220 LAURENCE F. FOSTER 



Methods 



Ammonia was determined by the Folin air current method 

 (1912) using 2 cc. of culture and collecting the gas in n/50 acid 

 after which the residual acid was determined by back titration 

 with n/50 .base. Results are expressed as miUigrams per 100 

 cc. of culture. 



Amino acids were determined by the formol titration method 

 of Sorensen previously described (see section I) . 



Gkicose was determined by the method of Bertrand (Hawk, 

 1918). As the presence of peptone and protein material in the 

 medium rendered the application of the method impossible, the 

 following procedure, devised by Dr. Marjorie W. Cook, was 

 employed to free the cultures from interfering substances: 



Twenty cubic centimeters of culture was diluted to 100 cc. 

 with distilled water and precipitated with 10 to 15 cc. of saturated 

 tannic acid solution. After filtering, 5 to 7 grams of lead acetate 

 were added to the filtrate to remove excess of tannic acid and 

 this mixture was filtered. If the filtrate was turbid more lead 

 acetate was added. To the filtrate from this treatment was 

 added 2 to 3 grams of sodium oxalate. This removed the lead 

 as Pb(C204). The filtrate from this last treatment should be 

 perfectly clear and colorless. It is important throughout the 

 whole procedure to keep the containers and funnels covered thus 

 minimizing evaporation and reducing the error 'from this source. 

 Two 10 cc. portions of this liquid were now used for determina- 

 tions of glucose. 



Bacterial counts were made by the method of Wright and 

 logarithms of the values so obtained were employed in plotting 

 growth curves. It must be borne in mind that the method 

 of Wright gives only approximate results and that the values 

 represent the total number of organisms rather than the number 

 of viable cells. 



Titrations of hemotoxin were made with sterile tubes, pipettes, 

 etc., to maintain the purity of the streptococcus cultures. Rab- 

 bit corpuscles which had been washed three times in 0.85 per 

 cent NaCl and made up in a 1 per cent suspension in beef infusion 



