WATER, AND ITS IMPURITIES. 163 
If all the organic matter present in some waters were to be 
removed by the ordinary processes of putrefaction, decomposi- 
tion, and the formation of offensive and deleterious gases and 
compounds, incalculable mischief would constantly ensue. To 
obviate this, nature has ordained that some of the organic 
matter of impure water, in place of undergoing decomposition, 
should be imbibed by other and living forms, and, these dying, 
that other generations should take their place, and fulfil a simi- 
lar important office. 
In the deep well, where the conditions necessary to animal 
life do not exist, the same end is accomplished by the formation, 
out of the nitrogen of the organic matter, of nitrates, to which 
we have already referred. 
The purposes fulfilled by living vegetable and animal pro- 
ductions in water are, then, of an eminently useful and preserva- 
tive character ; and this observation is true particularly of the 
vegetable productions contained in it, including both the lower 
forms, as the confervse and other algae, and the higher plants, 
as the flowering ; for these not only remove the organic matter 
dissolved in water, by absorbing it into their own tissues for 
appropriation, but they still further purify water by the effect 
of their respiration. 
Animals, during respiration, exhale a large amount of car- 
bonic acid ; vegetables absorb this, fix the carbon in their own 
tissues, and restore the oxygen to the air, if the plant be aerial, 
and to the water, if it be submerged and aquatic. 
Inorganic Impurities. 
Water is a substance formed by the chemical combination of 
two gases, oxygen, and hydrogen, in certain proportions : these 
alone being necessary to its constitution, all other inorganic 
matters contained in it are to be regarded as unnecessary, ex- 
traneous, and frequently injurious additions. 
Under the head “ Inorganic Impurities,” therefore, we in- 
clude, first, all those inorganic substances which are merely 
suspended in the water, as sand and grit; and, second, those 
substances which are in a state of actual solution, as the saline, 
earthy, and metallic salts, present in most waters, and the gases. 
Inorganic Impurities in Suspension. 
The inorganic impurities which are merely suspended in the 
water are the least important of all, since they do not impart 
any deleterious properties ; they consist chiefly of particles of a 
stony nature, which are therefore heavier than water, and are 
readily separated, either by subsidence or filtration. 
