390 APPLIED BACTERIOLOGY 



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. 37. — 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. 



The following procedure is recommended in the notifica- 

 tion of the Imperial German Health Department, issued 

 in regard to the filtration of surface waters used for public 

 water supplies : 



' In order to secure uniformity of method, the following 

 is recommended as the standard method for bacterial 

 examination : 



'The nutrient medium consists of 10 per cent, meat 



