THE BACTERIA OF SOIL, AIR, WATER, ETC. 143 



it is caught; for more or less change probably takes place at 

 all temperatures. If the temperature is too high, and the 

 water freezes, more or less of the bacteria may be killed; if, on 

 the contrary, the temperature is not low enough there will be 

 a multiplication of the bacteria in transit. The value of exam- 

 ination of water shipped in any way is of, at least, doubtful 

 utility. 



The number of bacteria may be determined by making 

 plates of a definite quantity of the water with gelatin or agar.* 

 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. Or better, 

 measure the water into the Petri dish, and pour the melted 

 medium in, and mix. 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 cer- 

 tain 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 al- 

 ways 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. 



*For preparation of culture media for water analysis, see p. 71. 



