130 
J. HOPKINSON—WATER AND WATER-SUPPLY 
fully compensated for by chalk being more cracked and fissured 
than is sandstone. It is also easily acted on by water, mechanically 
by attrition, and chemically, with the aid of carbonic acid gas, by 
dissolution. 
Absolutely pure water consists of two gases, oxygen and hydro¬ 
gen, chemically combined, a fact first experimentally proved by 
Cavendish in 1782. Exactly one volume of oxygen combines with 
exactly two volumes of hydrogen to form water; but the water 
thus produced equals in volume only about the 2000th part of the 
gases producing it, or, more exactly, one volume of water is the 
product of 655 volumes of oxygen and 1310 volumes of hydrogen. 
As oxygen is sixteen times, or, again to be precise, 15*96 times 
heavier than hydrogen (the lightest element known), it follows 
that nine parts by weight of water are composed of about eight 
parts of oxygen and one part of hydrogen. Air, on the other hand, 
is only a mechanical mixture of oxygen and nitrogen, in the pro¬ 
portion of one volume of oxygen to nearly four of nitrogen, with 
a small portion of carbonic acid gas. These gases, not being in 
chemical combination, vary in their proportions, and the oxygen of 
the air being more soluble in water than the nitrogen, rain in falling 
carries with it the greater amount of oxygen, so that the air always 
contained in rain-water has little more than two volumes of nitrogen 
to one of oxygen. Carbonic acid gas is more soluble in water than 
is either oxygen or nitrogen; but it exists in such a small proportion 
in the air,—about 0*04 per cent.,—that there can be but very little 
in fresh rain-water. In falling on the surface of the ground, how¬ 
ever, the rain removes and carries with it much decaying animal 
and vegetable matter, which is rich in carbon, in fact consists almost 
entirely of organic carbon and water. The free oxygen in the water, 
that is, the surplus oxygen dissolved from the air, combines with 
this carbonaceous matter, forming carbon dioxide (carbonic acid gas), 
which is a chemical compound of carbon and oxygen in the propor¬ 
tion of one equivalent of carbon to two equivalents of oxygen, as 
its name implies. This oxidation or slow burning of animal and 
vegetable matter is a chemical process which purifies the water 
by depriving it of organic carbon, and, as a flowing river, especially 
where it is rapid, is constantly dissolving out of the air a greater 
proportionate amount of oxygen than of nitrogen, the longer the 
run of the river, the less organic carbon and the more carbon dioxide 
will its water contain, provided that there be no further access of 
decaying animal or vegetable matter. While, however, dead organic 
matter is thus rendered innocuous, this cannot be said of living 
matter, for we have no more reason to think that any length of 
run in a river will destroy the living germs of disease than that 
it will destroy aquatic plants or fish. 
It is due to the presence of carbon dioxide, thus produced from 
the union of the oxygen of the air and the carbon of organic matter, 
that water percolating either directly into the chalk, or first 
through a porous bed of either gravel or sand, has the power of 
dissolving the chalk through which it passes. Chalk is a carbonate 
