}, SnO_{2}. Many intermediate oxides appear in
this and the preceding and following types.
R_{2}O_{5}; salts principally of the types ROX_{3}, RO_{2}X,
RO(OH)_{3}, RO_{2}(OH), rarely RX_{5}. The basic character (X, a
halogen, simple or complex; for instance, NO_{3}, Cl, &c.) is
feeble; the acid character predominates, as is seen in N_{2}O_{5},
P_{2}O_{5}, Cl_{2}O_{5}; then X = OH, OK, &c., for example
NO_{2}(OK).
R_{2}O_{6} or RO_{3}; salts and hydroxides generally of the type
RO_{2}X_{2}, RO_{2}(OH)_{2}. Oxides of an acid character, as
SO_{3}, CrO_{3}, MnO_{3}. Basic properties rare and feebly
developed as in UO_{3}.
R_{2}O_{7}; salts of the form RO_{3}X, RO_{3}(OH), acid oxides; for
instance, Cl_{2}O_{7}, Mn_{2}O_{7}. Basic properties as feebly
developed as the acid properties in the oxides R_{2}O.
R_{2}O_{8} or RO_{4}. A very rare type, and only known in OsO_{4} and
RuO_{4}.
It is evident from the circumstance that in all the higher types the
_acid hydroxides_ (for example, HClO_{4}, H_{2}SO_{4}, H_{3}PO_{4}) and
salts with a single atom of one element contain, like the higher saline
type RO_{4}, _not more than four atoms of oxygen_; that the formation of
the saline oxides is governed by a certain common principle which is best
looked for in the fundamental properties of oxygen, and in general of the
most simple compounds. The hydrate of the oxide RO_{2} is of the higher
type RO_{2}2H_{2}O = RH_{4}O_{4} = R(HO)_{4}. Such, for example, is the
hydrate of silica and the salts (orthosilicates) corresponding with it,
Si(MO)_{4}. The oxide R_{2}O_{5}, corresponds with the hydrate
R_{2}O_{5}3H_{2}O = 2RH_{3}O_{4} = 2RO(OH)_{3}. Such is orthophosphoric
acid, PH_{3}O_{3}. The hydrate of the oxide RO_{3} is RO_{3}H_{2}O =
RH_{2}O_{4} = RO_{2}(OH)_{2}--for instance, sulphuric acid. The hydrate
corresponding to R_{2}O_{7} is evidently RHO = RO_{3}(OH)--for example,
perchloric acid. Here, besides containing O_{4}, it must further be
remarked that _the amount of hydrogen in the hydrate is equal to the
amount of hydrogen in the hydrogen compound_. Thus silicon gives SiH_{4}
and SiH_{4}O_{4}, phosphorus PH_{3} and PH_{3}O_{4}, sulphur SH_{2} and
SH_{2}O_{4}, chlorine ClH and ClHO_{4}. This, if it does not explain, at
least connects in a harmonious and general system the fact that _the
elements are capable of combining with a greater amount of oxygen, the
less the amount of hydrogen which they are able to retain_. In this the
key to the comprehension of all further deductions must be looked for,
and we will therefore formulate this rule in general terms. An element R
gives a hydrogen compound RH_{_n_}, the hydrate of its higher oxide will
be RH_{_n_}O_{4}, and therefore the higher oxide will contain
2RH_{_n_}O_{4} - _n_H_{2}O = R_{2}O_{8 - _n_}. For example, chlorine
gives ClH, hydrate ClHO_{4}, and the higher oxide Cl_{2}O_{7}. Carbon
gives CH_{4} and CO_{2}. So also, SiO_{2} and SiH_{4} are the higher
compounds of silicon with hydrogen and oxygen, like CO_{2} and CH_{4}.
