In order that all four affinities of each carbon atom may come into play, it is assumed that the linking in the ring is alternately by one and by two affinities. In recent times this comparatively minor detail of the hypothesis has given rise to much discussion.

In

Kekulé's formula explains the whole behaviour of benzene and its derivatives in a remarkably satisfactory manner. the first place it indicates that each mono-substitution product' obtained by the replacement of one of the hydrogen atoms can exist in one form only, and that isomerides cannot exist. But if a second atom or radical is substituted for a hydrogen atom, the perfect symmetry of the molecule is changed, and the second atom may take one of three positions, namely, next to the first or separated from it by one or by two carbon atoms. If the first atom takes the position at 1, the second may occupy the position at 2 and 6 (which are identical), or at 3 and 5, or finally at 4.

Strictly speaking, the positions 2 and 6 are not absolutely identical on account of the double linking; this has led some authors to assume that the fourth affinities are free or else united

to the opposite carbon atoms. These modifications of the hypothesis have not up to the present acquired any particular practical signification.

In spite of the repeated endeavours of many chemists to discover four isomeric di-substitution products of benzene, e.g. dichloro-benzenes, not more than three have ever been discovered. It is this circumstance which has gained for Kekulé's hypothesis a general recognition of its value. The three isomeric disubstitution products are distinguished by the prefixes 'ortho,' 'meta,' and 'para.'

The problem now arises, Which of the three positions 1. 2, 1. 3, and 1 . 4, or which of the three formulæ

is to be assigned to each of these compounds?

Owing to the inherent difficulties of the problem, this question was debated for years, and our views on the subject have frequently been altered. One remarkable fact is, that on further substitution the para-derivative only yields one tri-substitution product, but both of the other derivatives yield more than one trisubstitution product. As it is only in the third formula that the four remaining hydrogen atoms occupy similar positions, this formula has been assigned to the para-compounds 1.4. In the first formula, 1. 2, the four remaining hydrogen atoms are in two different relations to the chlorine atoms, 3 and 6 are adjacent to the chlorine atoms, 4 and 5 are separated from them by a carbon atom. In the second formula, 1. 3, the third chlorine atom may occupy three different positions-e.g. 2, between the two chlorine atoms; 4 and 6, adjacent to the chlorine atoms;

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and 5, separated by a carbon atom. As it has been proved by experiment that an ortho-di-substitution product can only yield two tri-substitution products, and the meta-compound yields three tri-substitution products, the formula 1.2 is assigned to the ortho- and 1 . 3 to the meta-derivatives.

Numerous comparisons have fully confirmed these hypotheses, and it is now seventeen years since the accuracy of these views was disputed.

These examples suffice to give an idea of the methods by which our intimate knowledge of the constitution of organic compounds has been acquired.

We may in this see a confirmation of the saying of Bacon:

Nec manus nuda, nec intellectus sibi permissus ad inveniendam veritatem multum valet. Instrumentis et auxiliis res perficitur, quibus opus est non minus ad intellectum quam ad manum.

§ 52. Physical Isomerism. Allotropy. As the investigation of the constitution of organic compounds was extended, many instances were observed in which a larger number of isomeric bodies were discovered than could be accounted for by means of the formulæ derived according to the laws of atomic linking. In most of these cases the isomeric substances differed less in their chemical than in their physical properties, such as density, melting point, crystalline form, &c.

Such cases of isomerism which cannot be explained by chemical formulæ are termed 'physical isomerides' to distinguish them from the chemical isomerides,' which are caused by a difference in the mode of linking. This physical isomerism is very closely related to and is almost identical with ' allotropism.' The latter expression was introduced by Berzelius, and applied by him to describe the occurrence of elements in different forms or conditions, or allotropic modifications.' Before the molecules of elementary bodies were regarded as compounds of similar atoms, the existence of one and the same element in different modifications could not be explained in the same way as the isomerism of compounds. Hence the necessity of a special term to be applied to this class of phenomena. At the present time the expression allotropism' is also applied to compounds, and is synonymous with physical isomerism. There are several kinds of physical isomerism.

