12 VETERINARY HYGIENE 



ralise the acidity. For public supplies the water coming from a 

 known acid area is usually cut out or provision is made for the 

 exclusion of the first of the flood water as it comes from the moors. 



Action on other Metals. Water containing much carbonic acid 

 and oxygen has an eroding action on iron and causes pitting of 

 delivery pipes; the action is more marked in soft waters than in 

 those containing much lime. The addition of lime acts as a pre- 

 ventive. The carbon dioxide in the water causes the formation of 

 ferrous carbonate which, in the presence of air, becomes oxidised 

 into the oxide so that CO 2 is again liberated. The calibre of the 

 pipes may be greatly reduced by the incrustations of the oxide 

 or the iron may become so pitted and thinned that the pipes burst. 

 There is no reason to believe that iron-containing water is harmful 

 to animals, but the presence of iron is favourable, and indeed 

 necessary, for the growth of Crenothrix which is often very 

 troublesome in water pipes. 



Zinc is acted on by some waters, yet zinc-lined troughs are used 

 extensively without apparently causing illness. 



Other metals are sometimes found in water. Thus arsenic used 

 in the manufacture of sheep-dips and weed-killers may gain access 

 to water supplies if carelessly used in the vicinity of wells or if 

 the dripping ground at a dipping tank should drain through the sub- 

 soil by fissures into a well. 



THE STORAGE OF WATER. 



Wherever there is a public water supply to a community, pro- 

 vision is made for the collection and storage of such a quantity 

 as will meet the demand during the dry season. Storage of water 

 besides maintaining the supply for service greatly improves its 

 quality, unless the water is already pure, in which case it may 

 deteriorate. 



Storage reservoirs are of two kinds, impounding and service. 



Impounding reservoirs are the lakes, natural or artificial, that 

 hold the drainage of a watershed. From these the water is con- 

 veyed to service reservoirs. The water as it flows into an impound- 

 ing reservoir is more or less contaminated according to the condition 

 of the ground it has washed. If the area is not heavily manured 

 and is not polluted by human excreta the organic matter is mainly 

 of vegetable origin. Having entered the reservoir, the organic 

 matter in the water settles to the bottom and carries down with 

 it a very large proportion of the bacteria. 



The number of bacteria is also reduced by the lack of food, 

 and they themselves act as food for the infusoria present in the 



