THE MICROBES OF WATER 303 



(c) Filtration through porous porcelain. This is 

 the last method for the absolute sterilisation of 

 water. The Chamberland filter, which is univer- 

 sally used in bacteriological laboratories, is a device 

 by means of which water is forced through porous 

 porcelain. This filter may be attached to an ordinary 

 water-tap if the pressure of the water-supply is suf- 

 ficient to force the water through the porcelain ; if 

 not, a small force pump is required. 



Certain bacteriologists classify waters according to 

 the number of colonies revealed on cultivation. For 

 instance, Miquel gives the following standard for 

 the classification of waters : 



A water excessively pure yields from to 10 colonies per cc. 



,, very pure ,, , 10 to 100 



pure 



ordinary (mediocre) 

 impure 



very impure , , 



100 to 1,000 

 1,000 to 10,000 

 10,000 to 100,000 

 100,000 or more 



Such standards as this one are of little value, because 

 a water which reveals only 10 to 100 colonies may 

 be a worse water for drinking purposes than one 

 which reveals 1000 colonies. The former may be 

 contaminated with sewage, and, consequently, would 

 form a suitable medium for the further development 

 of microbes, whereas the latter may be free from 

 sewage; therefore it would be the better water of 

 the two. The fitness or otherwise of water for 

 drinking purposes cannot be pronounced from the 

 number of colonies obtained in a few quantitative 

 determinations. 'It must never be forgotten that 

 the sanitary examination of water is surrounded 

 with such difficulties that it is only by bringing to 



