4 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



In the case of spongy platinum and other finely divided 

 metals the chemical change is accelerated in a large degree by 

 physical causes ; a finely divided metal presents an extended 

 surface on which the reacting substances are brought into 

 intimate union. Chemical agencies may be at work at the 

 same time, e.g., the formation of unstable intermediate 

 compounds such as oxides or hydrides ; but the physical 

 conditions are probably the governing factor. 



It is otherwise with certain other catalytic changes, notably, 

 e.g., the combination of sulphur dioxide with oxygen through 

 the intervention of nitric oxide, which is the basis of the cham- 

 ber process for the manufacture of sulphuric acid. Sulphur 

 dioxide does not combine directly with oxygen, but when 

 oxygen is presented to it in combination as nitrogen peroxide, 

 it is easily oxidised with simultaneous formation of nitric 

 oxide. Nitric oxide, on the other hand, readily combines 

 with the oxygen of the air, again producing nitrogen peroxide. 

 The changes are expressed in the following equations : 



NO + - N0 2 



S0 2 + N0 2 = S0 3 + NO 



It will thus be seen that in presence of oxygen, or of course 

 of air, a very small amount of nitric oxide (NO) is capable 

 of converting an indefinite quantity of S0 2 into S0 3 , itself 

 remaining unchanged at the end of the process. 



On the large scale this change takes place in the vast 

 leaden chambers which cannot fail to be noticed in centres of 

 chemical industry, such as Widnes. The various gases are 

 introduced into these chambers, together with steam. The 

 steam, H 2 0, and S0 3 together form sulphuric acid, H 2 S0 4 , 

 which collects on the floor of the chamber. 



The catalytic action of nitrous .fumes can be readily shown 

 in the laboratory by shaking a solution of ferrous sulphate 

 (green copperas) with a little nitrite of soda and sulphuric 

 acid in a bottle nine-tenths full of air. The green colour 



