BACTERIOLOGICAL EXAMINATION- OF WATER. 167 



must judge by numbers arrived at empirically. It 

 may be stated that there should never be more than 

 ten different species in good drinking water. It may, 

 however, happen that a water containing fewer must 

 be condemned, or that even if more than ten species 

 are present, the water may be consideted fit for 

 drinking purposes. 



If there are many species which liquefy the gelatine, 

 and moreover a large number of colonies of them, 

 the water must be considered impure, even if the 

 number of species be less than ten. If ten or fifteen 

 colonies which liquefy the gelatine are present in one 

 cubic centimetre of water, the outlook is bad ; if 

 several different species are present amongst these 

 colonies, then no doubt the water must have been in 

 some manner contaminated. If the colonies of kind-s 

 which liquefy the gelatine are still more numerous, 

 the water must immediately be condemned as impure. 



On the other hand, it may happen that more than 

 ten kinds of bacteria are present in water which is 

 quite fit for drinking purposes. In these cases they 

 are such as multiply only slowly, and do not liquefy 

 the gelatine. These bacteria are mostly chromogenic, 

 and probably fall into the water with the dust out of 

 the air ; they are able to exist even in the purest water, 

 although with some difficulty. As a rule the colonies 

 are only few. 



As regards the nu mber of colonies, a limit has been 