Here the amounts of oxygen and hydrogen are equivalent. Nitrogen combines
with a large amount of oxygen, forming N_{2}O_{5}, but, on the other
hand, with a small quantity of hydrogen in NH_{3}. _The sum of the
equivalents of hydrogen and oxygen_, occurring in combination with an
atom of nitrogen, is, as always in the higher types, equal to _eight_. It
is the same with the other elements which combine with hydrogen and
oxygen. Thus sulphur gives SO_{3}; consequently, six equivalents of
oxygen fall to an atom of sulphur, and in SH_{2} two equivalents of
hydrogen. The sum is again equal to eight. The relation between
Cl_{2}O_{7} and ClH is the same. This shows that the property of elements
of combining with such different elements as oxygen and hydrogen is
subject to one common law, which is also formulated in the system of the
elements presently to be described.[7]
[7] The hydrogen compounds, R_{2}H, in equivalency correspond with the
type of the suboxides, R_{4}O. Palladium, sodium, and potassium
give such hydrogen compounds, and it is worthy of remark that
according to the periodic system these elements stand near to each
other, and that in those groups where the hydrogen compounds R_{2}H
appear, the quaternary oxides R_{4}O are also present.
Not wishing to complicate the explanation, I here only touch on the
general features of the relation between the hydrates and oxides
and of the oxides among themselves. Thus, for instance, the
conception of the ortho-acids and of the normal acids will be
considered in speaking of phosphoric and phosphorous acids.
As in the further explanation of the periodic law only those oxides
which give salts will be considered, I think it will not be
superfluous to mention here the following facts relative to the
peroxides. Of the _peroxides_ corresponding with hydrogen peroxide,
the following are at present known: H_{2}O_{2}, Na_{2}O_{2},
S_{2}O_{7} (as HSO_{4}?), K_{2}O_{4}, K_{2}O_{2}, CaO_{2}, TiO_{3},
Cr_{2}O_{7}, CuO_{2}(?), ZnO_{2}, Rb_{2}O_{2}, SrO_{2},
Ag_{2}O_{2}, CdO_{2}, CsO_{2}, Cs_{2}O_{2}, BaO_{2}, Mo_{2}O_{7},
SnO_{3}, W_{2}O_{7}, UO_{4}. It is probable that the number of
peroxides will increase with further investigation. A periodicity
is seen in those now known, for the elements (excepting Li) of the
first group, which give R_{2}O, form peroxides, and then the
elements of the sixth group seem also to be particularly inclined
to form peroxides, R_{2}O_{7}; but at present it is too early, in
my opinion, to enter upon a generalisation of this subject, not
only because it is a new and but little studied matter (not
investigated for all the elements), but also, and more especially,
because in many instances only the hydrates are known--for
instance, Mo_{2}H_{2}O_{8}--and they perhaps are only compounds of
peroxide of hydrogen--for example, Mo_{2}H_{2}O_{8} = 2MoO_{3} +
H_{2}O_{2}--since Prof. Schöne has shown that H_{2}O_{2} and
BaO_{2} possess the property of combining together and with other
oxides. Nevertheless, I have, in the general table expressing the
periodic properties of the elements, endeavoured to sum up the data
respecting all the known peroxide compounds whose characteristic
property is seen in their capability to form peroxide of hydrogen
under many circumstances.
In the preceding we see not only the regularity and simplicity which
govern the formation and properties of the oxides and of all the
compounds of the elements, but also a fresh and exact means for
recognising the analogy of elements. Analogous elements give compounds of
analogous types, both higher and lower. If CO_{2} and SO_{2} are two
gases which closely resemble each other both in their physical and
chemical properties, the reason of this must be looked for not in an
analogy of sulphur and carbon, but in that identity of the type of
combination, RX_{4}, which both oxides assume, and in that influence
which a large mass of oxygen always exerts on the properties of its
compounds. In fact, there is little resemblance between carbon and
sulphur, as is seen not only from the fact that CO_{2} is the _higher
form_ of oxidation, whilst SO_{2} is able to further oxidise into SO_{3},
but also from the fact that all the other compounds--for example, SH_{2}
and CH_{4}, SCl_{2} and CCl_{4}, &c.--are entirely unlike both in type
and in chemical properties. This absence of analogy in carbon and sulphur
is especially clearly seen in the fact that the highest saline oxides are
of different composition, CO_{2} for carbon, and SO_{3} for sulphur. In
Chapter VIII. we considered the limit to which carbon tends in its
compounds, and in a similar manner there is for every element in its
compounds a tendency to attain a certain highest limit RX_{_n_}. This
view was particularly developed in the middle of the present century by
Frankland in studying the metallo-organic compounds, _i.e._ those in
which X is wholly or partially a hydrocarbon radicle; for instance, X =
CH_{3} or C_{2}H_{5} &c. Thus, for example, antimony, Sb (Chapter XIX.)
