582 PRINCIPLES OF GENERAL PHYSIOLOGY 



two different kinds, capable of reacting together ; the impacts will, in a certain 

 proportion of cases, be between different kinds of molecules, and some of them will 

 give rise to combination. If there is also a third kind of molecule present, the 

 compound molecule fonned by the first two will occasionally meet with this third 

 kind, and combination of three kinds of molecules result. But the chances are 

 almost infinity to one against three different kinds of molecules meeting 

 simultaneously in such a way as to combine together. We are, no doubt, 

 justified in extending these considerations to substances in solution. It appear^. 

 therefore, to be extremely improbable that a reaction between three different 

 molecules ever occurs in one stage, or, indeed, a reaction between two of one kind 

 and one of another kind, or between three of one kind. All reactions should, if 

 possible, be represented as taking place in stages between two molecules only at a 

 time. Nernst (1913, pp. 475 and 595) calls attention to the same fact with 

 reference to the improbability that reactions of a higher order than bimolecular 

 should be anything but very rare. In any case, their velocity must be very small. 

 Reactions which appear to be trimolecular are often found, on investigation, to 

 take place in two bimolecular stages (see also van't HofTs Lectures, 1901, 

 Heft 1, p. 196). 



PEROXIDES AND THEIR CATALYSTS 



The peroxide, ozone, which is produced in such autoxidations as that of 

 phosphorus, has a powerful oxidation potential, so that, for example, it liberates 

 iodine from potassium iodide with great rapidity. Now, the peroxides which we 

 find produced in living cells have an oxidation potential which is not so higli as 

 this ; they consist either of hydrogen peroxide, or have a similar constitution. 

 Their appearance in the photo-chemical reactions of the green leaf has been met 

 with in Chapter XIX. In fact, in the presence of water, the organic peroxides of 

 the latter type readily form hydrogen peroxide. 



Peroxides of this type only liberate iodine from potassium iodide very slowly 

 and their power of oxidising such substances as sugar is practically nil. We have, 

 however, already seen an example of a typical catalytic process, with formation 

 of an intermediate compound, in the increased action of hydrogen peroxide 

 on hydriodic acid when minute amounts of molybdic acid are added (Erode). 

 We note that the molybdic acid is found at the end unchanged, so that, to all 

 appearances, it may have been merely an onlooker in the reaction. This is 1>\ 

 no means the case, as we saw. 



There are other substances, such as ferrous iron in the well-known Fenton's 

 reaction (1894), which act as catalysts on hydrogen peroxide with the separation 

 of what we may, for convenience, continue to call "active" oxygen. Moreover, 

 from various animal and plant tissues, enzymes have Ixvn prepared which have 

 the same effect. These have been called by Bach (1903) " peroxidases." 



The nature and properties of the numerous substances concerned with 

 physiological oxidations and reductions have led to much work and caused much 

 difficulty in the interpretation of the complex phenomena observed. A consistent 

 and intelligible theory was first proposed by Bach and Chodat (1904), to whom we 

 owe the greater part of the accurate investigation of the subject (see the article l>v 

 Bach, 1913). In the following pages I describe the phenomena on the Imsis of 

 this theory, although further research may make necessaiy some modification in it, 

 and it cannot be regarded as altogether complete as yet. 



The intervention of cell structures will come up for discussion in a later paragraph. 



There is an enzyme, to which we have already referred, called catafase, which has the 

 property of decomposing hydrogen peroxide without activating the oxygen given off. The 

 result of its action is molecular oxygen merely, given off as gas. Although catalase is of very 

 common occurrence in plants and animals, the part it plays is, at present, somewhat uncertain. 

 In any case, it does not directly concern us here, since it does not bring about oxidation, 

 although, according to Bach (1913, p. 182), it plays an important part in protecting sensitive 

 parts of the cell mechanism from the easily diffusible hydrogen peroxide, formed in the 

 oxidative processes. Bach states, also, that in a mixture of hydrogen peroxide with both 

 catalase and peroxidase, part of the peroxide is decomposed with production of active oxygen, 



