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.* 



NUMBER OF COLONIES FROM WATER THAT DEVELOPED UPON 



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



310 170 



280 140 



310 > (180 



340$ U60 



650) (210 



630 / (320 



380) (290 



400 i (210 



1000) J100 



890> *130 



340) f 280 



370 ) (210 



490) (110 



580 * '100 



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, on the whole, 

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

 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- 



1 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. 



