BACTERIOLOGICAL STUDY OF WATER 619 



Another point of very great importance (already men- 

 tioned) is the effect of temperature upon the number of 

 colonies of bacteria that will develop on the plates made 

 from water. It must always be remembered that a larger 

 nimiber of colonies appear on gelatin plates made from water 

 and kept at 18° to 20° C. than on agar-agar plates kept in 

 the incubator. The following table, illustrative of this 

 point, gives the results of parallel analyses of the same waters, 

 the one series of counts having been made upon gelatin 

 plates at the ordinary temperature of the room, the other 

 upon plates of agar-agar 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 quantita- 

 tive bacteriological analysis of water is too apparent to 

 require further comment. 



Table Comparing the Results Obtained by the use of Gelatin at 

 18°-20° C. AND Agab-Agar at 37°-38° C. in Quantitative Bac- 

 teriological Analyses of Water. Results Recorded are the 

 Number of Colonies That Developed From the Same Amount op 

 Various Waters 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 / 1 160 



650 \ / 210 



630 / I 320 



380 1 I 290 ' 



400 / ■ \ 210 



1000 \ / 100 



890 / \ 130 



340 \ / 280 



370 / \ 210 



490 \ / 110 



580 / I 100 



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

 1892-1893), University of Pennsylvania, for the results presented in this 

 table. 



