CATALYSIS AND CATALYSTS 197 



made by later experimenters proved that the effect was pro- 

 ducible by many substances beside platinum. 



Catalysis, then, is a process in which a chemical change, which 

 without assistance proceeds either not at all or very slowly, is 

 greatly accelerated by contact of the materials with a small 

 quantity of some agent, called the catalyst, which remains after 

 the reaction undiminished. 



The catalyst in some cases retains in its appearance some 

 evidence of having suffered change, but in most cases this can 

 be fairly attributed to the action of the heat which is developed 

 during the reaction. It is, however, possible frequently to 

 account for the starting and continuance of the observed 

 chemical action by the hypothesis of the alternate formation and 

 decomposition of a compound of the catalyst with some con- 

 stituent of the materials engaged in the change. Thus in the 

 process formerly employed for procuring oxygen by heating 

 potassium chlorate with a relatively small quantity of manganic 

 oxide, there can be little doubt that the action consisted essen- 

 tially in the formation of an oxide of manganese containing a 

 larger proportion of oxygen than the dioxide employed, and its 

 subsequent decomposition so that the dioxide remained at the 

 end. 



In this case evidence is derived from the altered appearance 

 of the oxide left behind, and also from the two facts that the 

 oxygen thus obtained always contains a trace of chlorine, while 

 the residual potassium chloride is alkaline from the formation 

 of a trace of potassium oxide. 



An example of this kind seems to differ from that of the first 

 mentioned, namely, the action of platinum on oxyhydrogen gas. 

 And on enquiry among the numerous and various instances of 

 catalytic action, among carbon compounds especially, it appears 

 that no explanation can be found which is applicable generally. 

 All that can be said is that the action is in the majority of cases 

 chemical and that it involves the alternate formation and 

 destruction of an unstable compound. 



As an example of the effect of a very small quantity of acid may 

 be cited the action of almost any acid on common sugar, whereby 

 it is converted into " invert " sugar, a mixture of equal quantities 

 of dextro- and laevo-glucose, or the action of a few drops of sul- 

 phuric acid on aldehyde whereby in a few minutes it is converted, 

 with evolution of heat, into paraldehyde, a compound having the 



