5O BACTERIA 



upon the Koch's plate, and the colonies counted in one little 

 square or set of squares and multiplied. 



By using flat, shallow, circular glass dishes, generally known 

 as Petris dishes, instead of Koch's plates, much manipulation 

 and time is saved, and, on the whole, less risk of pollution 

 occurs. Moreover, these are easily carried about and trans- 





PETRI'S DISH 



ferred from place to place. When counting colonies in a 

 Petri's dish it is sufficient to divide the circle into eight equal 

 divisions, and counting the colonies in the average divisions, 

 multiply and reduce to the common denominator of one 

 cc. For example, if the colonies of the plate appear to be 

 distributed fairly uniformly we count those in one of the 

 divisions. They reach, we will suppose, the figure of 60; 

 60 X 8=480 micro-organisms in the amount taken from the 

 suspected water and added to the melted gelatine from 

 which the plate was made. This amount was .25 cc. There- 

 fore we estimate the number of micro-organisms in the sus- 

 pected water as 60 X 8=480 X 4=1920 m.-o. per cc., which 

 is over standard by about 1500. A water might then be 

 condemned upon its quantitative examination alone or qual- 

 itative alone, or both. If the quantity were even that of an 

 artesian well, say 4-10 m.-o. per cc., but those four or ten 

 were all Bacillus typhosus, it would clearly be condemned on 

 its quality, though quantitatively it was an almost pure 

 water. If, on the contrary, the water contained 10,000 m.-o. 

 per cc., and none of them disease-producing, it would still be 

 condemned on the ground that so large a number of organ- 

 isms indicated some kind of organic pollution to supply 

 pabulum for so many organisms to live in one cc. of the water. 

 It is not the number per se which condemns. The large 



