1866. ] with the supply of water to Calcutta. 215 
water proper, the mean indeed being nearly as much as the maximum 
of the Hooghly before tidal influence begins. The amount of variation 
is much smaller in these waters: of the Thames water the highest 
quantity of solid matter was 32.62 grs. the lowest 18.78, and the well 
waters vary less. And it was observed that the quantity of solid 
matter tended to increase after heavy rain fall. 
Other river waters contain more or less of such constituents, dependent 
on the nature of the rocks and soil they traverse. Mr. Sterry Hunt 
gives an analysis of the Ottawa water, taken before the melting of the 
snows, containing 6.12 ers. solid in 100,000. Bischoff, in his Chemi- 
cal Geology, gives a pretty large list of analyses of river waters, show- 
ing a variation of from 2.61 to 54.5 grains solid matter in 100,000. 
The nature of their mineral constituents also varies greatly, but that 
will not engage our attention at present as it is more a geological 
question than a sanitary one. We shall proceed to the point more 
immediately connected with the object of the paper. 
The substances treated of can scarcely be called impurities with 
reference to natural waters. They are rather constituents, and are 
only to be considered impurities in a sanitary point of view when 
they are excessive in quantity, as for instance exceeding 40 or even 
50 grains in 100,000. The remaining substances to be noticed may 
in a purely chemical point of view be called constituents also with 
quite as much truth, but with reference to sanitary considerations 
may with propriety be termed impurities. They were enumerated 
before as organic matter, ammonia and nitric acid. It may be better 
to consider them as organic matter of vegetable origin and organic 
matter of animal origin, with the respective products of their decom- 
position. 
Vegetable substances of all kinds mixed with the soil, exposed to 
air and moisture or immersed in water, dead animal bodies of every 
variety in similar circumstances, all rotting, fermenting and putrefying, 
with the excrementitious matters from living animals, constitute the 
materials from which river water derives that portion of its consti- 
tuents called organic matter. Its nature is so heterogeneous and its 
quantity so small, that it would be hopeless to attempt to separate it 
into its proximate constituents. All we can attempt is to get some 
general idea of its nature, from which to form some judgment of its 
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