236 THEORIES AS TO ACTION OF 



than the time of the direct non-catalysed reaction. Otherwise 

 it is clear that the reaction will run more quickly without the inter- 

 mediate body being formed. 



It is clear that this condition is satisfied in those cases in which 

 the reaction does not go at all in the absence of the catalyst, or 

 does not go within appreciable experimental limits, as in the case 

 of sulphur dioxide and atmospheric oxygen mentioned above. 

 Here it is obvious that the reaction without the catalyst runs more 

 slowly than the sum of the formation of nitrosyl-sulphuric acid, 

 and the decomposition of this to form sulphuric acid and nitric 

 oxide, without any quantitative work on the substrate. Also in 

 one case of a reaction which does run at a measurable rate by itself 

 viz., that between hydriodic acid and hydrogen peroxide in which 

 water and iodine are formed, but which is catalysed into much 

 greater velocity by molybdic acid it has been shown by Erode 

 that an intermediate compound permolybdic acid can be isolated, 

 and also that the sum of the times of formation of (1) permolybdic 

 acid from molybdic acid and peroxide, and (2) the oxidation of 

 hydriodic acid by permolybdic acid to iodine with regeneration of 

 molybdic acid, is less than the time of direct oxidation of hydriodic 

 acid by peroxide in the absence of molybdic acid. 



Although a proof that the sum of the times was greater than 

 the direct time would rule out the intermediate compound, it must 

 be clearly pointed out, however, that the times being less does not 

 necessarily prove that the intermediate compound formation 

 actually occurs and is the cause of the catalysis. What it does 

 prove is that the formation of the intermediate compound would 

 increase the velocity of the reaction by a definite amount; but 

 such increase might be due to other causes, and the increase by 

 such other means might be greater than that by the path of the 

 intermediate reaction. 



Still, an experimental proof that the path by intermediate 

 reaction is quicker is presumptive evidence that the reaction 

 probably occurs by such intermediate reaction. 



It is only in a comparatively few of the known catalytic reac- 

 tions, however, that such intermediate compounds have been shown 

 to exist, and in a few others they have been introduced as an 

 hypothesis to explain irregularities of reaction. In the majority 

 of cases there is no experimental proof for the existence of such 

 compounds. Further, as Ostwald has pointed out, the theory 



