180 PHYSIOLOGY 



re-formed by shaking up the cuprous solution with air. It has been 

 thought that many or all of the catalytic reactions occur in the same 

 way by two stages, i.e. by the formation of an intermediate product. 

 Thus, in the ordinary process for the manufacture of sulphuric acid, 

 the nitric oxide may be supposed to combine with the oxygen of the 

 air to form nitrogen peroxide. This interacts with sulphur dioxide, 

 giving sulphur trioxide and nitric oxide once more. The nitric oxide, 

 which we alluded to before as the catalyser, may in this way be regarded 

 as the carrier of oxygen from air to sulphur dioxide. It has been 

 suggested that the action of spongy platinum or colloidal platinum 

 rests on the same process, and that in the oxidation of hydrogen, for 

 instance, PtO or Pt0 2 is formed and at once reduced by the hydrogen 

 with the formation of water. 



There is a certain amount of experimental evidence in favour of this hypo- 

 thesis. According to Engler and Wohler,* platinum black, which has been 

 exposed to oxygen, in virtue of the gas which it has occluded, has the power 

 of turning potassium iodide and starch blue. This power is not destroyed 

 by heating to 260 in an atmosphere of CO 2 , or by washing with hot water. 

 On exposure of the platinum black to hydrochloric acid, a certain amount is 

 dissolved, and the substance loses its effect on potassium iodide. The amount 

 dissolved corresponds with the amount of iodine liberated from potassium 

 iodide, and also with the amount of oxygen occluded, the (soluble) platinum 

 and oxygen being in the proportions necessary to form the compound PtO. 



But why should a reaction take place more quickly if it occurs 

 in two stages instead of one ? As Ostwald has pointed out, the 

 formation of an intermediate compound can be regarded as a suffi- 

 cient explanation of a catalytic process only when it can be demon- 

 strated by actual experiment that the rapidity of formation of the 

 intermediate compound and the rapidity of its decomposition into 

 the end-products of the reaction are in sum greater than the velocity 

 of the reaction without the formation of the intermediate body. In 

 the case of one reaction this requirement has been fulfilled. The 

 catalytic effect of molybdic acid on the interaction of hydriodic acid 

 and hydrogen peroxide has been explained by assuming that the 

 first action which takes place is the formation of permolybdic acid, 

 and that this then interacts with the hydrogen iodide to form water 

 and iodine. Now it has been actually shown (1) that permolybdic 

 acid is formed by the action of hydrogen peroxide on molybdic acid ; 

 (2) that permolybdic acid with hydriodic acid produces water and 

 iodine ; (3) that the velocity with which these two reactions occur 

 is much greater than the velocity of the interaction of hydrogen per- 

 oxide and hydriodic acid by themselves. 



Although we may find it difficult to explain why a reaction should occur 

 more quickly in the presence of a catalyser by the formation of these inter - 



* Quoted by Mellor, " Chemical Statics." 



