498 BACTERIOLOGY. 



ill 0.25 c.c. of the origiaal water we had 180 X 100 = 

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

 cetitimetre(0.25 =18,000, 1 c.c. =18,000X4 =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. 



