The Romance of Modern Chemistry

FROM SOLUTIONS TO CRYSTALS enough, if left to itself, the writing fades away, for the blue salt gradually absorbs moisture from the air, re- generating the pink salt, which is almost invisible. On the same lines the reader will himself be able to explain the behaviour of certain artificial flowers which ai e said to be made in Paris. Their petals are tinted with cobalt chloride, with the result that while the flowers are usually of a rose colour, they turn blue in a very dry atmosphere. Nothing has as yet been said about the strikingly regular and beautiful forms in which dissolved substances crystallise out from their solutions. These must be seen to be appreciated. As a rule, each dissolved salt separates in a definite shape, peculiar to itself, and it is, in fact, this regularity of form which is the main distinguishing feature of the crystalline state. If the separate crystals are large, it is easy not only to see distinctly the various shiny faces, but also to count them, and when the chemist has become familiar with the crystalline habits of a particular substance, he can afterwards identify it, even amongst many others, merely by its appearance. Ihc process of crystallisation consists in an ordered fitting and packing together of the molecules of the solid. This regularity of arrangement is evident not only from a study of large, well-formed crystals, but also from the appearance under the microscope of minute quantities of crystallised solutions. If, for instance, a drop of am- monium chloride solution is crystallised under a microscope slide, the crystals are seen on close examination to have assumed a regular fern-like shape (see Fig. 14). The reader must not suppose that it is only from solutions in water and other similar liquids that crystals are formed. Fused substances, as already indicated 321 y