gives, with chlorine, compounds SbCl_{3} and SbCl_{5} and corresponding
oxygen compounds Sb_{2}O_{3} and Sb_{2}O_{5}, whilst under the action of
CH_{3}I, C_{2}H_{5}I, or in general EI (where E is a hydrocarbon radicle
of the paraffin series), upon antimony or its alloy with sodium there are
formed SbE_{3} (for example, Sb(CH_{3})_{3}, boiling at about 81°),
which, corresponding to the lower form of combination SbX_{3}, are able
to combine further with EI, or Cl_{2}, or O, and to form compounds of the
limiting type SbX_{5}; for example, SbE_{4}Cl corresponding to NH_{4}Cl
with the substitution of nitrogen by antimony, and of hydrogen by the
hydrocarbon radicle. The elements which are most chemically analogous are
characterised by the fact of their giving compounds of similar form
RX_{_n_}. The halogens which are analogous give both higher and lower
compounds. So also do the metals of the alkalis and of the alkaline
earths. And we saw that this analogy extends to the composition and
properties of the nitrogen and hydrogen compounds of these metals, which
is best seen in the salts. Many such groups of analogous elements have
long been known. Thus there are analogues of oxygen, nitrogen, and
carbon, and we shall meet with many such groups. But an acquaintance with
them inevitably leads to the questions, what is the cause of analogy and
what is the relation of one group to another? If these questions remain
unanswered, it is easy to fall into error in the formation of the groups,
because the notions of the degree of analogy will always be relative, and
will not present any accuracy or distinctness Thus lithium is analogous
in some respects to potassium and in others to magnesium; beryllium is
analogous to both aluminium and magnesium. Thallium, as we shall
afterwards see and as was observed on its discovery, has much kinship
with lead and mercury, but some of its properties appertain to lithium
and potassium. Naturally, where it is impossible to make measurements one
is reluctantly obliged to limit oneself to approximate comparisons,
founded on apparent signs which are not distinct and are wanting in
exactitude. But in the elements there is one accurately measurable
property, which is subject to no doubt--namely, that property which is
expressed in their atomic weights. Its magnitude indicates the relative
mass of the atom, or, if we avoid the conception of the atom, its
magnitude shows the relation between the masses forming the chemical and
independent individuals or elements. And according to the teaching of all
exact data about the phenomena of nature, the mass of a substance is that
property on which all its remaining properties must be dependent, because
they are all determined by similar conditions or by those forces which
act in the weight of a substance, and this is directly proportional to
its mass. Therefore it is most natural to seek for a dependence between
the properties and analogies of the elements on the one hand and their
atomic weights on the other.
This is the fundamental idea which leads _to arranging all the elements
according to their atomic weights_. A periodic repetition of properties
is then immediately observed in the elements. We are already familiar
with examples of this:--
F = 19, Cl = 35·5, Br = 80, I = 127,
Na = 23, K = 39, Rb = 85, Cs = 133,
Mg = 24, Ca = 40, Sr = 87, Ba = 137.
The essence of the matter is seen in these groups. The halogens have
smaller atomic weights than the alkali metals, and the latter than the
metals of the alkaline earths. Therefore, _if all the elements be
arranged in the order of their atomic weights, a periodic repetition of
properties is obtained_. This is expressed by the _law of periodicity_,
_the properties of the elements, as well as the forms and properties of
their compounds, are in periodic dependence or (expressing ourselves
algebraically) form a periodic function of the atomic weights of the
elements_.[8] Table I. of _the periodic system of the elements_, which is
placed at the very beginning of this book, is designed to illustrate this
law. It is arranged in conformity with the eight types of oxides
described in the preceding pages, and those elements which give the
oxides, R_{2}O and consequently salts RX, form the 1st group; the
elements giving R_{2}O_{2} or RO as their highest grade of oxidation
belong to the 2nd group; those giving R_{2}O_{3} as their highest oxides
form the 3rd group, and so on; whilst the elements of all the groups
which are nearest in their atomic weights are arranged in series from 1
to 12. The even and uneven series of the same groups present the same
forms and limits, but differ in their properties, and therefore two
contiguous series, one even and the other uneven--for instance, the 4th
and 5th--form a period. Hence the elements of the 4th, 6th, 8th, 10th,
and 12th, or of the 3rd, 5th, 7th, 9th, and 11th, series form analogues,
like the halogens, the alkali metals, &c. The conjunction of two series,
one even and one contiguous uneven series, thus forms one large _period_.
