160 WATER, AND ITS IMPURITIES. 
in the amount of nitrogen which they contain, and in their 
subsistence on decomposing animal matter. 
But other differences may be observed between acid and 
alkaline animal solutions: while the animal matter contained in 
the alkaline fluids quickly undergoes putrefactive changes, and 
begins to smell disagreeably, that in the acid fluid scarcely 
alters at all, and emits no unpleasant odour. 
From these facts, it may be fairly concluded that these two 
waters stand in very different relations to each other in a sani- 
tary point of view, and from them important practical applica- 
tions may be deduced. 
But there are occasionally found in nature, waters which, 
although largely impregnated with dead organic matter, yet 
contain neither animal nor vegetable productions in a living 
state; these are generally well waters, contaminated by sewage 
or the contents of cesspools. The reason of the absence of 
life in these cases it is not difficult to divine : from the water 
in the deep and covered well, air, light, and warmth are ex- 
cluded, conditions essential to development. 
The kinds of vegetable and animal life present in water vary 
much, according to their nature and chemical composition. 
In the still waters of ponds, cisterns, and reservoirs, which 
contain dead vegetable matter, in the spring and summer, which 
is the great season of the development of vegetation, we en- 
counter living vegetable forms, especially confervse. 
In the same waters, holding in solution much animal matter, 
we meet with living animal productions, particularly infusoria 
and entomostracese. 
The forms of life inhabiting the running waters of springs 
and rivers are usually very different from those which dwell in 
still waters ; they belong to different species and genera, and 
are provided with an organization by which they are adapted 
to the very different life they are destined to lead. 
The quantity of animal and vegetable life contained in water 
also varies according to its nature and composition. 
Thus pure distilled water contains no organic matter, either 
dead or alive. 
Rain water, caught at a distance from houses, towns, or 
other sources of contamination, contains none. 
Uncontaminated well water contains no organic matter. 
Neither does pure spring water contain any form or trace of 
organic matter. 
In river water there is present, usually, a considerable but 
variable amount of organic matter; this amount varies according 
to certain well determined circumstances : thus, at the source of 
