BACTERIA AND ICE 151 



There are numerous circumstances which come 

 into play in determining the density of the bac- 

 terial population in ice. First, of course, the 

 initial quality of the water from which the ice is 

 derived is a factor of great importance, for the 

 purer the water the fewer will be the bacteria 

 found in the resulting ice. 



Again, if the ice field is wind-swept by air 

 bearing an unduly rich complement of bacteria, 

 as may be expected in the vicinity of populous 

 cities, for example, then the ice will reflect in its 

 bacterial contents the undesirable neighbourhood 

 in which it was produced. Water in repose, again, 

 yields purer ice than water in movement during 

 freezing, for during rest opportunity is given for 

 the bacteria present in suspension to subside, the 

 process of sedimentation or deposition of bacteria 

 which takes place under these conditions playing 

 an important part in water-purification ; when, 

 however, the water is disturbed by swift currents, 

 or agitated by storms, this process is interrupted, 

 and the bacteria become entangled in the ice and 

 frozen in situ. 



The importance attaching to the physical condi- 

 tions under which ice is produced in enabling an 

 estimate to be formed of the safety or otherwise 

 of the same for consumption may be gathered 



