112 BACTERIOLOGY 



The first of these was the titration of a known volume 

 of the culture medium with alkaline solutions of known 

 strength and the use of indicators believed at the time to 

 be trustworthy in revealing very minute deviations from 

 the neutral point. From the amount of alkali used in estab- 

 lishing the neutral point of a small fraction of the mass 

 under consideration it was easily possible to calculate the 

 amount needed for the whole. 



The indicator commonly employed in such titrations is 

 phenolphthalein, a compound which in neutral or faintly 

 acid solutions is practically colorless but which shades from 

 a delicate pink into deep magenta as alkalinity gradually 

 increases. 



The adoption of still more refined physical methods to 

 the study of solutions, together with the fact laid down 

 by Arrhenius that many substances when dissolved in 

 water no longer retain their molecular structure but in 

 part or whole are dissociated into atomic modifications 

 that he terms ions, some of which carry positive and others 

 negative electrical charges, have led to a complete revision 

 of our views on the questions of "the reactions of solutions" 

 as the expression had hitherto been employed. 



As a result of all this two general plans for determining 

 acidity or alkalinity of culture media are now in use the 

 one the titration method, the other the estimation of the 

 aggregate electro-positive ions present in the solution, i. e., 

 the hydrogen-ion concentration of the solution. As there is 

 every reason for regarding the latter method as the more 

 exact, it is likely that in the near future it will supersede all 

 others, particularly as the mode of its application is becom- 

 ing constantly more and more simplified and adapted to the 

 routine needs of the bacteriologist. 



