BACTERIOLOGICAL EXAMINATION OF WATER. 163, 



in five of the squares through which the diameter 

 would pass, are then counted. The total is multiplied 

 by ten, and thus the approximate number of the 

 colonies contained in the whole plate is arrived at. 

 Of course it is taken for granted that the colonies are 

 pretty evenly distributed over the whole surface. 



Not much, however, can be deduced from the num- 

 ber of the colonies alone. This may be proved by ex- 

 amining some distilled water, which has been allowed to 



Fig. 9. 



stand for some time in the laboratory, and which some- 

 times contains an immense number of living bacteria. 

 If water is examined just after it has been distilled, 

 and then again eight days later, having been allowed 

 during the interval to stand in the laboratory at 

 ordinary temperatures, it is seen that in the first case 

 only isolated colonies make their appearance, 'whilst 

 in the second a great number are to be seen. Thus 

 the bacteria must have multiplied considerably during 

 the interval, and the infinitesimal amount of foreign 



