TRANSACTIONS OF SECTION I. — PRESIDENTIAL ADDRESS. 679 



SECTION I.— rHYSIOLOGY. 



Peesident of the Section: Professor W. ]\r. Bayliss, 

 M.A., D.Sc, F.R.S. 



WEDNERDA Y, SEPTEMBER 8. 

 The President rlolivei-ed the following Address : — 



The Physiological Importance of Phase Boundaries. 



Even a hasty consideration of the arrangements present in living cells is suffix 

 cient to bring conviction that the physical and chemical systems concerned 

 operate under conditions very different from those of reactions taking place 

 between substances in true solution. We become aware of the fact that there 

 are numerous constituents of the cell which do not mix with one another. In 

 other words, the cell system is one of many ' phases/ to use the expression 

 introduced by Willard Gibbs. 



Further, parts of this system which appear homogeneous under the ordinary 

 microscope are shown by the ultra-microscope to be themselves heterogeneous. 

 These are in what is known as the colloidal state. Some dispute has taken 

 place as to whether this state is properly to be called a heterogeneous one, but 

 it is sufficient for our purpose to note that investigation shows that the inter- 

 faces of contact between the components of such systems are the seat of the 

 various forms of energy which we meet with in the case of systems obviously 

 consisting of phases which can be separated mechanically, so that considerations 

 applying to coarsely heterogeneous systems apply also to colloidal systems. 

 Although the phases of a colloidal system cannot be so obviously and easily 

 sepai-ated as those of an ordinary heterogeneous one, this can be done almost 

 completely by filtration through membranes svich as the gelatin in Martin's 

 process. To avoid confusion, however, it has been suggested that the colloidal 

 state should be spoken of as ' micro-heterogeneous.' 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 complexity 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 before us was discussed by one of my 

 predecessors. Professor Hopkins, as also by Professor Macallum, but its 

 importance will, I think, warrant my asking your indulgence for a further 

 brief discussion. Permit me fir.«t to apologise for what may seem to some of 

 those present to be an unnecessarily elementary treatment of certain points. 



It is easy to realise 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 



