326 FREDERICK A. WOLF AND I. V. SHUNK 



peptone and 0.5 per cent sodium chlorid. They were then heated 

 in an autoclave, flasked, and sterilized, for fifteen minutes at 

 15 pounds pressure in the case of agar, and 10 pounds pressure 

 in the case of gelatin. No attempt was made to adjust the 

 reaction of the media to neutrality prior to sterilization. The 

 acid used was hydrochloric, with a specific gravity of 1.20 or 

 it possessed an HCl concentration of 39.11 per cent." The 

 sodium hydroxid had a specific gravity of 1.226 or an NaOH 

 concentration of approximately 20 per cent. Strong acid and 

 alkali were employed to eliminate the factor of dilution of the 

 media. Upon removal from the autoclave the agar was cooled 

 to about 50'C. and the gelatin to about 40°C. before the addition 

 of appropriate quantities of acid or alkali, and were maintained 

 at these temperatures while 10 cc. portions were withdrawn 

 with a pipette and put into test tubes. The acid or alkali w^as 

 added to these 10 cc. portions wdth a 1 cc. pipette graduated in 

 tenths. After the addition of the acid or alkali the tubes were 

 weU agitated and were further cooled with the results indicated 

 in the tabulations which follow. 



EXPERIMENTAL 



Only those proportions of agar or gelatin which are commonly 

 employed in making culture media were used in this study but 

 they indicate, as would be anticipated, that the jellifying power 

 is modified by the proportion of colloidal material added. The 

 results with 1 and 2 per cent agar are shown in table 1 . 



It wiU be noted that the limits of solidification of 1 per cent 

 agar are approximately 5.11 per cent acid and 0.39 per cent 

 alkali whereas 2 per cent agar does not lose its jellifying power 

 until 6.51 per cent acid or 0.58 per cent alkali has been added. 

 A better appreciation of the degree of acidity and alkalinity 

 of these limits can be gained when they are compared with pH 

 values determined by the colorimetric method of Clark and Lubs 

 (1917). It was found that the addition of 0.1 cc. of HCl to 

 10 cc. of agar gave a concentration of about pH 1.4 and 0.03 cc. 

 NaOH a concentration of about pH 9.2. In reaching the limits 