These periods, beginning with the alkali metals, end with the halogens.
The elements of the first two series have the lowest atomic weights, and
in consequence of this very circumstance, although they bear the general
properties of a group, they still show many peculiar and independent
properties.[9] Thus fluorine, as we know, differs in many points from the
other halogens, and lithium from the other alkali metals, and so on.
These lightest elements may be termed _typical elements_. They include--
H.
Li, Be, B, C, N, O, F.
Na, Mg....
In the annexed table all the remaining elements are arranged, not in
groups and series, but _according to periods_. In order to understand the
essence of the matter, it must be remembered that here the atomic weight
gradually increases along a given line; for instance, in the line
commencing with K = 39 and ending with Br = 80, the intermediate elements
have intermediate atomic weights, as is clearly seen in Table III., where
the elements stand in the order of their atomic weights.
I. II. III. IV. V. VI. VII. I. II. III. IV. V. VI. VII.
{ Even Series. } Mg Al Si P S Cl
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br
Rb Sr Y Zr Nb Mo -- Ru Rh Pd Ag Cd In Sn Sb Te I
Cs Ba La Ce Di? -- -- -- -- -- -- -- -- -- -- -- --
-- -- Yb -- Ta W -- Os Ir Pt Au Hg Tl Pb Bi -- --
-- -- -- Th -- U { Uneven Series }
The same degree of analogy that we know to exist between potassium,
rubidium, and cæsium; or chlorine, bromine, and iodine; or calcium,
strontium, and barium, also exists between the elements of the other
vertical columns. Thus, for example, zinc, cadmium, and mercury, which
are described in the following chapter, present a very close analogy with
magnesium. For a true comprehension of the matter[10] it is very
important to see that all the aspects of the distribution of the elements
according to their atomic weights essentially express one and the same
fundamental _dependence_--_periodic properties_.[11] The following points
then must be remarked in it.
[8] The periodic law and the periodic system of the elements appeared
in the same form as here given in the first edition of this work,
begun in 1868 and finished in 1871. In laying out the accumulated
information respecting the elements, I had occasion to reflect on
their mutual relations. At the beginning of 1869 I distributed
among many chemists a pamphlet entitled 'An Attempted System of the
Elements, based on their Atomic Weights and Chemical Analogies,'
and at the March meeting of the Russian Chemical Society, 1869, I
communicated a paper 'On the Correlation of the Properties and
Atomic Weights of the Elements.' The substance of this paper is
embraced in the following conclusions: (1) The elements, if
arranged according to their atomic weights, exhibit an evident
_periodicity_ of properties. (2) Elements which are similar as
regards their chemical properties have atomic weights which are
either of nearly the same value (platinum, iridium, osmium) or
which increase regularly (_e.g._ potassium, rubidium, cæsium). (3)
The arrangement of the elements or of groups of elements in the
order of their atomic weights corresponds with their so-called
_valencies_. (4) The elements, which are the most widely
distributed in nature, have _small_ atomic weights, and all the
elements of small atomic weight are characterised by sharply
defined properties. They are therefore typical elements. (5) The
_magnitude_ of the atomic weight determines the character of an
element. (6) The discovery of many yet unknown elements may be
expected. For instance, elements analogous to aluminium and
silicon, whose atomic weights would be between 65 and 75. (7) The
atomic weight of an element may sometimes be corrected by aid of a
knowledge of those of the adjacent elements. Thus the combining
weight of tellurium must lie between 123 and 126, and cannot be
128. (8) Certain characteristic properties of the elements can be
foretold from their atomic weights.
