EXAMINATION OF WATER. 135 



Plate cultures or Esmarch rolls are made on gelatin, agar, 

 or glycerin-agar. If the water contains many bacteria per 

 cubic centimeter, it is necessary to dilute the water with sterile 

 water; or use only a small quantity for the inoculation by 

 means of pipettes (Figs. 42 and 43). 



Wolf huegel counts the colonies by placing the dish or plate 

 on a large plate of glass (Fig. 44) divided into many small 



FIG. 44. 



Wolffhuegel's apparatus for counting colonies. 



squares. The colonies in a certain number of squares are 

 counted with the aid of a hand lens (Fig. 45), and the num- 

 ber of bacteria per cubic centimeter estimated accordingly. 



FIG. 45. 



Lens for counting colonies (Abbott.) 



If the dilution has been sufficient, only a small number of 

 colonies appear, and these are easily counted. Divide the 

 result by the number of squares counted, and multiply this 

 average by the number of square centimeters in the plate. 

 The result is the entire number of colonies which have devel- 

 oped from the quantity of water used. Or Pakes's apparatus 

 may be used : a black disk, ruled with white lines (Fig. 46), 

 is printed on a sheet of white paper ; a Petri dish, containing 

 the colonies, with the cover removed, is then placed over the 



