Natural Waters. 73 



who drink them. The presence of calcium sulphate renders wa- 

 ter incapable of this dangerous action upon the lead. In the 

 presence of calcium sulphate the metal becomes coated with a 

 film of the very insoluble lead sulphate, which protects it from 

 further contact with the water. 



Organic matter. Of greater importance than the mineral mat- 

 ter in drinking water, is the amount and nature of the organic 

 matter. This in itself is comparatively harmless. Its import- 

 ance lies in the influence it may have upon the kinds of micro- 

 organisms which accompany it. Animal excreta is the most dan- 

 gerous contamination, since the micro-organisms which cause 

 various diseases, as for example, typhoid, cholera, etc., are liable 

 to be thus introduced into the water. Animal organic matter is 

 richer in nitrogen than most vegetable refuse, so that in practice 

 the detection of much combined nitrogen, whether as organic 

 matter, ammonium salts, or nitrates, is regarded as sufficient to 

 indicate that the water has been contaminated with sewage or 

 other animal matter. If much organic matter of animal origin 

 be present there must always be considerable risk of disease pro- 

 ducing organisms finding their way into the bodies of those who 

 drink it ; and though such contaminated water may be, and often 

 is, drunk for years with impunity, its consumption is decidedly 

 dangerous. 



Another substance characteristic of sewage is common salt; 

 consequently the presence of much chlorine in a water is gen- 

 erally indicative of sewage contamination, unless the water is 

 derived from some rock which contains salt, or is collected near 

 the sea. 



What has been said has an important bearing upon the loca- 

 tion of the farm wells. Dangers of seepage from the out door 

 privy and the barn-yard must be avoided by locating the well 

 at a proper distance from both and on higher ground. Even 

 these precautions may not always entirely remove the danger of 

 contamination. 



