The Romance of Modern Chemistry

SMALL CAUSES; GREAT EFFECTS barium, strontium, and calcium, and, as the name “ phos- phorescent11 implies, these sulphides are luminous in a dark room after they have been brought out of the light. Luminous paints or luminous compositions generally are dependent for their characteristic behaviour on the pre- sence of such a phosphorescent substance. Curiously enough, however, the pure materials do not appear to be phosphorescent; it is only when minute traces of other matter are present that they are stimulated to luminous activity. Incandescent mantles and phosphorescent substances illustrate very well the striking modifications of properties which are attributable to small quantities of foreign material. But it is not only the properties of particular compounds which are affected by impurities; as we have seen in the case of imperfectly dried hydrogen and oxygen, the speed at which a chemical change takes place may be remarkably modified by the presence of some alien substance, which keeps, so to speak, in the back- ground, and does not itself suffer any apparent alteration. Such an acceleration of chemical change by an alien substance is known as “ catalysis,11 and the alien substance itself is spoken of as a “ catalytic agent.11 Chemical changes which take place under the influence of a catalytic agent are not merely laboratory curiosities— they are of the utmost importance in the technical world. The most modern method of manufacturing sulphuric acid, for instance, depends on just such a change, and if the reader tries to realise the fact that about 3000 tons of sulphuric acid are made in England every day, he may appreciate the bearing of catalysis on chemical industry. The main thing to be done in making this important product is to persuade sulphur dioxide—the 330