6 SCIENCE PROGRESS. 



studying the simpler chemical phenomena. Since then we 

 have had the modern physico-chemical researches on elec- 

 trolysis and solution, on electrical resistance and lowering 

 of freezing points, on vapour pressures and superficial 

 tensions in solutions. This latter question, an obvious 

 borderland where physics touches on chemistry, deserves 

 careful investigation. Catalytic actions, dyeing, crystal- 

 lisation, diffusion, all may depend on the peculiar properties 

 of the superficial layer between two liquids, or the layer of 

 liquid next a solid. A good deal has been done on the 

 electrical side of these questions — very little on the thermal 

 and chemical sides — and yet, considering the small size 

 of organic structures, the minute volumes and enormously 

 extended surfaces, actions of this kind must have an 

 important effect in organic life. The solubility of a sub- 

 stance in the layer of a liquid where it is in contact with 

 another substance — liquid or solid — is sometimes quite 

 different from its solubility in the general body of the liquid. 

 A layer of porous solid may act as a semi-permeable mem- 

 brane, and permit only a comparatively small diffusion of 

 one of the constituents of a solution to pass through by 

 collecting the other constituent upon its enormously ex- 

 tended surface layer. The physical conditions of the 

 material within these surface layers are very different from 

 outside. It has, for instance, been suggested that in the 

 surface layer between a liquid and its vapour, in which some 

 state of kinetic equilibrium holds between evaporation and 

 condensation, all the various states represented by the 

 sinuous part of the theoretical continuous isothermal which 

 can be drawn, connecting the liquid and gaseous states, 

 coexist at different depths in it. The conditions of chemical 

 equilibrium under such abnormal circumstances can hardly 

 be the same as within the liquid ; indeed we know that 

 the solubility, a very important question of simple chemi- 

 cal equilibrium, is different within this layer. The con- 

 ditions of diffusion through continuous membranes like the 

 case of hydrocarbons diffusing through india-rubber and 

 other similar cases of semi-permeable membranes, may be 

 more strictly chemical questions, but being so closely 