The entire periodic law is included in these lines. In the series
of subsequent papers (1870-72, for example, in the _Transactions_
of the Russian Chemical Society, of the Moscow Meeting of
Naturalists, of the St. Petersburg Academy, and Liebig's _Annalen_)
on the same subject we only find applications of the same
principles, which were afterwards confirmed by the labours of
Roscoe, Carnelley, Thorpe, and others in England; of Rammelsberg
(cerium and uranium), L. Meyer (the specific volumes of the
elements), Zimmermann (uranium), and more especially of C. Winkler
(who discovered germanium, and showed its identity with
ekasilicon), and others in Germany; of Lecoq de Boisbaudran in
France (the discoverer of gallium = ekaaluminium); of Clève (the
atomic weights of the cerium metals), Nilson (discoverer of
scandium = ekaboron), and Nilson and Pettersson (determination of
the vapour density of beryllium chloride) in Sweden; and of Brauner
(who investigated cerium, and determined the combining weight of
tellurium = 125) in Austria, and Piccini in Italy.
I consider it necessary to state that, in arranging the periodic
system of the elements, I made use of the previous researches of
Dumas, Gladstone, Pettenkofer, Kremers, and Lenssen on the atomic
weights of related elements, but I was not acquainted with the
works preceding mine of De Chancourtois (_vis tellurique_, or the
spiral of the elements according to their properties and
equivalents) in France, and of J. Newlands (Law of Octaves--for
instance, H, F, Cl, Co, Br, Pd, I, Pt form the first octave, and O,
S, Fe, Se, Rh, Te, Au, Th the last) in England, although certain
germs of the periodic law are to be seen in these works. With
regard to the work of Prof. Lothar Meyer respecting the periodic
law (Notes 12 and 13), it is evident, judging from the method of
investigation, and from his statement (Liebig's _Annalen, Supt.
Band 7_, 1870, 354), at the very commencement of which he cites my
paper of 1869 above mentioned, that he accepted the periodic law in
the form which I proposed.
In concluding this historical statement I consider it well to
observe that no law of nature, however general, has been
established all at once; its recognition is always preceded by many
hints; the establishment of a law, however, does not take place
when its significance is recognised, but only when it has been
confirmed by experiment, which the man of science must consider as
the only proof of the correctness of his conjectures and opinions.
I therefore, for my part, look upon Roscoe, De Boisbaudran, Nilson,
Winkler, Brauner, Carnelley, Thorpe, and others who verified the
adaptability of the periodic law to chemical facts, as the true
founders of the periodic law, the further development of which
still awaits fresh workers.
[9] This resembles the fact, well known to those having an acquaintance
with organic chemistry, that in a series of homologues (Chapter
VIII.) the first members, in which there is the least carbon,
although showing the general properties of the homologous series,
still present certain distinct peculiarities.
[10] Besides arranging the elements (_a_) in a successive order
according to their atomic weights, with indication of their
analogies by showing some of the properties--for instance, their
power of giving one or another form of combination--both of the
_elements_ and of their compounds (as is done in Table III. and in
the table on p. 36), (_b_) according to periods (as in Table I. at
the commencement of volume I. after the preface), and (_c_)
according to groups and series or small periods (as in the same
tables), I am acquainted with the following methods of expressing
the periodic relations of the elements: (1) By a curve drawn
through points obtained in the following manner: The elements are
arranged along the horizontal axis as abscissæ at distances from
zero proportional to their atomic weights, whilst the values for
all the elements of some property--for example, the specific
volumes or the melting points, are expressed by the ordinates.
