WATER 17 



1 1 cat. Except for domestic purposes and for sterilising water 



a small scale heat is not used. It is too expensive and the 

 product, when the water is boiled, is devoid of gas and is con- 

 sequently insipid. When boiled under pressure it " boils " at a 

 higher temperature than when exposed to the air, and all its gas con- 

 tent is not dissipated. For general purposes the use of heat may be 

 discounted. It is, however, of great value under certain circum- 

 stances as when there is reason to believe the filters have become 

 temporarily faulty or when, owing to heavy flooding, surface water 

 has gained entrance to wells or springs. Where such an accident 

 as the latter occurs warning is usually given by the turbid appear- 

 ance of the water. 



Light. The ultra-violet rays of direct sunlight are germicidal, 

 but their action is limited to a very little way below the surface of 

 the water. The ultra-violet rays are sometimes used for the final 

 purification of public water supplies. There are many difficulties 

 attending its use. It is practically useless with anything but clear 

 water as suspended matters arrest the rays. 



Chemical Sterilisation. Sterilisation of water by the use of 

 chemicals should always be preceded by some form of filtration 

 so as to remove the grosser particles of organic matter. The 

 chemical substance used must not impart a disagreeable taste to the 

 water nor make it harmful to animals or people, while at the same 

 time it must possess sufficient germicidal power to effect its purpose. 

 The following substances are those most commonly used. 



Potassium Permanganate. Permanganate of potassium decom- 

 poses in the presence of organic matter yielding nascent oxygen. 



It is a feeble disinfectant in water, but has a specific action on 

 the cholera vibrio owing to the lethal effect of oxygen on this parti- 

 cular organism. A better reaction is obtained by the addition to 

 the water of some dilute acid. This treatment is not used on a 

 large scale but is sometimes adopted for the disinfection of wells 

 and water tanks. The amount of permanganate added must obvi- 

 ously vary with the degree of pollution. Lelean* says " The 

 treatment of wells is effected by adding to each gallon of water 60 

 grains of permanganate with 3 drachms of strong hydrochloric 

 acid; leaving for twenty- four hours; pumping until the water is 

 colourless; removing dead aquatic fauna." 



Sodium Bisulphate. The addition of sodium bisulphate to 

 water has for its object the liberation of sulphuric acid. The salt 

 i> put up in the form of tablets and is flavoured with lemon and 

 saccharin. The strength recommended is 15 grains of the salt 



* Sanitation in War, 1919. 

 2 



