CHEMICAL CHANGES IN LIVING MATTER. FERMENTS 177 



place rapidly at a temperature of 30 C. In the same way a solution 

 of protein may be kept almost indefinitely without undergoing 

 hydrolysis, which, however, can be induced by heating the solution 

 under pressure. The action of the ferments in these two cases is 

 to quicken a process of hydrolysis which without their presence 

 would take an infinity of time for its accomplishment, 



In this respect their action is similar to that of acids, and indeed 

 of a whole class of bodies which are spoken of as catalysers or catalysts. 

 A catalyser is a substance which will increase (or diminish) the velocity 

 of a reaction without adding in any way to the energy changes 

 involved in the reaction, or taking any part in the formation of the end- 

 products. Since the catalyser is unchanged in the process, a very 

 small quantity is able to influence reactions involving large quantities 

 of other substances. By adding acids to a watery solution of the 

 food-stuffs, the process of hydrolysis is quickened in proportion to 

 the strength and concentration of the acid. The effective catalytic 

 agents in this process appear to be the hydrogen ions of the free acid. 

 There are many other bodies, besides the free acids, which may act 

 as catalysers, and a study of the conditions under which catalysis 

 takes place may throw some light on the essential nature of the action 

 of ferments. 



The velocity of almost any reaction in chemistry can be altered 

 by the addition of some catalytic agent, and there are few of 

 the ordinary reactions in which catalysis does not play some part. 

 Among such processes we may instance the action of spongy platinum 

 on hydrogen peroxide. Hydrogen peroxide undergoes slow spon- 

 taneous decomposition into water and oxygen. If a little spongy 

 platinum be added to it, it is at once seen to decompose rapidly with 

 the evolution of bubbles of oxygen, and the action does not cease 

 until the whole of the hydrogen peroxide has been destroyed. Spongy 

 platinum is able in the same way to quicken a very large number of 

 chemical reactions. Thus sulphur dioxide and oxygen when heated 

 together will combine very slowly ; the combination becomes rapid if 

 a mixture of the two gases be passed over heated platinum. The 

 same reaction, namely, the combination of sulphur dioxide with 

 oxygen, may be quickened by the addition of a small trace of nitric 

 oxide, and this fact is made use of in the manufacture of sulphuric 

 acid on a commercial scale by the ordinary lead- chamber process. 

 Hydrogen peroxide and hydriodic acid slowly interact with the 

 formation of water and iodine. This reaction may be quickened by 

 the addition of many substances, among which we may mention 

 molybdic acid. 



There is moreover a specificity in the action of catalysers, though 

 not so well marked as with ferments. Whereas all the disaccharides 



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