498 BACTERIOLOGY. 



in 0.25 c.c. of the original water we had 180 X 100 = 

 18,000 bacteria, which will be 72,000 bacteria per cubic 

 centimetre (0.25 =18,000, 1 c.c.=18,000 X4 =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 men- 

 tioned) is the effect of temperature upon the number 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 following table, illus- 

 trative of this point, gives the results of parallel anal- 

 yses of the same waters, the one series of counts having 

 been made upon gelatin plates at the ordinary tempera- 

 ture 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 quan- 

 titative bacteriological analysis of water is too apparent 

 to require further comment. 



