366 Dr. A. Ransome on some of the 



cules of the dissolved substance drop from their grasp and are 

 thrown down. 



Catalysis. — The degree to which molecular affinity is exerted 

 upon different substances must play a most important part in de- 

 termining the results of those actions ordinarily termed catalytic, 

 in some cases bringing about decomposition, in others synthesis. 



When contact with a substance produces decomposition, as 

 when binoxide of manganese facilitates the evolution of oxygen 

 from chlorate of potash, the molecular affinity of the catalyte is 

 probably greatest for one of the constituents of the substance 

 acted upon, and it will attract this to such an extent as to enable 

 it to gain its freedom. 



When, however, synthesis is the result, as in the action of pla- 

 tinum-black upon alcohol, assisting its oxidation, the catalyte 

 in this case may draw the particles together, or it may be able 

 to concentrate upon itself one of the elements so exclusively that 

 it may appear in an undiluted or liquid form, and hence able to 

 act with greater energy upon the substances presented to it. 



Osmose. — The phenomena of osmose are probably dependent 

 purelyupon molecular affinity. Osmose takes place when a porous 

 but, usually, water-tight septum or membrane separates two mis- 

 cible fluids. In this case the fluids both pass through the mem- 

 brane, but at different rates. The structure and composition of the 

 septum influence the rate of diffusion, and determine which of 

 the two fluids shall pass through most rapidly. Thus, when 

 water and dilute alcohol are separated by an animal membrane, 

 the water will pass through most rapidly ; but when the septum 

 is composed of caoutchouc, the alcohol diffuses at much the 

 greater rate. 



The more rapid rate of diffusion of one 



fluid (A) over that of another fluid (B) may 

 be accounted for by supposing that the sep- 

 tum has a greater molecular affinity for fluid 

 A than for fluid B. The particles of fluid r; 

 A may perhaps fill the pores of the septum, (I 

 and if there were no other fluid with which p 

 it could mix on the other side of the septum, pE-^-z-z-z 1 !-:-!-;^^ 

 its attraction for the septum would suffice fc z^g| 



to keep it from exuding; but since on this "^Ms^iSiSBss^ 

 other side of the septum there is a fluid with 

 which it is miscible, and since, as we have before seen, miscibi- 

 lity seems to assume that the particles of one of the mixing fluids 

 must have a greater molecular attraction for the particles of the 

 other mixing fluid than they have for one another, it follows 

 that when the particles of fluid B come into contact with those 

 of fluid A they will at once mix with them and drive out some 



