78 



SCIENCE 



[N. S. Vou XLVI. No. 1178 



From its very beginning, Arrhenius's 

 theory of ionization emphasized the pecu- 

 liar importance of the ions of hydrogen and 

 hydroxy!. As products of the electrolytic 

 dissociation of water these ions must be 

 present in all aqueous liquids. As products 

 of the dissociation of acids in one case and 

 of bases in the other, they must be essen- 

 tial factors, or at least the only constant 

 factors, of acidity and alkalinity in aqueous 

 solutions. 



Methods for the estimation of the concen- 

 tration of these ions were presently found, 

 and before long successfully, if rather 

 roughly, applied to physiological problems. 

 Thus it was proved that the reaction of 

 blood is nearly neutral and very constant. 



Meanwhile the theory was extended, 

 with the help of the mass law, until it be- 

 came a quantitative theory of acidity, neu- 

 trality and alkalinity. The principal re- 

 sults of this development of the subject, so 

 far as they concern the biologists, are as 

 follows : 



First, the product of the concentrations 

 of hydrogen and hydroxyl ions (at constant 

 temperature) is approximately constant. 



(H) . (OH) = c. 



Therefore the concentrations of these two 

 ions always vary inversely. 



(H): 



(OH) 



Secondly, if for convenience, just as the 

 histologist uses microns instead of meters, 

 we adopt as unit concentrations of hydro- 

 gen and hydroxyl ions a very small quan- 

 tity, viz., the concentration of these ions in 

 neutral solutions, the value of this constant 

 becomes unity. 



(H).(OH)=l, 



(^> = (OHy 



It may be noted that, using this unit of 



concentration, an ordinary decinormal so- 

 lution of hydrochloric acid has a concen- 

 tration of hydrogen ions of nearly 1,000,- 

 000; and a decinormal solution of sodium 

 hydroxide a corresponding concentration 

 of hydroxyl ions. Other common dilute 

 acid and alkaline solutions are only less 

 remote from the concentrations of neutral 

 solutions and of blood. 



Thirdly, upon this basis the definitions of 

 neutrality, acidity and alkalinity are as 

 follows : 



For neutrality, 



(H)=l=(OH). 

 For acidity, 



(H) > 1 > (OH). 

 For alkalinity, 



(H) < 1 < (OH). 



Finally, in any solution containing a 

 weak acid and its salts with one or more 

 bases, regardless of the other components of 

 the solution, the concentration of hydrogen 

 ions is approximately proportional to the 

 ratio of free acid to combined acid. 



This relation, however, holds only when the 

 ratio of acid to salt is neither very large nor 

 very small. 



It is therefore evident that in the solution 

 of any weak acid, when the quantities of 

 free and combined acid are equal, the value 

 of (H) is &; if the ratio of acid to salt be 

 10 : 1, (H) is 10 k, if the ratio be 1 : 10 (H) 

 is 0.1 k. 



This is the total outcome of the theoret- 

 ical analysis so far as it is necessary for a 

 general understanding of the biological 

 problem. 



We may now turn to the special case of 

 carbonic acid. For this substance the value 

 of k, expressed in our present units, is 

 about 5. Accordingly, in a solution of car- 



