146 MASS. EXPERIMENT STATION BULLETIN 167. 



contains hydrochloric acid and sodium hydroxide is taken, it can be ex- 

 pressed in the following manner : — 



+ - + - + - 



H CI + Na OH =Na Cl+HOH 



hydrochloric sodium hydroxide salt water 



This solution is neutral only when it contains just as many hydrogen 

 as hydroxyl ions, or when both the acid and alkali are equally dissociated. 



It is understood, therefore, that the "true acidity, alkahnity and neu- 

 trality" are not determined by the amount of such substances present, 

 but entirely by the H and OH ion concentration. 



With the above facts in mind it becomes possible to enter upon a more 

 intelligent discussion of the methods involved. It has been stated pre- 

 viously that most bacteriologic experiments, having for their purpose 

 the study of reaction upon bacterial life, fall under the following pro- 

 cedures : — 



(a) Kisch's method. ^ 



(b) Ordinary titration method. 



(c) Colorimetric method. 



It is well known that Kisch's method is a dilution method wherein a 

 certain number of gram molecules of an acid or alkali are diluted to a 

 definite quantity for the purpose of ascertaining the influence of the re- 

 action upon the life of bacteria. There are two distinct ways to apply 

 Kisch's method, namely: (a) immersing the bacteria in different dilutions 

 of acids or alkalis in pure water for different periods of time by means of 

 s ilk threads or any other convenient agents, and then testing their vitality; 

 or (6) adding a known percentage of acids or alkalis directly to the culture 

 medium (usually solution). In either case the results obtained by Kisch's 

 method indicate neither the influence of "true reaction" upon bacterial 

 life nor the influence of molecular concentration, because, as Lingelheim* 

 has shown, different acids of the same molecular concentration have 

 varying influence upon bacteria, and the degree of influence is parallel 

 to the dissociation constant of an acid or alkali. This is especially true 

 in the case of the second manner of application, (b), where adsorption is 

 caused by the culture medium. 



The ordinary titration method is generally employed in adjusting 

 reaction of culture medium, and also to measure the amount of acid or 

 alkali produced in the course of physiologic tests. This method is inac- 

 curate in the study of physiologic Hquids containing more or less ampho- 

 teric substances and a comparatively small quantity of H or OH ions. 

 In other words, it is impossible to determine the " true reaction " in such a 

 liquid by this method. FuUer's ' and Schiiltz's * methods of adjusting the 

 scale of reaction of culture media are scientifically condemned by the 



» Biochem. Zeitschr., 40, 152, 1912. 



» Zeitschr. f. Hyg., 8, 201. 



» Jour. Amer. Pub. Health Assn., 1895, 20, 381. 



« Centralbl. f. Bakteriol., O, 1891, 10, 52. 



