WATER, AND ITS IMPURITIES. 161 
the river, where the stream is comparatively pure and un- 
polluted, the quantity of organic matter, either dead or living, 
is but small. 
As the stream flows along, however, it becomes exposed to 
numberless sources of contamination; it receives the contents of 
still smaller streams, and the impurities of innumerable ditches, 
the refuse of the dwellings of persons who live near its banks, 
the contents of drains, and very frequently the sewage of whole 
and populous towns, so that at last, when it reaches its outlet 
into the sea, great and manifold are the corruptions and large 
is the amount of dead and living organic matter pervading its 
waters. 
Were it not that for the continually increasing impurities of 
river water there is a daily means of escape by the ocean, that 
description of water would be the most contaminated of all. 
The water of ponds usually contains a large quantity of or- 
ganic matter, because these also are exposed to many sources 
of contamination ; the leaves of trees which grow near fall each 
year into them, and successive crops of aquatic vegetation are 
developed and pass to decay in their waters. 
The water of reservoirs , likewise, usually contains much 
organic matter ; into these, water which is itself very impure 
is poured from time to time ; on each occasion a portion of the 
contained organic matter subsides to the bottom, and thus at 
last a great accumulation of organic impurities occurs, sufficient, 
in the course of a very short time, to spoil and render unfit for 
use even the very best water. 
Lastly, the water of cisterns generally contains much organic 
matter. A cistern may be compared to a reservoir ; it is, in 
fact, a reservoir on a small scale, and in it also organic im- 
purities increase from day to day, both by accumulation and 
growth. 
It should be remembered, that the actual quantity of organic 
matter present in a large body of water, or even in a river, 
cannot be estimated from an examination of samples of water 
obtained by merely dipping a jar or bottle into it, because by 
far the greatest portion of the organic matter is contained in 
the mud or other deposit at the bottom of the reservoir or river. 
This fact has very generally been overlooked, and hence the 
proportion of organic matter given by chemists in many cases 
as present in certain waters is by far too low. 
The effect of the presence of organic matter in the sediment 
of a reservoir or river on the entire body of the water contained 
in it is shewn by placing a little of such sediment in distilled 
water in a bottle; in a short time, especially if the weather be 
warm, the water will begin to smell offensively. 
VOL. XXIV. z 
