BACTERIOLOGICAL STUDY OF WATER. 499 



TABLE ILLUSTRATING THE PROPORTION BETWEEN THE RE- 

 SULTS OBTAINED BY THE USE OF GELATIN AND AGAR-AGAR 

 IN QUANTITATIVE BACTERIOLOGICAL ANALYSIS OF WATER. 

 RESULTS RECORDED ARE THE NUMBER OF COLONIES THAT 

 DEVELOPED FROM THE SAME AMOUNT OF WATER IN EACH 

 SERIES.! 



NcxBKR OF Colonies from Water that dktelopbd rros— 



GeUtin plates at 18° to 20° C. Agar-agar plates at 37° to 38° C. 



310 170 



280 140 



31(r) fl80 



340 i 1160 



- 650) »210 



630/ 1320 



380) f290 



400> 1210 



1000) JlOO 



890' 1130 



340) f280 



370) (210 



490 > (110 



580 S I1OO 



Another point of equal importance in its influence 

 upon the number of colonies that develop is the reac- 

 tion of the gelatin. A marked excess of either alka- 

 linity or acidity always has a retarding effect upon 

 many species found in water. Experience at Law- 

 rence has shown that gelatin of such a degree of acidity 

 as to require the further addition of from 15 to 20 c.c. 

 per litre of a normal caustic alkali solution to bring it to 

 the phenolphtalein neutral point gives, ou the whole, 

 the best results. Thus, by way of illustration, FuUer 

 found that a sample of Merrimac River water gave 

 5800 colonies per c.c. on phenolphtalein neutral gel- 

 atin, 15,000 colonies on gelatin that would need 20 c.c. 

 of normal alkali solution to bring it up to the phenol- 



• I am indebted to Dr. James Homer Wright, Thomas Scott Fellow in 

 Hygiene (1892-'93), University of Pennsylvania, for the results presented in 

 this table. 



