This operation, when carried out in a chemical laboratory, is a very simple one. Fig. 4 shows the necessary apparatus. Sulphur dioxide from a siphon of the liquefied gas and air from a gasholder are passed into the Woulff’s bottle A, containing concentrated sulphuric acid; this removes moisture from the gases. The drying process is completed in the tower B, which contains pumice stone soaked in sulphuric acid. The mixed gases then pass through the tube C, containing platinized asbestos heated to about 400° C.: the sulphur trioxide collects in the cooled receiver D. [Illustration: Fig. 4. SULPHUR TRIOXIDE—THE CONTACT PROCESS] Platinized asbestos is made by soaking long-fibred asbestos in a solution of platinum chloride. The material is then dried and subjected to a gentle heat. In this way, metallic platinum in an exceedingly fine state of subdivision is deposited on the asbestos fibre, which merely serves as a convenient support. Catalytic or Contact Action. The influence of the finely divided platinum is a very important factor in the reaction. It cannot, however, be said to _cause_ the union of sulphur dioxide with oxygen, for the gases combine to a very slight extent when it is not present. What the platinum actually does is to influence the rate of formation to such a degree that, under favourable conditions, practically the whole of the sulphur dioxide is changed to sulphur trioxide instead of an exceedingly small fraction of it. The most interesting, and at the same time the most perplexing, feature of the reaction is that the platinum itself does not appear to undergo any change. It is not diminished in quantity, for only a very small amount is necessary for the conversion of a very large amount of the mixed gases. Its activity lasts for a very long time, and even when it does become inactive, it can be shown that this is due to some external cause, such as the presence of dust and certain impurities in the gases. Many other similar cases are known in which the presence of a small quantity of a third substance greatly influences the course of a chemical reaction without appearing in any other way to be necessary to the reaction. These substances, which are often metals in a very fine state of subdivision, are called catalytic or contact agents. The Contact Process for making sulphuric acid is nothing more nor less than the simple laboratory operation which we have described above, carried out on a larger scale.