162 PHYSIOLOGY 



by the formation of an intermediate product. Thus, in the old lead chamber 

 process for the manufacture of sulphuric acid, the nitric oxide may be sup- 

 posed 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 hypothesis. 

 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 sufficient explanation of a 

 catalytic process only when it can be demonstrated 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 inter- 

 action of hydrogen peroxide 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 intermediate bodies, 

 certain simple analogies may help us to comprehend how a factor which introduces no 

 energy can yet assist the process. Thus a man might stand to all eternity before a 

 perpendicular wall twenty feet high. Since he cannot reach its top at one jump, he is 

 unable to get there at all. The introduction of a ladder will not in any way alter the 

 total energy he must expend on raising his body for twenty feet, but will enable him 

 to attain the top. Or we might imagine a stone perched at the top of a high hill. The 

 passive resistance of the system, the friction of the stone, and its inertia will tend to 

 keep it at rest, even though it be on a sloping surface and therefore tending to slide 

 or roll to the bottom. If, however, it be rolled to a point where there is a 

 sudden increase in the rapidity of slope, it may roll over, and having once started 

 * Quoted by Mellor, " Chemical Statics." 



