510 



SCIENCE 



[N. S. Vol. XLII. No. 1085 



Martin's process. To avoid confusion, 

 however, it has been suggested that the 

 colloidal state should be spoken of as 

 "microheterogeneous." There are, in fact, 

 certain phenomena more or less peculiar to 

 the colloidal state and due to the influence 

 of the sharp curvature of the surfaces of 

 the minutely subdivided phase. The effect 

 of this curvature is a considerable pressure 

 in the interior of the phase, owing to the 

 surface tension, and it adds further com- 

 plexity to the properties manifested by it. 



We see, then, that the chemical reactions 

 of chief importance to us as physiologists 

 are those known as "heterogeneous." 

 This class of reactions, until comparatively 

 recent times, has been somewhat neglected 

 by the pure chemist. 



In some of its aspects, the problem be- 

 fore us was discussed by one of my pre- 

 decessors. Professor Hopkins, as also by 

 Professor Maeallum, but its importance 

 will, I think, warrant my asking your in- 

 dulgence for a further brief discussion. 

 Permit me first to apologize for what may 

 seem to some of those present to be an un- 

 necessarily elementary treatment of certain 

 points. 



It is easy to realize that the molecules 

 which are situated at the interface where 

 two phases are in contact are subject to 

 forces differing from those to which the 

 molecules in the interior of either phase 

 are subject. Consider one phase only, the 

 molecules at its surface are exposed on the 

 one side to the influence of similar mol- 

 ecules ; on the other side, they are exposed 

 to the influence of molecules of a nature 

 chemically unlike their own or in a differ- 

 ent physical state of aggregation. The 

 result of such asymmetric forces is that 

 the phase boundary is the seat of various 

 forms of energy not present in the interior 

 of the phase. The most obvious of these is 

 the surface energy due to the state of ten- 



sion existing where a liquid or a gas forms 

 one of the phases. It would lead us too far 

 to discuss the mode of origin of this sur- 

 face tension, except to call to mind that it is 

 due to the attractive force of the molecules 

 for one another, a force which is left par- 

 tially unbalanced at the surface, so that the 

 molecules here are pulled inwards. The 

 tension is, of course, only the intensity 

 factor of the surface energy, the capacity 

 factor being the area of the surface. We 

 see at once that any influence which alters 

 the area of the surface alters also the mag- 

 nitude of that form of energy of which we 

 are speaking. 



This is not the only way in which the 

 properties of substances are changed at 

 phase boundaries. The compressibility of a 

 solvent, such as water, are altered, so that 

 the solubilities of various substances in it 

 are not the same as in the interior of the 

 liquid phase. It is stated by J. J. Thom- 

 son that potassium sulphate is 60 per cent, 

 more soluble in the surface film. The ways 

 in which the properties of a solvent are 

 changed are sometimes spoken of as "lyo- 

 tropic," and they play an important part 

 in the behavior of colloids. We meet also 

 with the presence of electrical charges, of 

 positive or negative sign. These are due, 

 as a rule, to electrolytic dissociation of the 

 surface of one phase, in which the one ion, 

 owing to its insolubility, remains fixed at 

 the surface, while the opposite ion, although 

 soluble, can not wander away further than 

 permitted by electrostatic attraction. Thus 

 we have a Helmholtz double layer produced. 



Before we pass on to consider how these 

 phenomena intervene in physiological proc- 

 esses, there is one fact that should be re- 

 ferred to on account of its significance in 

 connection with the contractile force of 

 muscle. Surface tension is found to de- 

 crease as the temperature rises, or, as it is 

 sometimes put, it has a negative tempera- 



