86 DR. R. ANGUS SMITH ON THE EXAMINATION 



neral waters, will seldom, after all, cause any error with a 

 careful person. 



Nitrates are very common in small quantities. They are 

 found in water from manured land, in gardens, wells near 

 houses, and, as a consequence, in nearly all town wells ; 

 in great abundance near churchyards, if the drainage is 

 direct. They are not necessarily found in sewage which 

 has not flowed through strata. The amount from the 

 atmospheric sources is so minute that it will not interfere 

 with any inquiry regarding the wholesomeness of water at 

 present. 



Although caution must be exercised in drawing con- 

 clusions from the presence of the chlorides, their absence 

 may be held as conclusive against the presence of decom- 

 posed animal matter and excretions of animals in large 

 quantities. 



If chlorides and nitrates are found together in water, we 

 may take it for granted that animal matter has existed 

 there or does exist in the water. Of course a very rigid 

 scientific inquiry shows at once that vegetable matter may 

 be present, especially from grain or seeds ; but practically 

 this need not aflPect the question, especially in a sanitary 

 point of view, because, if this accumulation of vegetable 

 matter occurred capable of giving as much nitrogen as 

 animal matter, it would be sufficiently and perhaps equally 

 hurtful when putrid. 



If chlorides and nitrates are found in water still capable 

 of decomposing chameleon, the presence of animal matter, 

 or injurious gases or nitrites, may be assumed, a part only 

 being oxidized. 



If the chlorides and nitrates are present, but the power of 

 decomposing chameleon absent, then animal matter in a 

 putrefying state is absent; but whether it disappeared 

 minutes or ages before is not shown by chemistry. 



