34 VETERINARY HYGIENE 



throw down all the calcium and magnesium salts, so that this method 

 is not strictly correct. 



Interpretation of the Results of Analysis. The interpretation 

 of the results obtained from an examination of a water is most 

 difficult, and can only be done correctly by a water analyst of con- 

 siderable experience. Hereunder is given a brief resume of the 

 indications that lead to the formation of an opinion as to the fitness 

 or otherwise of water for drinking purposes. 



Appearance. The absence of colour is no indication of purity. 

 A yellow or green tint would lead one to suspect that the water 

 is polluted, but confirmation of this should be sought in the chemical 

 or bacteriological examination. Turbidity is always suspicious 

 of surface water, and especially if this occurs after heavy rain. 



Odour. With the exception of peaty water and water contain- 

 ing sulphuretted hydrogen, water should have no smell. Absence 

 of smell is no sign of purity. 



Reaction. Acid waters act on metals. This is of special im- 

 portance where lead pipes are used. 



Residue left on Evaporation. A small amount of white residue 

 which shows little or no change on ignition indicates a water free 

 from organic impurity. If the residue has a brown tint and chars 

 on ignition, the indication is that organic matter is present. If 

 a disagreeable odour is given off when the residue is strongly heated 

 to ignition, organic matter of animal origin is to be suspected. 

 Waters containing much nitrates with organic matter may not char, 

 while the residue of peaty water does char. 



Chlorides. Chlorides are found in all waters, chiefly as sodium 

 chloride, with occasionally magnesium, potassium and calcium 

 chloride. In practice the chlorine is estimated and either 

 expressed as so many parts of chlorine per 100,000 or, on the 

 assumption that all the chlorine is derived from sodium chloride 

 (the remainder being so small), by multiplying the chlorine by 1 65 

 and stating it as grains of sodium chloride. Any amount of 

 chlorine by itself has no hygienic significance except that much 

 excess of salt would make it unsuitable for drinking or washing 

 purposes. By itself it is no indication of pollution either past or. 

 recent, but if two wells sunk in the same district and penetrating 

 similar strata yield waters differing markedly in chlorine content, 

 then it may be assumed that the one with the high content is polluted 

 with sewage or manurial matter, the chlorine being derived from 

 the urine. Waters containing chlorine and nitrates in large 

 quantity with much organic matter have almost certainly been 

 polluted with organic matter of animal origin, and that at no distant 



