500 STUDIES IN GENERAL PHYSIOLOGY 
means that it allows certain substances to pass through and 
others not. In my opinion, the working out of a theory of 
semi-permeability is one of the main physiological problems 
of the day. 
The theory of the dissociation of electrolytes is of funda- 
mental importance in the analysis of the constitution of living 
matter. Pharmacology will feel its influence most directly. 
Everything seems to indicate that the specific physiological 
effects of inorganic acids are due to the number of positively 
charged hydrogen ions in the unit of solution, and the spe- 
cific physiological effects of alkalies to the negatively charged 
hydroxyl ions. But the universal bearing of the theory of 
dissociation upon physiology will perhaps be best seen in the 
field of animal electricity. An active element of living mat- 
ter is negatively electric to its surrounding resting parts. 
We may assume that an acid is formed in the active part, and 
that the passive parts are neutral. The positive hydrogen 
ions of the acid have a much greater velocity of migration 
than the anions. Hence the former will diffuse more rapidly 
into the passive tissue than the anions, and the active tissue 
will remain negatively charged.’ 
At no time since the period immediately following the 
discovery of the law of conservation of energy has the out- 
look for the progress of physiology appeared brighter than 
at present. But in order to reap the full benefit of our 
opportunities, we must bear in mind that the fundamental 
problem of physiology is the determination of the constitu- 
tion of living matter, and that in order to accomplish our 
task we must make adequate use of comparative physiology 
as well as physical chemistry. Pathology, in particular, will 
be benefited by such a departure. 
1As far as I am aware, this was the first attempt at applying the principle of 
batteries of concentration to the explanation of the current of action. [1903] 
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