This method, although graphic, has the theoretical disadvantage
that it does not in any way indicate the existence of a limited
and definite number of elements in each period. There is nothing,
for instance, in this method of expressing the law of periodicity
to show that between magnesium and aluminium there can be no other
element with an atomic weight of, say, 25, atomic volume 13, and
in general having properties intermediate between those of these
two elements. The actual periodic law does not correspond with a
continuous change of properties, with a continuous variation of
atomic weight--in a word, it does not express an uninterrupted
function--and as the law is purely chemical, starting from the
conception of atoms and molecules which combine in multiple
proportions, with intervals (not continuously), it _above all_
depends on there being but few types of compounds, which are
arithmetically simple, _repeat themselves_, and offer no
uninterrupted transitions, so that each period can only contain a
definite number of members. For this reason there can be no other
elements between magnesium, which gives the chloride MgCl_{2}, and
aluminium, which forms AlX_{3}; there is a break in the
continuity, according to the law of multiple proportions. The
periodic law ought not, therefore, to be expressed by geometrical
figures in which continuity is always understood. Owing to these
considerations I never have and never will express the periodic
relations of the elements by any geometrical figures. (2) _By a
plane spiral._ Radii are traced from a centre, proportional to the
atomic weights; analogous elements lie along one radius, and the
points of intersection are arranged in a spiral. This method,
adopted by De Chancourtois, Baumgauer, E. Huth, and others, has
many of the imperfections of the preceding, although it removes
the indefiniteness as to the number of elements in a period. It is
merely an attempt to reduce the complex relations to a simple
graphic representation, since the equation to the spiral and the
number of radii are not dependent upon anything. (3) _By the lines
of atomicity_, either parallel, as in Reynolds's and the Rev. S.
Haughton's method, or as in Crookes's method, arranged to the
right and left of an axis, along which the magnitudes of the
atomic weights are counted, and the position of the elements
marked off, on the one side the members of the even series
(paramagnetic, like oxygen, potassium, iron), and on the other
side the members of the uneven series (diamagnetic, like sulphur,
chlorine, zinc, and mercury). On joining up these points a
periodic curve is obtained, compared by Crookes to the
oscillations of a pendulum, and, according to Haughton,
representing a cubical curve. This method would be very graphic
did it not require, for instance, that sulphur should be
considered as bivalent and manganese as univalent, although
neither of these elements gives stable derivatives of these
natures, and although the one is taken on the basis of the lowest
possible compound SX_{2}, and the other of the highest, because
manganese can be referred to the univalent elements only by the
analogy of KMnO_{4} to KClO_{4}. Furthermore, Reynolds and Crookes
place hydrogen, iron, nickel, cobalt, and others outside the axis
of atomicity, and consider uranium as bivalent without the least
foundation. (4) Rantsheff endeavoured to classify the elements in
their periodic relations by a system dependent on solid geometry.
He communicated this mode of expression to the Russian Chemical
Society, but his communication, which is apparently not void of
interest, has not yet appeared in print. (5) _By algebraic
formulæ_: for example, E. J. Mills (1886) endeavours to express
all the atomic weights by the logarithmic function A = 15(_n_ -
0·9375_t_), in which the variables _n_ and _t_ are whole numbers.
For instance, for oxygen _n_ = 2, _t_ = 1; hence A = 15·94; for
antimony _n_ = 9, _t_ = 0; whence A = 120, and so on. _n_ varies
from 1 to 16 and _t_ from 0 to 59. The analogues are hardly
distinguishable by this method: thus for chlorine the magnitudes
of _n_ and _t_ are 3 and 7; for bromine 6 and 6; for iodine 9 and
9; for potassium 3 and 14; for rubidium 6 and 18; for cæsium 9 and
20; but a certain regularity seems to be shown. (6) A more natural
method of expressing the dependence of the properties of elements
on their atomic weights is obtained by _trigonometrical
functions_, because this dependence is periodic like the functions
of trigonometrical lines, and therefore Ridberg in Sweden (Lund,
1885) and F. Flavitzky in Russia (Kazan, 1887) have adopted a
similar method of expression, which must be considered as worthy
of being worked out, although it does not express the absence of
intermediate elements--for instance, between magnesium and
aluminium, which is essentially the most important part of the
matter. (7) The investigations of B. N. Tchitchérin (1888,
_Journal of the Russian Physical and Chemical Society_) form the
first effort in the latter direction. He carefully studied the
alkali metals, and discovered the following simple relation
between their atomic volumes: they can all be expressed by A(2 -
0·0428A_n_), where A is the atomic weight and _n_ = 1 for lithium
