ITS SCOPE AND METHOD 69 



It is physiological variations of osmotic pressure which 

 determine, on the one hand, currents of water across the 

 cells, and therefore across the tissues of the body, and, 

 on the other, the diffusion of substances suitable for 

 drafting off into definite channels of the body. It is 

 noteworthy that the relation between osmotic energy and 

 the products which the cells form takes no note as it 

 were of the chemical nature of the products, but only of 

 the number of their molecules present per unit of solvent- 

 It has therefore no selective action in that sense. Yet 

 it does act selectively in a certain sense, because the 

 lower the stage to which a product belongs in the dis- 

 integration of a large complex molecule pari passit the 

 greater the number of molecules of it which will arise 

 from the parent compound. Therefore diffusion by 

 osmotic energy will act more in regard to final products 

 of protoplasmic disintegration than to earlier. 



One consequence of the cell-theory is therefore a recog- 

 nition of the construction of the body as including vast 

 numbers of surfaces of separation, dividing minute chemi- 

 cal and physical fields one from another. Membranes 

 and surfaces deserve special consideration therefore in 

 physiology. In the limiting layer separating two media 

 forces act which depend on mutual relations between the 

 two media in contact. It is the internal energy of such a 

 limiting layer which is usually called the energy of surf ace. 

 Energy of surface is therefore a form of potential energy. 

 It cannot therefore derive directly from chemical energy, 

 which is also a form of potential energy. A free form 

 of energy must intervene ; perhaps heat ; more probably 

 electrical energy. Colloid solutions are not homogeneous 

 systems, but suspensions, of ultra-microscopic fineness. 

 That is to say, they are diphasic systems offering vast 

 surface ; the limiting surface between the two, the 



