84 A. P. MATHEWS 



necessary inference, if this were true, that the ionic potential must be 

 of decisive value in determining the precipitation of colloids by electro- 

 lytes. An investigation of this possibility showed that this was 

 indeed the case. McGuigan 1 then investigated the relation between 

 the ionic potential and the power of salts to prevent the action of the 

 diastatic ferment upon starch, and found here also a remarkably close 

 agreement with the theoretical anticipations. 



The theory of the importance of the ionic potential has, therefore, 

 been abundantly confirmed. It is the more surprising that it has 

 met with little acceptance or attracted little notice, since its general 

 bearings are exceedingly important, involving as they do the nature 

 of chemical affinity on the one hand, and the basis of pharmacology 

 on the other. 



Owing to the importance of the subject, the slight attention it has 

 received, and to the fact that my own ideas have become during the 

 course of the work more clear and definite, it seemed to me desirable 

 that the results previously presented both by myself and by others be 

 summarized and put in a more definite, and perhaps a more com- 

 prehensible, form, together with new observations in the same direc- 

 tion. Since the solution tension and ionic potential are properties 

 with which physiologists are not generally very familiar, since they 

 lie in another field not hitherto brought into relationship with physio- 

 logical processes, I have tried to get these ideas clear at the outset. 



a) General physical principles involved in chemical stimulation and 

 toxicity. Any physiological response to an external agent, however 

 that response is produced, implies a change in motion or in state of 

 the atoms, molecules, and masses composing the protoplasmic system. 

 Now such a change in state means that work has been done in pro- 

 ducing these movements, and this work must have been done at the 

 expense either of the internal energy of the system itself, or of the 

 energy of the environment in this case of the substance causing the 

 change. There are accordingly two possible ways in which an exter- 

 nal agent such as a salt might produce a change in the protoplasmic 

 system. It may itself supply the energy, in whole or in part, which 

 is necessary to bring to pass the internal movements of the system; 

 or it may by its presence facilitate the transference of the potential 



1 McGuiGAN, Amer. Jour. Physiol., 1904, 10, p. 444. 