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§ 53. Polymorphism.-Dimorphism and polymorphism are common forms of physical isomerism. When one and the same substance crystallises in two or more distinct forms, it is said to be dimorphous or polymorphous. Both elements and compounds exhibit this peculiar phenomenon. Well-known examples of polymorphism are exhibited by carbon, which crystallises in the regular system as the diamond, and in the hexagonal system as graphite. Sulphur is deposited from fusion in monoclinic crystals and from solution in carbon bi-sulphide in rhombic crystals. Calcium carbonate (CaCO3) occurs in rhombohedral crystals as calcite and in rhombic crystals as arragonite. Silica is met with in two distinct hexagonal forms as quartz and tridymite. Titanium dioxide (TiO2) exists in three distinct forms as rutile, brookite, and anatase. Stannic oxide (SnO2), which is isomorphous with titanium dioxide, assumes the same forms as rutile and brookite, and perhaps anatase. These bodies are consequently isodimorphous' or 'isotrimorphous.'

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There are many other examples of isodimorphism, such as that of the oxides of arsenic and antimony, As,O, and Sb,O,, the sulphides of copper and silver, Cu,S and Ag,S. Many organic compounds are also dimorphous.

The form that a di- or poly-morphous body assumes on crystallisation depends chiefly on the temperature, and also on certain other external conditions. If the crystallisation takes place from a solution, the nature of the solvent, the presence of other substances, especially of such as are isomorphous with one of the forms of the body in question, influence the form that body assumes.

The conditions under which many forms are produced are entirely unknown. We do not know under what conditions carbon crystallises as diamond, in spite of the numerous attempts which have been made to produce this valuable gem. In the case of many organic compounds one modification has been accidentally obtained, but the conditions under which it is formed still remain unknown.

The allotropic modifications of a substance differ considerably in their stability. Some modifications when once formed are very stable, but others can only exist within narrow limits of the conditions under which they are produced. As examples

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of the first class we have carbon as diamond or graphite; sulphur is an example of the second class.

The diamond and graphite can exist unaltered side by side, and it is only at a very high temperature that the diamond is converted into graphite. On the other hand the rhombic form of sulphur is only stable below, and the monoclinic form above, a temperature of 95°-6 C.; both modifications can exist unchanged for some time outside these limits But they are in a state of unstable equilibrium, which is easily upset by heating, or shaking, or more particularly by contact with a crystal of that modification which is stable at the prevailing temperature, and the whole mass is converted into this form.

Many dimorphous organic compounds behave like sulphur in this respect, and as a rule only one modification is stable, and the other unstable above and below a certain definite temperature.

This kind of physical isomerism is supposed to be due to a difference in the arrangement of the particles or molecules, which are in themselves identical. The accuracy of this hypothesis cannot be proved, as we do not possess any method by which the nature, or even the size, of the molecules of solid bodies can be ascertained. But when we see that under suitable conditions crystals of both modifications can be obtained from one and the same liquid, it seems probable that these modifications are composed of similar molecules, just as different kinds of buildings can be constructed from the same kinds of bricks. class of isomerism may be termed 'isomerism of aggregation.'

This

§ 54. Physical Isomerism of the Molecules.-There are also cases of physical isomerism caused by a difference in the molecules. The examples of real polymerism belong to this class, e.g. when a body has different molecular weights in the gaseous and liquid states. In the case of sulphur the molecules at temperatures near the boiling point consist of six atoms, S6, which are split up at higher temperatures into molecules consisting of two atoms, S2. Many organic and inorganic compounds, such as certain aldehydes, acetic acid, nitrogen peroxide, &c., exhibit analogous behaviour.

The allotropic modifications of phosphorus are probably due to differences in the number of atoms composing the molecules.

H