CATALYSTS AND ENZYMES 235 



substance, and by analogy it is probable that in a certain number 

 of cases the explanation of catalysis may lie in molecular kinetics. 



In any case the theory does not deserve dismissal in the cavalier 

 fashion in which it is often treated by modern physical chemists. 



Other instances of mechanical movements effecting chemical 

 or physical change are the effects of motion in causing crystal- 

 lisation from supersaturated solutions. While this action is not 

 marked in some cases, it is conspicuous in others, as in the two 

 cases quoted by Liebig, of acid potassium tartrate on shaking 

 mixed solutions of a potassium salt and tartaric acid, or on stir- 

 ring with a glass rod the sides of a beaker containing ammonio- 

 magnesium phosphate, when the crystals separate out on the parts 

 rubbed by the rod. Similar effects are seen in the crystallisation 

 out from amorphous solids (or solid solutions) of sugar, glass, 

 or sulphur on mechanical stimulation. 



II. The Theory of Intermediate Reactions. This theory of 

 catalytic action, in its application at least to one classical reaction 

 viz., that of production of sulphur trioxide from sulphur dioxide 

 and oxygen by the catalytic action of nitric oxide was introduced 

 even before the previous one, although it does not appear to have 

 occurred to anyone to invoke it as a general theory of catalysis 

 until long after Liebig's theory had been propagated. As early 

 as 1806 Clement and Desormes described this process in the leaden 

 chamber sulphuric acid process as arising from an alternating 

 reduction and oxidation of the nitrogen oxides. Another classical 

 example is the formation of the intermediate ethyl- hydrogen 

 sulphate shown by Williamson to occur in the etherification process 

 for the production of ethylic ether from ethyl alcohol. 



Although bodies corresponding to such intermediate reactions 

 have been isolated in some cases by slightly varying the condi- 

 tions of reaction, it has frequently been disputed whether or not 

 they actually occur in the reaction as it takes place under usual 

 conditions. Further, Ostwald has rightly pointed out that the 

 mere fact of the occurrence of such a body is by no means a proof 

 that it is the cause, or a step in the process, of the quicker catalytic 

 reaction. It may not be an intermediate product : it may merely 

 be a by-product. In order that such a body can be shown to 

 be a part of the catalytic reaction, it must be shown not only that 

 it occurs, but that the sum of the times, for its formation and its 

 breaking up again to form the final products of the reaction, is less 



