THe PHYSIOLOGICAL EFFECTS oF Ions 451 
the work done in growth has never once been considered. 
We need not go far to find the reason for this. Even in the 
newer text-books of physiology the attraction of salts for 
water is still spoken of as was the case fifty years ago; the 
idea of osmotic pressure and the work of Van ’t Hoff have 
not yet worked their way into this territory.’ 
One of the most fertile results of the theory of dissocia- 
tion is the idea which has been brought forward, chiefly by 
Ostwald, that those reactions of acids in aqueous solutions 
which are common to all acids, and only to these, are 
dependent upon the activity of the positively charged hydro- 
gen ion, and that in a similar way the universally specific 
effects of bases are determined by the negatively charged 
OH ion. The relative strength of acids and bases is there- 
fore dependent upon the number of H and OH ions con- 
tained in the unit volume of the solution, and this number 
is determined by the degree of dissociation of the electrolyte 
under consideration. A further important result of the 
theory of dissociation is the fact that in completely disso- 
ciated salt solutions the properties of the solution are the 
sum of the properties of the ions contained in it. The ion 
itself, however, represents a new species of molecules, namely, 
atoms or groups of atoms which are charged with a definite 
amount of electricity. It is the object of this and further 
papers to determine the physiological effects of individual 
ions. 
Kahlenberg and True were the first to make such experi- 
ments in physiology.” These authors investigated the toxic 
properties of acids and salts in dilute aqueous solutions upon 
growing plants. As a measure of the toxic effects these 
authors used that concentration of the given electrolyte in 
water which just allowed germinating beans to grow sixteen 
1Since these lines were written matters have changed. [1903] 
2 Botanical Gazette, Vol. XXII (1896), p. 81. 
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