EXAMINATION OF ^YATER 301 



squares ; an average is then taken, and the number of 

 organisms present thus calculated. With a little practice 

 very close approximations are to be obtained with this 

 apparatus. 



In the bacteriological examination of drinking-water it 

 is very important to note the character of the species 



Fig. 30. — Wolffhugbl's Apparatus. 



present. It is as well to know approximately the number 

 of organisms which liquefy the gelatine. These kinds are 

 almost invariably putrefaction bacteria, and can only grow 

 where there is plenty of organic matter, they being almost 

 absent from pure waters. 



The time and labour involved in ascertaining the char- 

 acters and number of the species of micro-organisms by 

 means of subcultures renders this operation prohibitive 

 in the ordinary bacteriological examination of a drinking- 

 water. This generally resolves itself into the enumeration 

 of the bacteria present, and an examination for a specific 

 pathogenic organism, typhoid or cholera, as the case 

 may be. 



With reference to the general question of the bacterio- 

 logical examination of drinking-water, much information 

 as to the character of a water is gained by incubating a 

 small quantity of the sample at blood-heat for twenty-four 

 hours. The number of organisms is then ascertained by 

 an ordinary gelatine plate culture. The number of organ- 

 isms so found is compared with the number of organisms 



