No. 4.] STABLE MANAGEMENT. 177 



to drinking water with positive advantage. But, on the 

 other hand, chlorine derived from the chloride of sodium 

 of urine and fa?ces would he a very serious thing. We 

 determine the chlorine, therefore, to see whether urine or 

 other organic matter has obtained entry into the drinking 

 water; we know the normal amount of chlorine which 

 should exist, and all over and above this must either be 

 derived from organic matter or from a salt stratum. 



It must not be inferred that the amount of contamination 

 of a stream, as indicated by the excess of chlorine over the 

 normal of the region, is necessarily the result of direct 

 access of sewage to the stream. The amount of chlorine 

 in a stream above the normal is in direct proportion to the 

 population on the area on which it is drained, whether the 

 region has sewers or not ; provided, of course, that the 

 sewage is not carried outside the drainage area. But with 

 regard to the organic matters the case is very different. 

 When sewage flows directly into a stream, there is direct 

 pollution of the water by decomposing organic matter ; 

 whereas, when house drainage reaches the stream, after 

 filtering through porous earth, the organic matter may have 

 been entirely oxidized, and the water be purer, organically, 

 than the stream into which it flows. But the chlorine in 

 the waste waters suffers no change in filtering through the 

 earth, and hence this evidence of "previous pollution" 

 remains to tell the origin of the water. 



Nitrogen as Nitrates and Nitrites. 



The oxidation of ammonia which goes on in natural 

 waters under the influence of micro-organisms results in 

 the conversion of all of the hydrogen into water and 

 the nitrogen into nitric acid, which combines with soda, 

 potash or lime in tin' water. Intermediate between the 

 ammonia and nitric acid is nitrous acid, a lower state of 

 oxidation, through which (presumably) the nitrogen must 

 pass before it attains its highest stage of oxidation. The 

 combinations of nitrons acid with bases are called nitrites ; 

 those of nitric acid with bases, nitrates. 



High free ammonia and high nitrites together are charac- 

 teristic of recent pollution, and when they arc uniformly 



