642 STABILITY OF BACTERIAL SUSPENSIONS. H 



fifteen measurements due to the gradual removal of the film. Control experiments 

 with clean glass surfaces showed no significant variation under the conditions of 

 the experiment. The values obtained in this way were surprisingly reproducible. 

 They have been expressed as milligrams required to separate two surfaces each 

 2 cm. square. The results are not exactly comparable to the measurements of 

 the potential since the organisms have been subjected to dry heat. It will be 

 noted, in fact, that the results do not conform exactly to those expected from the 

 potential measurements. In the case of NaCl, for instance, the concentration 

 required to affect the cohesive force noticeably, is slightly higher than would be 

 expected from the potential curve. 



It has usually been considered that this force is a surface tension effect, but there 

 does not appear to be any conclusive evidence as to its nature. It is better, per- 

 haps, to refer to it simply as "cohesive" without an exact definition of its nature. 



Measurement and Regulation of the Hydrogen Ion Concentration. — The pH 

 determinations were made electrometrically, using a saturated calomel cell and 

 taking the pH value of 0.10 N HCl as 1.04 at 33° as the standard. 



Buffers Used. — It was found that a very convenient buffer could be made by 

 combining sodium phosphate, sodium acetate, and glycine. It may be used over 

 a range of pH from 1 to 13 and has the further advantage that the nature of ions 

 present is not varied. The only variation is a change in concentration of the 

 CI and Na ions. The composition and the titration curve of the buffer are given 

 in Fig. 1. This is referred to as G. P. A. Buffer. The pH measurements were 

 made at 33°. In some experiments Walpole's^° acetate series was used. 



Cultures. — The culture of the bacillus of rabbit septicemia used was that pre- 

 viously isolated and described by one of the writers.^^ The typhoid culture was 

 the Pfeiffer strain obtained through the kindness of Dr. Charles Krumwiede to 

 whom we are also indebted for the strong antityphoid horse serum. 



Measurement of the Degree of Agglutination. — No satisfactory method could be 

 found for measuring the agglutination quantitatively. Several degrees of agglu- 

 tination were, therefore, selected and the determinations made on this basis. 

 They were recorded as foDows: 

 — No agglutination. 



+ Distinct particles visible with a lens, 8 diameters magnification. 

 + + Particles visible with the eye alone. 



+ + + Suspension almost completely agglutinated and settled but cloudy 

 appearance in the supernatant liquid. 



C. Supernatant liquid perfectly clear. 



The stage marked C. is the easiest to detect with certainty and was used as the 

 end-point. 



The degree of agglutination increases with time at first but after 24 hours re- 

 mains constant. All readings were therefore made after 24 hours at 20° to elimi- 



" Walpole, G. S., /. Chem. Soc, 1914, cv, 2501. 

 " De Kruif, P. H., /. Exp. Med., 1921, xxxiii, 773. 



