then being the difference in relative position. This kind of
isomerism has been denominated stereoisomerism (q.v.) often
stereomerism. But there is a last group belonging here in which
identity of structure goes farthest. There are substances such
as sulphur, showing difference of modification in crystalline
state—the ordinary rhombic form in which sulphur occurs as a
mineral, while, after melting and cooling, long needles appear
which belong to the monosymmetric system. These differences,
which go hand in hand with those in other properties, e.g.
specific heat and specific gravity, are absolutely confined to
the crystalline state, disappearing with it when both modifications
of sulphur are melted, or dissolved in carbon disulphide
or evaporated. So it is natural to admit that here we have
to deal with identical molecules, but that only the internal
arrangement differs from case to case as identical balls may be
grouped in different ways. This case of difference in properties
combined with identical composition is therefore called polymorphism.
To summarize, we have to deal with polymerism, metamerism, stereoisomerism, polymorphism; whereas phenomena denominated tautomerism, pseudomerism and desmotropism form different particular features of metamerism, as well as the phenomena of allotropy, which is merely the difference of properties which an element may show, and can be due to polymerism, as in oxygen, where by the side of the ordinary form with molecules O2 we have the more active ozone with O3. Polymorphism in the case of an element is illustrated in the case of sulphur, whereas metamerism in the case of elements has so far as yet not been observed; and is hardly probable, as most elements are built up, like the metals, from molecules containing only one atom per molecule; here metamerism is absolutely excluded, and a considerable number of the rest, having diatomic molecules, are about in the same condition. It is only in cases like sulphur with octatomic molecules, where a difference of internal structure might play a part.
Before entering into detail it may be useful to consider the nature of isomerism from a general standpoint. It is probable that the whole phenomenon of isomerism is due to the possibility that compounds or systems which in reality are unstable yet persist, or so slowly change that practically one can speak of their stability; for instance, such systems as explosives and a mixture of hydrogen and oxygen, where the stable form is water, and in which, according to some, a slow but until now undetected change takes place even at ordinary temperatures. Consequently, of each pair of isomers we may establish beforehand which is the more stable; either in particular circumstances, a direct change taking place, as, for instance, with maleic acid, which when exposed to sunlight in presence of a trace of bromine, yields the isomeric fumaric acid almost at once, or, indirectly, one may conclude that the isomer which forms under greater heat-development is the more stable, at least at lower temperatures. Now, whether a real, though undetected, change occurs is a question to be determined from case to case; it is certain, however, that a substance like aragonite (a mineral form of calcium carbonate) has sensibly persisted in geological periods, though the polymorphous calcite is the more stable form. Nevertheless, the theoretical possibility, and its realization in many cases, has brought considerations to the front which have recently become of predominant interest; consequently the possible transformations of isomers and polymers will be considered later under the denomination of reversible or dynamical isomerisms.
Especially prominent is the fact that polymerism and metamerism are mainly reserved to the domain of organic chemistry, or the chemistry of carbon, both being discovered there; and, more especially, the phenomenon of metamerism in organic chemistry has largely developed our notions concerning the structure of matter. That this particular feature belongs to carbon compounds is due to a property of carbon which characterizes the whole of organic chemistry, i.e. that atoms attached to carbon, to express it in the atomic style, cling more intensely to it than, for instance, when combined with oxygen. This explains a good deal of the possible instability; and, from a practical point of view, it coincides with the fact that such a large amount of energy can be stored in our most intense explosives such as dynamite, the explanation being that hydrogen is attached to carbon distant from oxygen in the same molecule, and that only the characteristic resistance of the carbon linkage prevents the hydrogen from burning, which is the main occurrence in the explosion of dynamite. The possession of this peculiar property by carbon seems to be related to its high valency, amounting to four; and, generally, when we consider the most primitive expression of isomerism, viz. the allotropy of elements, we meet this increasing resistance with increasing valency. The monovalent iodine, for instance, is transformed by heating into an allotropic form, corresponding to the formula I, whereas ordinary iodine answers to I2. Now these modifications show hardly any tendency to persist, the one stable at high temperatures being formed at elevated temperatures, but changing in the reverse sense on cooling. In the divalent oxygen we meet with the modification called ozone, which, although unstable, changes but slowly into oxygen. Similarly the trivalent phosphorus in the ordinary white form shows such resistance as if it were practically stable; on the other hand the red modification is in reality also stable, being formed, for instance, under the influence of light. In the case of the quadrivalent carbon, diamond seems to be the stable form at ordinary temperatures, but one may wait long before it is formed from graphite.
This connexion of isomerism with resistant linking, and of this with high valency, explains, in considerable measure, why inorganic compounds afforded, as a rule, no phenomena of this kind until the systematic investigation of metallic compounds by Werner brought to light many instances of isomerism in inorganic compounds. Whereas carbon renders isomerism possible in organic compounds, cobalt and platinum are the determining elements in inorganic chemistry, the phenomena being exhibited especially by complex ammoniacal derivatives. The constitution of these inorganic isomers is still somewhat questionable; and in addition it seems that polymerism, metamerism and stereoisomerism play a part here, but the general feature is that cobalt and platinum act in them with high valency, probably exceeding four. The most simple case is presented by the two platinum compounds PtCl2(NH3)2, the platosemidiammine chloride of Peyrone, and the platosammine chloride of Jules Reiset, the first formed according to the equation PtCl4K2+2NH3 = PtCl2(NH3)2+2KCl, the second according to Pt(NH3)4Cl2 = PtCl2(NH3)2+2NH3, these compounds differing in solubility, the one dissolving in 33, the other in 160 parts of boiling water. With cobalt the most simple case was discovered in 1892 by S. Jörgensen in the second dinitrotetramminecobalt chloride, [Co(NO2)2(NH3)4]Cl, designated as flavo—whereas the older isomer of Gibbs was distinguished as croceo-salt. An interesting lecture on the subject was delivered by A. Werner before the German chemical society (Ber., 1907, 40, p. 15). (See Cobalt; Platinum.)
Dealing with organic compounds, it is metamerism that deserves chief attention, as it has largely developed our notions as to molecular structure. Polymerism required no particular explanation, since this was given by the difference in molecular magnitude. One general remark, however, may be made here. There are polymers which have hardly any inter-relations other than identity in composition; on the other hand, there are others which are related by the possibility of mutual transformation; examples of this kind are cyanic acid (CNOH) and cyanuric acid (CNOH)3, the latter being a solid which readily transforms into the former on heating as an easily condensable vapour; the reverse transformation may also be realized; and the polymers methylene oxide (CH2O) and trioxymethylene (CH2O)3. In the first group we may mention the homologous series of hydrocarbons derived from ethylene, given by the general formula CnH2n, and the two compounds methylene-oxide and honey-sugar C6H12O6. The cases of mutual transformation are generally characterized by the fact
