88 BACTERIOLOGY 



this error becomes very appreciable. The discrepancy between 

 the end point for litmus and for phenolphthalein will vary for 

 different lots of media. Another source of error and misunder- 

 standing arises from the fact that the- reaction of a medium 

 changes somewhat after its neutralization, especially during 

 sterilization, but also upon standing afterward at ordinary tem-^ 

 perature. This change is toward decreased alkalinity and in- 

 creased acidity and its extent is not the same for different media, 

 being most marked, perhaps, in those rich in glucose. Where 

 particular importance is attached to the titre of a medium, it is 

 well, therefore, to determine this upon a sample of the medium 

 taken from the lot at the tinie it is used, rather than to quote 

 figures obtained before sterilization. The optimum reaction for 

 most microbes is very close to the neutral point for litmus and 

 for most pathogenic bacteria slightly alkaline to this indicator. 



A clear understanding of electrolytic dissociation and the 

 measurement of acidity in terms of hydrogen ion concentration, 

 requires a knowledge of the elementary principles of physical 

 chemistry, which the beginning student of bacteriology may not 

 possess. Inasmuch as it is customary to use such terms as 

 Ph — -T-o to indicate that a medium possesses neutral reaction, 

 P„ = 6.0 for a definite slight acidity, arid P^ = 8.0 for a definite 

 slight alkalinity, the student should be acquainted with them. 



When a crystalline salt, such as NaCl, is mixed with a quan- 

 tity of a solvent insufficient to dissolve all of it, the salt exists in 

 three states, (i) undissolved crystalline residue, (2) undissociated 

 salt in solution as NaCl and (3) electrolytically dissociated salt 

 in solution as the dissociated electrically charged ions, Na+ and 

 C1-. If all of the solid substance is dissolved then it exists in the 

 states indicated as (2) and (3). The speed of chemical reaction 

 into which such a solution will enter depends upon the concentra- 

 tion of the ionized portion. In the same way an acid, such as 

 HCl, in solution in water is more or less completely ionized into 

 H+ and CI" and the reactivity or chemical strength of such a 

 solution is measured by the concentration of the ions and its 