and sodium, 4/8 for potassium, 3/8 for rubidium, and 2/8 for
cæsium. If _n_ always = 1, then the volume of the atom would
become zero at A = 46-2/3, and would reach its maximum when A =
23-1/3, and the density increases with the growth of A. In order
to explain the variation of _n_, and the relation of the atomic
weights of the alkali metals to those of the other elements, as
also the atomicity itself, Tchitchérin supposes all atoms to be
built up of a primary matter; he considers the relation of the
central to the peripheric mass, and, guided by mechanical
principles, deduces many of the properties of the atoms from the
reaction of the internal and peripheric parts of each atom. This
endeavour offers many interesting points, but it admits the
hypothesis of the building up of all the elements from one primary
matter, and at the present time such an hypothesis has not the
least support either in theory or in fact. Besides which the
starting-point of the theory is the specific gravity of the metals
at a definite temperature (it is not known how the above relation
would appear at other temperatures), and the specific gravity
varies even under mechanical influences. L. Hugo (1884)
endeavoured to represent the atomic weights of Li, Na, K, Rb, and
Cs by geometrical figures--for instance, Li = 7 represents a
central atom = 1 and six atoms on the six terminals of an
octahedron; Na, is obtained by applying two such atoms on each
edge of an octahedron, and so on. It is evident that such methods
can add nothing new to our data respecting the atomic weights of
analogous elements.
[11] Many natural phenomena exhibit a dependence of a periodic
character. Thus the phenomena of day and night and of the seasons
of the year, and vibrations of all kinds, exhibit variations of a
periodic character in dependence on time and space. But in
ordinary periodic functions one variable varies continuously,
whilst the other increases to a limit, then a period of decrease
begins, and having in turn reached its limit a period of increase
again begins. It is otherwise in the periodic function of the
elements. Here the mass of the elements does not increase
continuously, but abruptly, by steps, as from magnesium to
aluminium. So also the valency or atomicity leaps directly from 1
to 2 to 3, &c., without intermediate quantities, and in my opinion
it is these properties which are the most important, and it is
their periodicity which forms the substance of the periodic law.
It expresses _the properties of the real elements_, and not of
what may be termed their manifestations visually known to us. The
external properties of elements and compounds are in periodic
dependence on the atomic weight of the elements only because these
external properties are themselves the result of the properties of
the real elements which unite to form the 'free' elements and the
compounds. To explain and express the periodic law is to explain
and express the cause of the law of multiple proportions, of the
difference of the elements, and the variation of their atomicity,
and at the same time to understand what mass and gravitation are.
In my opinion this is still premature. But just as without knowing
the cause of gravitation it is possible to make use of the law of
gravity, so for the aims of chemistry it is possible to take
advantage of the laws discovered by chemistry without being able
to explain their causes. The above-mentioned peculiarity of the
laws of chemistry respecting definite compounds and the atomic
weights leads one to think that the time has not yet come for
their full explanation, and I do not think that it will come
before the explanation of such a primary law of nature as the law
of gravity.
It will not be out of place here to turn our attention to the
many-sided correlation existing between the undecomposable
_elements and the compound carbon radicles_, which has long been
remarked (Pettenkofer, Dumas, and others), and reconsidered in
recent times by Carnelley (1886), and most originally in
Pelopidas's work (1883) on the principles of the periodic system.
Pelopidas compares the series containing eight hydrocarbon
radicles, C_{_n_}H_{2_n_ + 1}, C_{_n_}H_{2_n_} &c., for instance,
C_{6}H_{13}, C_{6}H_{12}, C_{6}H_{11}, C_{6}H_{10}, C_{6}H_{9},
C_{6}H_{8}, C_{6}H_{7}, and C_{6}H_{6}--with the series of the
elements arranged in eight groups. The analogy is particularly
clear owing to the property of C_{_n_}H_{2_n_+1} to combine with
X, thus reaching saturation, and of the following members with
X_{2}, X_{3} ... X_{8}, and especially because these are followed
by an aromatic radicle--for example, C_{6}H_{5}--in which, as is
well known, many of the properties of the saturated radicle
C_{6}H_{13} are repeated, and in particular the power of forming a
univalent radicle again appears. Pelopidas shows a confirmation of
the parallel in the property of the above radicles of giving
oxygen compounds corresponding with the groups in the periodic
system. Thus the hydrocarbon radicles of the first group--for
instance, C_{6}H_{13} or C_{6}H_{5}--give oxides of the form
R_{2}O and hydroxides RHO, like the metals of the alkalis; and in
the third group they form oxides R_{2}O_{3} and hydrates RO_{2}H.
