598 BACTERIOLOGY. 



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 AGAE-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 feom Wateb that developed upon— 



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



310 . . . 170 



280 . , . 140 



3101 . . fl80 



340 ) . 1 160 



6501 . . 1 210 



630 J I 320 



380) . . . f290 



400 J I 210 



1000 1 . f 100 



890 J . 1 130 



3401 ... f280 



370J . . . 1210 



490) . . . 1 110 



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- 



' I am indetted to James Homer Wright, Thomas Scott Fellow in 

 Hygiene (I892-'93), University of Pennsylvania, for the results pre- 

 sented in this table. 



