14 VETEEINAEY HYGIENE 



because the bodies are in themselves harmful, but because 

 they indicate the existence of nitrogenous organic matter. 

 This organic matter may be due to a minute portion of 

 enteric stool, or a piece of decomposing leaf. Which it is 

 the chemist certainly cannot determine, all he can do is 

 to draw attention to the existence of free and albuminoid 

 ammonia, point out that they exist in far larger amounts 

 than should be found in good water, and hazard a guess 

 from the remainder of the analysis as to their origin. 



Chlorine, in the same way, is used solely as an index of 

 the organic matter which may have obtained access to the 

 water. The amount of chlorides in a good drinking water 

 is known, and anything found over and above that, assum- 

 ing there is no question of the water being brackish, is 

 believed to be due to animal contamination, for example 

 urine. 



Chlorine, sulphuric, carbonic, phosphoric, nitric and 

 nitrous acids, combined with lime, magnesia, soda, potash, 

 ammonia, iron, and alumina are not isolated by the chemist 

 because in themselves they are harmful in water, for all 

 these might be added to drinking water in the proportion 

 found in London sewage without being in the least harmful ; 

 but they are used as an index of the purity of the water, 

 the assumption being that compounds of the above acids 

 and elements are only found in pure water to a well 

 ascertained amount, and anything over and above must 

 have been introduced from without and is an impurity. 



The examination of a sample of water consists of a 

 physical, microscopical, bacteriological and chemical inquiry. 



In the Physical Examination is determined the colour 

 clearness, sediment, lustre, taste, and smell of the water! 

 The colour and clearness of a water are obtained by putting 

 the sample in a tall glass vessel placed upon a piece of 

 white paper, and then looking down through the depth of 

 water which should, if possible, be not less than 18 inches. 

 Any marked colour by this means is readily detected" 

 Water should have a bluish tinge ; yellowish is perhaps the 

 most common, and is probably due to fine sand or clay. 



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