628 THE MECHANISM OF ABSORPTION 



electrode of a constant current, and we should expect for the 

 same reason the cation Na to have a depressing action owing 

 to its positive charge. The truth of this supposition is well 

 shown by the different action of acids and alkalies upon nerve ; 

 hydrates stimulate and acids depress it. The action of a hydrate 

 is due to the predominance of negatively charged OH ions which 

 it contains, and the action of an acid is similarly due to a 

 preponderance of positively charged H ions. Mathews, in fact, 

 has reached the generalisation that all anions have a stimulating 

 action, and all cations a depressing action, not only upon nerve 

 but upon bioplasm in general. The available evidence on the 

 whole strongly supports Mathews' s view. 



A salt consists of both an anion and cation. From what 

 has been said it would follow that the factor which determines 

 whether a salt as a whole stimulates or depresses, must be the 

 relative efficiency of its anion and cation ; if the anion pre- 

 dominates, as in the hydrates, the salt will stimulate, and if 

 the cation predominates the salt will depress. 



It would seem to follow also that there must be a connection 

 between the valency of an ion and its physiological action ; for, 

 the greater the valency the greater is the number of electrical 

 charges which the ion possesses. And on the whole this is found 

 to be the case ; the depressant action of a salt increases with an 

 increase in the number of charges on its cation, and the stimu- 

 lating power increases with the valency of the anion. But if the 

 valency of the ions were the real and sole explanation of the 

 physiological action of a salt, we should at once be landed in a 

 difficulty. For salts with ions of the same valency should stimu- 

 late or depress equally ; but this is not so. For instance, NaCl 

 as a whole stimulates nerve, i.e. the Cl overbalances the action 

 of the Na, but AgCl [depresses nerve, i.e. the Ag overbalances 

 the Cl. We have therefore to explain why the charge on Ag 

 appears to be so much more efficient than that on Na. Mathews 

 has pointed out that the efficiency of an ion depends upon the 

 potential of its charge, its ionic potential, that is, upon the 

 tendency of the ion to give up its charge and to change its 

 electrical state ; or, to put the point in another way, those 

 elements which are the most stable and hold their charges most 

 firmly, will have the least physiological action. Now the greater 

 the affinity of an element for its ionic charge, the greater will be 



