120 Drinking- Water and some of its Impurities. 



we have the oxidizing influence of one mile of porous 

 stratum, which, though much more effective than the same 

 distance in the open air, would be utterly unable to de- 

 stroy the fever germs. 



Mtrates and nitrites are produced by the oxidation of 

 nitrogenous organic matter and almost always from animal 

 matter. They are called in the Registrar General's Re- 

 port the test of ^' previous sewage contamination." They 

 are found in all waters, even in small quantities in rain 

 water, and though not injurious in themselves yet lead us 

 to suspect that the water containing them in solution, to 

 any extent, has at some time been contaminated largely 

 with organic matters. These organic matters have been 

 oxidized and the results are nitrates and nitrites, but it 

 may some time happen that they are not all oxydized, 

 and then we have them existing in a dangerous form, as 

 at Lausen. The chlorine is of considerable importance in 

 judging of the character of a water, as it is usually derived 

 from animal excreta, though in some cases it may come 

 from the soil. It is usually present as sodic chloride, but 

 occasionally as a calcic salt. Of course, attention should 

 be paid to the geological nature of the district from which 

 the water comes, its distance from the sea, etc., in order 

 to decide on the origin of the chlorine. 



Unpolluted river and spring waters contain a little less 

 than one part in 100,000 of chlorine : average sewage in 

 water-closeted towns, about ten parts. Its amount is scarcely 

 affected by any amount of filtration through soil. Thus 

 the water flowing from land irrigated by sewage, contains 

 as much chlorine in proportion as the sewage itself, unless 

 it has been diluted by subsoil water or concentrated by 

 evaporation. Under ordinarj^ circumstances, a water con- 

 taining more than two or three parts of chlorine in 100,000, 

 should be regarded with suspicion. 



When the free ammonia exceeds 8 parts in 100,000 it 



