170 PATHOGENIC BACTERIA. 



of bacteria in water is very simple, and can generally be 

 prosecuted without much apparatus. The principle de- 

 pends upon the equal distribution of a given quantity of 

 the water to be examined through a sterile liquid medium, 

 and the subsequent solidification of this medium in a 



FlG. 47. Heyroth's instrument for counting colonies of bacteria in Petri dishes. 



thin layer, so that all the colonies which develop may 

 be counted. 



The method, which originated with Koch, may be per- 

 formed with the Koch plates or with Petri dishes or 

 with Esmarch rolls. It is always best to make a num- 

 ber of these plate-cultures with different amounts of the 

 water to be examined, using, for example, o.oi, o.i, 0.5, 

 and i.o c.cm. added to a tube of gelatin, agar-agar, or 

 glycerin agar-agar. 



The exact method must depend somewhat upon the 

 quality of the water to be examined. If the number of 

 bacteria per cubic centimeter is small, large quantities 

 may be used, but if there are millions of bacteria in 

 every cubic centimeter, it may be necessary to dilute the 



