BACTERIOLOGICAL STUDY OF WATER. 453 



in 0.25 c.c. of our 1 : 100 dilution, therefore in 0.25 c.c. 

 of the original water we had 180 X 100 = 18,000 bac- 

 teria, which will be 72,000 bacteria per cubic centimetre 

 (0.25 = 18,000, 1 c.c. = 18,000 X 4 = 72,000). The re- 

 sults 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. 



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