BACTERIOLOGICAL STUDY OF WATER. 597 



mean of the number of colonies that develop upon 

 them taken as the basis from wliich to calculate the 

 number of organisms per volume in the original water. 



For example : from a sample of water 0.25 c.c. is 

 added to a tube of liquefied gelatin, carefully mixed and 

 poured as a plate. When development occurs the 

 number of colonies are too numerous to be accurately 

 counted. One cubic centimetre of the original water 

 is then to have added to it, under precautions that pre- 

 vent contamination from without, 99 c.c. of sterilized 

 distilled water — that is, ^ve have now a dilution of 

 1 : 100. Again, 0.25 c.c. of this dilution is j^lated, 

 and we find 180 colonies on the plate: Assuming that 

 each colony develops from an individual bacterium, 

 though this is perhaps not strictly true, we had 180 

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

 centimetre (0.25 c.c. = 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 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 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 



