In order to learn the manner in which water may be broken up into its
elements or decomposed, you must turn to the Primer on Chemistry. But as
a preliminary to the study of that science, it may be useful to consider
some simple cases of composition and decomposition which are exemplified
by water.
If half a pint of water, coloured by putting a little ink into it, is
added to the same quantity of clean water, the two will readily mingle;
the total quantity of water will be a pint; and its colour will be just
half as dark as that of the coloured half-pint. This is a case of simple
=mixture=. The volume of the mixture equals the sum of the volumes of
the things mixed, and there is no change in the properties of these
things. So when water evaporates, the gaseous water or vapour mixes with
the air in the same way, the molecules of the one body dispersing
themselves between the molecules of the other until there is the same
proportion of each everywhere. In like manner, sand and sugar may be
(and unfortunately often are) mixed, without any change in the
properties of either, or in the space which they primitively occupied.
On the other hand, oil and water will not mix, however much you may stir
the two together; and the oil, being the lighter, rises to the top as
soon as the fluid is quiet. Nor will quicksilver and water mix, but the
quicksilver, being very much heavier than the water, rushes to the
bottom of the vessel into which the two are put. Neither will sand nor
iron filings mix with water; as heavier bodies, they also sink to the
bottom. Nor does powdered ice, though it is water in another shape, mix
with ice cold water; as a lighter body it floats at the top.