2 3 2 



Bacteriology of Water 



to make a number of cultures with different quantities of the 

 water, using, for example, o.oi, o.i, 0.5, and i.o c.c., re- 

 spectively, to a tube of liquefied gelatin, agar-agar, or gly- 

 cerin agar-agar. 



The details of the method depend upon the quality of the 

 water to be examined. If the number of bacteria per cubic 

 centimeter be small, large quantities may be used ; but if 

 there be millions of bacteria in every cubic centimeter, it 

 may be necessary to dilute the water to be examined in the 

 proportion of i : 10 or i : 100 with sterile water, mixing well, 

 and making the plate cultures from the dilutions. 



It is best to count all the colonies developed upon the cul- 

 ture, if possible; but when hundreds or thousands are scat- 

 tered over it, an estimate made 

 by counting and averaging the 

 number in each of the small 

 squares of some counting appa- 

 ratus, such as that devised by 

 Wolfhiigel (Fig. 68) or that of 

 Ksmarch (Fig. 69). In count- 

 ing the colonies a lens is indis- 

 pensable. 



The majority of the water 

 bacteria rapidly liquefy gelatin, 

 on which account it is better to 

 employ both gelatin and agar- 

 agar in making the cultures. 



In ordinary city hydrant- 

 water the bacteria number from 

 2 to 50 per cubic centimeter; 



in good pump- water, 100 to 500; in filtered water from 

 rivers, according to Gtinther, 50 to 200 ; in unfiltered river- 

 water, 6000 to 20,000. According to the pollution of the 

 water the number may reach as many as 50,000,000. 



The waters of wells and springs are dependent for their 

 purity upon the character of the earth or rock through which 

 they filter, and the waters of deep wells are much more pure 

 than those of shallow wells, unless contamination take place 

 from the surface of the ground. 



Ice always contains bacteria if the water contained them 

 before it was frozen. In Hudson River ice Prudden found 

 an average of 398 colonies in a cubic centimeter. 



A sample of water when collected for examination should 



Fig. 69. Esmarch's instru- 

 ment for counting colonies of 

 bacteria in Esmarch tubes. 



