32 



ELEMENTS OF WATER BACTERIOLOGY. 



EFFECT OF THE LENGTH OF INCUBATION OF WATER BACTERIA IN GEL- 

 ATIN UPON THE NUMBER OF COLONIES DEVELOPING. 

 (Miquel and Cambier, 1902.) 



Length of Incubation. 



day . 

 days. 



3 



4 

 5 

 6 



7 

 8 



9 

 10 

 11 

 12 

 13 

 14 

 15 



Colonies Developed. 



20 



136 



2S4 



387 



53° 



637 



72S 



780 



821 



859 



892 



921 



9Si 



976 



1000 



Counting. — The number of bacteria is determined by 

 counting the colonies developed upon the plate, either 

 with the naked eye or preferably with a low-power lens. 

 This is actually done by placing the plate upon a glass 

 plate ruled in centimeter squares and over a black tile; 

 or the tile itself may be ruled. The colonies then appear 

 as whitish round or oval specks in and upon the medium. 

 As has already been said, it is desirable that this number 

 should not exceed 200, for when the number is very high 

 the colonies grow only to a small size and make counting 

 laborious and inaccurate, and many do not develop at all. 

 On the other hand, too few bacteria upon the plate prob- 

 ably give inaccurate results also. The greatest accuracy 

 is probably obtained with numbers ranging from 50 to 

 200. 



