VIII 



.] SOLUTION-PRESSURE 149 



between these dissociated ions is the mutual attraction of their 

 -f- and charges. Chlorides, bromides, and iodides, etc., of 

 sodium, potassium, and ammonium, etc., in dilute solution and 

 decimolecular solutions such as we find it convenient to use in 

 the physiological laboratory are in this connection " dilute " 

 exist in this active state of ionisation as positively charged 

 kations and negatively charged anions. I said this active state 

 of ionisation, since it is to this state of ionisation that their chief 

 chemical and physical actions are due, and among these actions 

 no doubt we must reckon their action upon living matter. 



Now, if we take a number of salts, such as those just named, 

 in dilute equimolecular (or, better, in isosmotic) solution, 

 and systematically compare their respective effects upon some 

 convenient physiological reaction of living matter, we shall find 

 that differences of the kation produce far greater differences of 

 reaction than do differences of the anion. 



I do not pretend that this is in itself any proof that action 

 and action currents are essentially characterised by increased 

 solution-pressure and actual discharge into solution of electro- 

 positive ions. But I think we may provisionally conclude that 

 in the action of dilute saline solutions upon living matter, it is 

 the electro-positive kation that takes the lead, acting presum- 

 ably in the sense that we should anticipate, viz., increasing the 

 kationic pressure of the lymph bath, and decreasing, therefore, 

 the difference between that pressure and the kationic solution- 

 pressure of the living matter. 



The state of things under the influence of the anode must be 

 regarded as the converse to that just pictured as obtaining under 

 that of the kathode. Anions are attracted, kations are repelled ; 

 the solution-pressure of the latter must be lowered, and the sur- 

 face of living matter must be rendered less zincative, less easily 

 made to discharge kations ; and, if forced to such discharge by 

 adequate excitation, manifesting greater effect than when the 

 initial pressure was higher. 



The post-anodic action current, which makes its appearance 

 at the moment of cessation of an exciting (or polarising) 

 current, is very easily accounted for. The kationic solution- 

 pressure, depressed under the influence of the anode, is suddenly 



