The Romance of Modern Chemistry

ELEMENTS WITH DOUBLE IDENTITY gases like oxygen and ozone. For, as has been pointed out already, the molecules of a gas are rushing hither and thither at a high speed, and any definite arrangement of these particles is quite out of the question. The few cases discussed in this chapter of elements existing in more than one form are simply illustrations of what has been observed all over the field of chemistry. It is very frequently found that two compound substances with the same chemical composition are quite distinct in their outward appearance and general behaviour. In some instances the difference is merely one of crystalline form, and is to be attributed to a different arrangement of the molecules. In other compounds, however, the origin of the distinction is far more subtle, and is found in a different arrangement of the atoms within the mole- cule. The story of the way in which chemists have discovered the internal structure and anatomy of the molecule is very fascinating, but it is long and intricate, and would detain us from excursions into interesting fields which are close at hand. One other curious phenomenon, however, deserves notice here. When a compound exists in two crystalline modi- fications, it is very commonly observed that one of the forms has but little persistence, and changes into the other on the slightest provocation. Some years ago the writer came across an interesting case of this kind. He obtained a substance which crystallised in the form of shiny leaflets, but these were no sooner produced than they began to change, entirely of their own accord, into little needle-shaped crystals. The first form of the substance, in fact, never existed for more than a few minutes. A host of similar interesting cases might be quoted- 59