METHODS USED IN CULTIVATION OF BACTERIA 163 



used, this is multiplied by 63.6, the number of squares in the area of the 

 dish, and then by the dilution originally used. 



Thus if twelve squares have been counted with a total nimiber of 

 one hundred and forty-four colonies — ^the average for each square is 

 twelve. Twelve times 63.6 is 763.2, which represents the total number 

 of colonies in the plate. Now if 0.1 c.c. of the original material 

 (water or milk) has been plated, this material may be assumed to have 

 contained 10 X 763.2, or 7,632 bacteria to each cubic centimeter. 



If dishes of an unusual size are employed, the square area must be 

 ascertained by measuring the radius and multiplying its square by iz 

 {nXB? = area) ( t^= 3.141592). 



