ARNOLD H. EGGERTH AND MARGARET BELLOWS 



671 



Lactic acid-lactate and acetic acid-acetate buffer mixtures were 

 made up according to the method of Beniasch (1912), which gives 

 mixtures of uniform salt concentration (0.025 m). This unifonnity of 

 salt concentration is essential, as the salt effect is very high in some 

 cases. The phosphate buffer mixtures were made up according to 

 the directions of Clark and Lubs (1917), except that they are made 

 twice as dilute, to make the salt concentration equal to that of the 

 Beniasch buffer mixtures. 



Flocculation of Bacteria with Gelatin. 



Isoelectric gelatin was prepared by the method of Loeb (1919). 

 Suspensions of Bacterium coli were incubated with varying dilutions 

 of gelatin, both in buffer mixtures (Table I) and in the absence of 

 salt (Table II). 



TABLE I. 



Bacterium coli Suspension with Gelatin. 

 1.0 cc. buffer mixture + 0.5 cc. gelatin solution + 0.5 cc. coli suspension. 



Temperature = 40°C. X = agglutination within 30 minutes. + = agglu- 

 tination within 6 hours. 



In Table I it is to be noticed that with the highest concentration of 

 gelatin, agglutination occurs only at pH 4.7, the isoelectric point of 

 the gelatin. In the 1:4,000 dilution, the zone of flocculation widens, 

 especially on the acid side, but it also includes the pH 5.0 tube. 

 Beginning with the 1:40,000 dilution of gelatin, the zone of floccula- 

 tion shifts over to the acid side, even becoming more acid than the 

 flocculation zone of the control. It will be noted that there is an 

 absence of flocculation with the higher concentrations of gelatin in 

 the more acid tubes, forming a so called "proagglutinoid zone." 



