BACTERIA AND ICE 153 



on the ice-supply of Turin, part of which is 

 derived from a much-polluted portion of the 

 River Dora, found that about 90 per cent, less 

 bacteria were present in the ice than were present 

 in the water from which it was produced. In the 

 making of ice, therefore, a remarkable removal 

 of bacteria may be effected which approaches 

 very nearly the degree of bacterial purification 

 which is achieved during the best-conducted sand- 

 filtration of water. 



Uffreduzzi's results have been repeatedly con- 

 firmed by other researches. Thus, in regard to ice 

 obtained from the River Merrimac, water which 

 contained originally about 38,600 bacteria per 

 cubic centimetre, on its conversion into ice had 

 only from three to six. Sewage, again, containing 

 about a million and a half bacteria per cubic 

 centimetre after being frozen only contained under 

 74,000. It should be mentioned that this last 

 figure represented the number of bacteria obtained 

 by thawing the outside of the sewage ice-cake ; 

 inside the cake there were more found about 

 121,000. The difference in these figures is due to 

 the fact that, whereas the outer layers of ice 

 looked quite clear, towards the centre the ice 

 contained sewage sludge and hence more bacteria 

 had become arrested ; but in spite of this the 

 bacterial purification effected is very striking, 



