We have seen that pure water is perfectly clear and transparent. The
naked eye can discern no difference between one part and another. In
other words, it has no visible texture or =structure=. It does not
follow that it really possesses none, however, for there are many things
which seem to be the same throughout, or =homogeneous=, which yet show
structure if they are examined with a magnifying glass. Thus the surface
of a sheet of fine white paper looks perfectly even and smooth to the
eye; but a magnifying glass of no great power will show the minute woody
fibres of which it is made up; while, under a powerful microscope, the
paper looks like a coarse matting.
But if we put a small drop of water on a slide, such as is used for
microscopic objects, and cover it over with a thin glass so as to spread
it out into a film, perhaps not more than 1/10000th of an inch thick, it
may be examined with the very highest magnifying powers we can command,
and yet it looks as completely homogeneous and shows as little evidence
of being made up of separate parts as before. However, this is still no
proof that the water is not made up of little parts, or particles,
distinctly separated from one another. It may merely mean that the
particles are so extremely small that they cannot be distinguished even
by microscopes which magnify four or five thousand diameters.
It is certain that solid bodies may be divided into particles so minute
that the best microscopes show no trace of them. Common gum-mastic
cannot be dissolved by water, but it readily dissolves in strong spirit
or alcohol, and mastic varnish is an alcoholic solution of gum-mastic.
If you add water to mastic varnish, the alcohol takes away the water and
the mastic falls out, or =precipitates=, as a curdy solid composed of
very visible whitish particles. But if a drop of the varnish is added to
a good deal, say half a pint, of water and well stirred at the same
time, the mastic, though it is still precipitated as a solid, is in a
state of extremely minute division. No separate solid particles of
mastic are visible to the naked eye, but the water assumes a faint milky
tinge.
This milkiness arises from the presence of solid particles of mastic
diffused through the water; and yet, if the experiment has been properly
managed, a drop of the fluid may be spread out as before and examined
with the highest magnifying powers, and nothing can be seen of such
particles. So far as vision goes it might be a drop of pure water. Now
our best microscopes are able to show us anything solid which has a
diameter of 1/100000th of an inch, quite distinctly; and probably solid
opaque particles of much smaller size would make themselves apparent as
a turbidity or cloudiness. The particles of mastic must be therefore so
much smaller than this that they remain invisible. Hence it follows that
if water were made up of separate particles, or droplets, 1/1000000th of
an inch in diameter, and thus had the structure of a mass of very fine
shot, no microscope that has yet been constructed would enable us to see
even a trace of that structure. We could not obtain any direct evidence
of it.