18 VETERINARY HYGIENE 



to a pint of water; this concentration is said to destroy vegetative 

 organisms in half an hour. This method of purification has been 

 used with success by troops and travellers, but it has no application 

 in veterinary practice. 



Chlorine. Bleaching Powder. Of the halogens chlorine is the 

 only one of practical interest for the sterilisation of water. It may 

 be used in one of three ways : as chlorine gas, as bleaching 

 powder, or as sodium hypochlorite. 



When chlorine is liberated in water a certain proportion is con- 

 sumed by the organic matter, and any iron and sulphur compounds 

 present, so that the amount that is available for the destruction 

 of bacteria, that is for sterilisation, will be the total quantity gener- 

 ated less that taken up by the other substances. In practice this is 

 carefully estimated. Bleaching powder when mixed with water 

 yields hypochlorous acid and nascent oxygen ; it is a more powerful 

 oxidising agent than chlorine gas and produces relatively more 

 oxygen. The following equations explain the reactions that take 

 place : 



CaOCl 2 + H 2 CO 3 =CaCO 3 +H 2 



2HC1O = 



When bleaching powder is used for the purification of water 

 it should be added in such quantity as to yield at least one part 

 per million of available chlorine, that is chlorine that is not used 

 other than for the destruction of bacteria. The time allowed 

 should be half an hour. Chlorinated water has often a disagreeable 

 taste. According to Lelean* two parts of chlorine per million of 

 water can be used without the risk of an objectionable taste. Sub- 

 sequent exposure to the air removes the taste and when water is 

 chlorinated on a large scale it is allowed to fall in cascades for 

 thorough aeration. 



Bleaching powder added to foul ponds readily purifies the water 

 and destroys the algae. 



Sulphate of Copper. Sulphate of copper has a very destructive 

 effect upon algae. One part of the salt to a million parts of water 

 is sufficient to rid a water of most of its algae. Provided that the 

 water does not contain too great an amount of organic matter, 

 one part to 400,000 will destroy typhoid bacilli in twenty- four hours. 



THE AMOUNT OF WATER REQUIRED BY DOMESTIC ANIMALS. 



The amount of water required daily by domestic animals 

 depends upon their size and functional activity, the nature of the 

 * Sanitation in War, 1919. 



