200 EXAMINATIONS OF WATER, AIR, AND SOIL. 



For counting colonies, a small hand lens, such as is repre- 

 sented in Fig. 72, is often necessary. 



FIG. 72. 



Lens for counting colonies. 



Instead of plates and Petri dishes, Esmarch tubes may 

 be used as follows : A definite quantity of the water is added 

 to melted gelatin in a test-tube and the same rolled as 

 described previously. 



A special counter for Esmarch tubes, provided with a mag- 

 nifying glass, is used in counting the colonies (Fig. 73). 



FIG. 73. 



Esmarch's apparatus for counting colonies in rolled tubes. (Abbott.) 



As previously mentioned, the value of this quantitative 

 examination of the bacteria of water is not very great, because. 



