16 VETERINARY HYGIENE 



bacterial content may be reduced 99 per cent. The micro-organisms 

 that are pathogenic to man are more easily killed in the film than 

 are the harmless ones, and the Bacillus coli, which is always found 

 in sewage, is taken as the index of the efficiency of the filtration, 

 because if this organism is absent from filtered water it may rightly 

 be presumed that pathogenic organisms are also absent. 



Sand filtration keeps back from the service pipes the fauna 

 that are found in storage reservoirs and which, if they gain entrance 

 to the pipes and cisterns, cause a lot of trouble and spoil the appear- 

 ance of the water, even if they do no actual harm. 



The film is sometimes impaired by the burrowing of worms, 

 small eels and sticklebacks, so that on occasion water may pass 

 through only partially filtered. Gas bubbles, the result of animal 

 life in the film, may leave a ruptured surface on the film when they 

 break loose. 



While the purification of water for domestic use is very import- 

 ant, the same precautions need scarcely be taken for farm animals 

 or for general use on farms, were it not that impure water, when 

 used for washing out dairy utensils, may be the means of starting 

 an epidemic of typhoid. 



If water containing much suspended solid matter, muddy water, 

 is habitually drunk by animals it leaves an ever-increasing deposit 

 of silica, mica, &c., in the alimentary tract. This is a cause of colic, 

 constipation and, in the case of mica which forms a coating on the 

 mucous membrane, mal-nutrition. 



Water of this nature can be roughly filtered by passing it through 

 a gravel and sand bed or, in cases of emergency, through coarse 

 canvas. 



STERILISATION OF WATER. 



Subsequent to filtration, the purification of the water may be 

 completed by sterilisation. 



This is not always necessary, but is probably advisable for a 

 public supply when the water is used for drinking. Each supply 

 must necessarily be considered on its own merits. In the case 

 of dairy farms situated in the country remote from public con- 

 veniences the water used for all purposes is often drawn from 

 shallow wells and other doubtful sources. Artificial filtration of 

 such an individual supply is not practicable, so that sterilisation must 

 be substituted. 



The methods adopted for the sterilisation of water may be 

 divided into two classes, physical and chemical. The physical 

 methods include the use of heat, light and electricity. 



