BACTERIOLOGICAL STUDY OF WATER. 633 



kept for the same length of time in the incubator at 

 from 37 to 38 C. It will be seen from the table 

 that much the larger number of colonies i. e., much 

 higher results were always obtained when gelatin was 

 employed. The importance of this point in the quan- 

 titative bacteriological analysis of water is too apparent 

 to require further comment. 



TABLE COMPARING THE RESULTS OBTAINED BY THE USE OF GEL- 

 ATIN AT 18-20 C. AND AGAR-AGAR AT 37-38 C. IN QUANTI- 

 TATIVE BACTERIOLOGICAL ANALYSES OF WATER. RESULTS 

 RECORDED ARE THE NUMBER OF COLONIES THAT DEVELOPED 

 FROM THE SAME AMOUNT OF VARIOUS WATERS IN EACH 

 SERIES.i 



NUMBER OF COLONIES FROM WATER THAT DEVELOPED UPON 



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



310 170 



280 . 140 



310) J180 



340) 1160 



650) 1 210 



630 1 1320 



380) f 290 



400) (210 



1000) flOO 



890) (130 



340) |280 



370 J (210 



490) rllO 



580) llOO 



Another point of equal importance in its influence upon 

 the number of colonies that develop is the reaction of the 

 gelatin. A marked excess of either alkalinity or acidity 

 always has a retarding effect upon many species found in 

 water. Fuller's experience at the Lawrence (Mass.) Ex- 



1 I am indebted to James Homer Wright, Thomas Scott Fellow in 

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

 sented in this table. 



