432 BACTERIOLOGY. 



CO. of the original water we had 180 X 100 = 18,000 

 bacteria, which will be 72,000 bacteria per cubic centi- 

 metre (0.25 = 18,000, 1 c.c. = 18,000 X 4 = 72,000). 

 The results are always to be expressed in terms of the 

 number of bacteria per cubic centimetre of the original 

 water. 



Another point of very great importance (already 

 mentioned) is the effect of temperature upon the num- 

 ber of colonies of bacteria that will develop on plates 

 made from water. It must always be remembered 

 that a larger number 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 follow^ 

 ing 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, are always obtained when 

 gelatin is employed. The importance of this point in 

 the quantitative bacteriological analysis of water is too 

 apparent to require further comment. 



