EXAMINATION OF WATER. 423 



such as mentioned, are extremely rare. On the other hand, 

 infections through impure water are unfortunately but too 

 common. By infection is meant the introduction into the 

 body of a specific, pathogenic, micro-organism. When im- 

 pure water is spoken of, it does not follow that the water 

 is unsightly, malodorous or repulsive to the taste. The 

 water may be perfectly clear, sparkling in character and a 

 chemical examination may show that it contains minimal 

 amounts of organic and inorganic constituents, and yet, that 

 water may be impure because of the presence of pathogenic 

 bacteria. 



A chemical examination will not detect the presence of 

 bacteria, much less assist in their identification. It can, 

 however, detect the presence of certain chemical substances 

 from the relative amounts of which an inference may be 

 drawn as to the existence of pollution with human or 

 animal excreta. A pollution of this kind once established 

 indicates that danger from infection may be expected when- 

 ever the pathogenic germ is actually present in such excreta. 

 From a chemical standpoint a good water should not con- 

 tain more of the several constituents, in parts per million 

 or mg. per liter, than what is shown in the following table: 



Total residue 500 Nitric anhydride 5-15 



Earthy bases *. . . 180-200 Nitrous acid traces 



Sulphuric anhydride .. 80-100 Albuminoid ammonia. 0.20 



Chlorine 20-30 



Moreover, it should not consume more than 8 to 10 parts, 

 per liter, of potassium permanganate. 



It should not be understood from these requirements 

 that a water containing an excess of these constituents 

 above the limits given will necessarily be dangerous. The 

 chlorine, nitrates, nitrites and ammonia are in themselves- 

 harmless and can be taken with impurity in relatively 

 large doses. The data given, however, are supposed to 

 represent the average composition of good water. An 



