THE BACTERIA OF SOIL, AIR, WATER, ETC. 1 29 



The amount examined ordinarily is i c.c. When the number 

 of bacteria is very large, a smaller quantity must be taken, and 

 it may be necessary to dilute the sample ten times or more with 

 sterilized water. The amount should be measured with a 

 sterilized, graduated pipette. The water is mixed with melted 

 gelatin or agar in a tube which has been allowed to cool after 

 melting. After thorough mixing, remove the plug, burn the 

 edge of the tube in the flame, hold in a nearly horizontal position 

 until cool and pour into a sterilized Petri dish. The number of 

 colonies may be counted on the third or fourth day; the later 

 the better, as some forms develop slowly and may not present 

 visible colonies for several days ; but the plates are often spoiled 

 after three or four days by the profuse surface growths of 

 certain forms or by the rapid liquefaction of gelatin, if that be 

 used, by other forms. The number of colonies that develop is 

 supposed to represent the number of individual bacteria con- 

 tained in the quantity measured. That will probably not 

 always be the case, however, as colonies may develop from a 

 clump of bacteria which have not been separated from one 

 another by the mixing process. Abbott has shown that the 

 number of colonies is usually larger on gelatin plates than upon 

 agar plates, and at the room temperature than in the incubator. 

 This observation illustrates the fact that there are doubtless 

 many kinds of bacteria that do not find favorable conditions 

 for development on ordinary culture-media. The reaction of 

 the medium has an important influence upon the development 

 of these water bacteria in plate-cultures. 



When the number of colonies is small, there is no difficulty 

 in counting them as they appear in the ordinary Petri dish. 

 When the number is large some kind of mechanical device may 

 be used to assist counting. The Wolffhiigel plate is a large 

 square of glass resting in a wooden frame painted black. The 

 glass plate is ruled in squares. It was designed particularly 

 with reference to the form of plate-cultures first made by Koch. 



