154 BACTERIA IN DAILY LIFE 



although not sufficient to render the use of ice 

 from such a polluted source either palatable or 

 desirable. 



It is, of course, a well-known fact that water 

 possesses the power of purifying itself during its 

 transformation into ice, and that the process of 

 crystallisation not only prevents a considerable 

 proportion of the matters in suspension from 

 becoming embodied in the ice, but also eliminates 

 a large percentage of the matters in solution, the 

 latter being driven from the water which is being 

 frozen into the water beneath. If, therefore, ice in 

 the act of forming can get rid of matters in solu- 

 tion, it is not difficult to understand how it can 

 eject bacteria, which though so minute are yet 

 bodies of appreciable dimension and in suspension. 

 But that there are limits to this power of exclud- 

 ing bacteria, and that, as in the case of other 

 mechanical processes, an overtaxing of the avail- 

 able resources is at once reflected in the inferiority 

 of the product, is shown by the frozen sewage 

 experiment, in which the ice, having had too large 

 a supply of bacteria in the first instance to deal 

 with, was unable to get rid of more than a certain 

 proportion, and was obliged to retain a very 

 considerable number. Hence great as is the 

 degree of purification achieved by ice in forming, 

 yet it must be recognised that its powers in this 



