Microchemical Test for the Oxygen Place in Tissues. 175 



and has nothing to do with the particular surface being one specially 

 prone to oxidation or reduction. In this connection I am not forgetting 

 that, when once completely condensed, the chemical potential of the 

 oxidisable substance is no higher than (it is in fact identical with) what 

 it is in the solution; but many instances show that during transition the 

 molecules are under stresses which may find relief in exceptional chemical 

 activity. For instance, the exceptional electrical and chemical properties 

 of gases entering or leaving the surface of platinum ; the high chemical 

 potential of condensing oxygen in Prof. Bone's experiments on surface com- 

 bustion, while by contrast a fully condensed film of oxygen on metallic iron 

 does not oxidise the iron. Eoberts' experiments on the volatilisation of 

 metals, perhaps, are also a case in point. 



During condensation local heat changes occur, the sign being determined 

 by whether the solution of the substance condensed is endothermic or 

 exothermic. Let heat be liberated when condensation occurs, the relation of 

 solvent and solute being such that heat is absorbed during solution. The local 

 liberation of heat will oppose condensation and the velocity of condensation 

 becomes a function of the rate of dissipation of heat. In the well known 

 case of an over-cooled fluid phase the dissipation of heat may be so slow as 

 completely to arrest the change of phase.* If the substance which is being 

 condensed under these conditions is chemically unstable, chemical change of 

 the nature of oxidation, reduction, dissociation or association may be caused 

 locally by the enormous molecular stresses. 



This third possibility considers a surface not as specially a place of oxidation 

 because, for instance, oxidation of Wurster's tetra-substance occurs there, but 

 as a surface which condenses basic substances. In this process oxidation 

 of a basic body may occur, but an equally oxidisable acid substance would 

 escape change. 



Mr. Drury's experiments clear the, ground for further discussion to this 

 extent — they prove conclusively that the condensation of a so-called test 

 substance for " active " oxygen or a simple basic dye not only will take 

 place on to a surface wholly devoid of oxygen, but is actually hindered by the 

 existence thereon of a film of oxygen 



It must always be remembered that an oxidation place is also a reduction 

 place, and it is to be called the one or the other according to the particular 

 zero which is chosen. A convenient zero is the chemical potential of 

 atmospheric oxygen, and a place would be an oxidation place if oxygen, whose 

 chemical potential is = that of atmospheric oxygen, is condensed to the intra- 

 molecular state. Such a region would then be a reduction place for chemical 

 * H. A. Wilson, 'Camb. Phil. Proe.,' vol. 10, p. 25 (1898). 