For example, in the series CH_{3} the corresponding compounds of
the third group will be the oxide (CH)_{2}O_{3} or
C_{2}H_{2}O_{3}--that is, formic anhydride and hydrate, CHO_{2}H,
or formic acid. In the sixth group, with a composition of C_{2},
the oxide RO_{3} will be C_{2}O_{3}, and hydrate
C_{2}H_{2}O_{4}--that is, also a bibasic acid (oxalic) resembling
sulphuric, among the inorganic acids. After applying his views to
a number of organic compounds, Pelopidas dwells more particularly
on the radicles corresponding with ammonium.
With respect to this remarkable parallelism, it must above all be
observed that in the elements the atomic weight increases in
passing to contiguous members of a higher valency, whilst here it
decreases, which should indicate that the periodic variability of
elements and compounds is subject to some higher law whose nature,
and still more whose cause, cannot at present be determined. It is
probably based on the fundamental principles of the internal
mechanics of the atoms and molecules, and as the periodic law has
only been generally recognised for a few years it is not
surprising that any further progress towards its explanation can
only be looked for in the development of facts touching on this
subject.
1. The composition of the higher oxygen compounds is determined by the
groups: the first group gives R_{2}O, the second R_{2}O_{2} or RO, the
third R_{2}O_{3}, &c. There are eight types of oxides and therefore eight
groups. Two groups give a period, and the same type of oxide is met with
twice in a period. For example, in the period beginning with potassium,
oxides of the composition RO are formed by calcium and zinc, and of the
composition RO_{3} by molybdenum and tellurium. The oxides of the even
series, of the same type, have stronger basic properties than the oxides
of the uneven series, and the latter as a rule are endowed with an acid
character. Therefore the elements which exclusively give bases, like the
alkali metals, will be found at the commencement of the period, whilst
such purely acid elements as the halogens will be at the end of the
period. The interval will be occupied by intermediate elements, whose
character and properties we shall afterwards describe. It must be
observed that the acid character is chiefly proper to the elements with
small atomic weights in the uneven series, whilst the basic character is
exhibited by the heavier elements in the even series. Hence elements
which give acids chiefly predominate among the lightest (typical)
elements, especially in the last groups; whilst the heaviest elements,
even in the last groups (for instance, thallium, uranium) have a basic
character. Thus the basic and acid characters of the higher oxides are
determined (_a_) by the type of oxide, (_b_) by the even or uneven
series, and (_c_) by the atomic weight.[11 bis] The groups are indicated
by Roman numerals from I. to VIII.
2. _The hydrogen compounds_ being volatile or gaseous substances which
are prone to reaction--such as HCl, H_{2}O, H_{3}N, and H_{4}C[12]--are
only formed by the elements of the uneven series and higher groups giving
oxides of the forms R_{2}O_{_n_}, RO_{3}, R_{2}O_{5}, and RO_{2}.
3. If an element gives a hydrogen compound, RX_{_m_}, it forms an
_organo-metallic compound_ of the same composition, where X =
C_{_n_}H_{2_n_ + 1}; that is, X is the radicle of a saturated
hydrocarbon. The elements of the uneven series, which are incapable of
giving hydrogen compounds, and give oxides of the forms RX, RX_{2},
R_{X}3, also give organo-metallic compounds of this form proper to the
higher oxides. Thus zinc forms the oxide ZnO, salts ZnX_{2} and zinc
ethyl Zn(C_{2}H_{5})_{2}. The elements of the even series do not seem to
form organo-metallic compounds at all; at least all efforts for their
preparation have as yet been fruitless--for instance, in the case of
titanium, zirconium, or iron.
4. The atomic weights of elements belonging to contiguous periods differ
approximately by 45; for example, K
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