_Saline Ammonia_ in water is commonly of animal origin, ammonia (NH₃) being one of the first products of decomposition of nitrogenous animal refuse. Upland surface water usually contains about ·002 parts per 100,000, but it may reach ·008 or more if the land over which the water passes has been manured. Shallow well-water may be free from ammonia, or this may be very excessive in amount. Deep well-water may contain no ammonia or any amount up to ·1 per 100,000. Its presence is suspicious if the albuminoid ammonia is above a trace, or if the oxygen absorbed is appreciable in amount. Generally water is suspicious if saline ammonia is up to ·01 per 100,000. _Albuminoid Ammonia_ indicates the amount of organic nitrogenous matter present in the water. It should not exceed ·005 parts per 100,000, while at the same time the saline ammonia should not usually exceed ·01 per 100,000. For _Oxygen consumed_ the following table of the weight of oxygen required for 100,000 parts of water is given by Clowes and Coleman:— ┌────────────────────┬─────────────────────┬─────────────────────────┐ │ │UPLAND SURFACE WATER.│WATER FROM OTHER SOURCES.│ │ _Water of_ ├─────────────────────┼─────────────────────────┤ │ _Great purity_ │Not exceeding ·1 │Not exceeding ·05 │ │ _Medium purity_ │From ·1 to ·3 │From ·05 to ·15 │ │ _Doubtful purity_│From ·3 to ·4 │From ·15 to ·20 │ │ _Impure_ │Exceeding ·4 │Exceeding ·20 │ └────────────────────┴─────────────────────┴─────────────────────────┘ The presence of more than 1 and still more so of 2 grains of _Chlorine_ per 100,000 of water is most suspicious, except in saline districts. _Nitrites_ if present in an appreciable quantity indicate comparatively recent contamination by sewage. In deep well-water they may be produced by deoxidation of nitrates. _Nitrates_ in upland surface waters should not be equivalent to more than ·03 of N. per 100,000; in shallow well-waters the amount varies greatly; in deep well-waters it may be excessive. As a rule it ought not to be equivalent to more than 5 parts of N. per 100,000 of water; but the significance of nitrates depends greatly on the source of the water and on the amount of the other constituents present. Chemical analysis alone cannot ascertain the safety of a given drinking water. A minute amount of impurity inappreciable to analysis may be competent to produce disease; while another water may be drunk with impunity, which contains considerable organic matter. Chemical analysis “can tell us of impurity and hazard, but not of purity or safety” (Buchanan). An accurate opinion as to the character of a drinking water can only be expressed when one knows the amount of each chief constituent (as above), and whether these amounts deviate from the same water at other times or from other waters in the vicinity. CHAPTER XII.