198 CHEMICAL DISCOVERY AND INVENTION 



same composition as aldehyde but three times the molecular 

 weight. The preservative effect may also be cited of a minute 

 quantity of sulphuric or phosphoric acid on hydrocyanic (prussic) 

 acid, which in its absence passes quickly into a mixture of 

 ammonium formate and a brown substance. 



One very interesting case in which a minute quantity of a 

 third substance affects the mutual behaviour of two others is 

 provided in those numerous instances in which the presence of 

 a minute quantity of water seems to be essential to interaction. 

 It certainly appears to be so when chlorine is brought into 

 contact with metals, for even metallic sodium may be preserved 

 for years in contact with chlorine gas at common temperatures 

 if the latter is perfectly dry, and the indifference of combustible 

 substances such as carbon monoxide gas, charcoal, and phos- 

 phorus to oxygen gas when all are free from moisture has been 

 the subject of much experimental enquiry. 



So striking are these phenomena that it has even been supposed 

 that chemical action cannot take place except in the presence of 

 a small quantity of some electrolyte. This generalisation is, 

 however, for the present too wide and many facts are known 

 which seem to oppose it. 



Catalytic agents are employed in many industrial operations, 

 and since the publication of the researches of Professor Sabatier 

 of Toulouse, less than twenty years ago, a stimulus has been 

 applied to the utilisation of catalytic change for manufacturing 

 purposes. 



An example of catalytic effect is to be found in the long- 

 established lead-chamber process for making sulphuric acid, in 

 which sulphur dioxide, water, and atmospheric oxygen are 

 enabled to interact rapidly in the presence of a relatively small 

 proportion of nitric peroxide N0 2 . Here several intermediate 

 nitrogenous compounds are undoubtedly formed, but whether 

 they are essential stages in the process by which sulphuric acid 

 is ultimately produced from its dissociated constituents is a 

 question which cannot be regarded as even yet finally settled. 

 The lead chamber survives, but of late years has found a serious 

 rival in the " Contact " process, which is merely the outcome of 

 a long known catalytic operation based on the use of finely 

 divided platinum. Theoretically sulphur dioxide requires one 

 atom of oxygen to convert it into sulphur trioxide, S0 2 +0 = 

 SO 3 . This combination is attended by the evolution of a con- 



