The same reasons which lead to the adoption of the hypothesis that water
is composed of separate particles justify its extension to all forms of
matter whatever.
The metal =mercury= or =quicksilver=, for instance, may be supposed to
be made up of distinct particles of mercury of extreme minuteness, and
according to the temperature, these associate themselves in the solid
(frozen mercury), liquid (ordinary quicksilver), or gaseous form (vapour
of mercury). To whatever treatment pure mercury may be subjected, we
cannot get anything but mercury out of it. The particles of mercury have
never been broken up. Hence they are generally termed =atoms=, or
particles that cannot be divided; and mercury is said to be an
=element=, or a substance which is not compounded of any other
substances.
Here is a case in which it is very useful to distinguish between fact
and hypothesis. The matter of fact is that, up to the present time, no
one has been able to get out of pure mercury anything but pure mercury.
The statement that mercury is a simple substance, and therefore never
can be broken up into any other substances, is a hypothesis which future
observation and experiment may or may not confirm.
A hundred and fifty years ago it was universally believed that water was
as much an element as mercury. But water is now well known to be a
compound. In fact, as has already been said, the particles of water may
be very readily broken up or =decomposed= (in what way, you will learn
when you study chemistry) into two totally distinct substances, =oxygen=
and =hydrogen=, which are gaseous at all known temperatures, though by
combining vast pressure with extreme cold they have recently been
liquefied. Each of these gases, according to our hypothesis, consists of
particles, and since these can by no known means be further broken up,
they are considered to be =atoms= like those of mercury.
Nine parts by weight of pure water always yield eight of oxygen and one
of hydrogen. The hypothetical particle, or molecule of water, therefore,
must be composed of atoms of oxygen and hydrogen having this relative
weight; and chemists have grounds for believing that one atom of oxygen
and two atoms of hydrogen exist in each molecule of water. If this be
so, the structure of water must be more complicated than we thought at
first; and each particle of water (the molecule) must be a system
composed of three separate atoms.
50. =Elementary Bodies are neither destroyed nor is their Quantity
increased in Nature.=
It has been seen that when a cubic inch of water is dissipated by heat,
it is not destroyed, but that it merely changes its form from the fluid
to the gaseous state, while its weight remains unaltered. If the same
cubic inch of water is decomposed into oxygen and hydrogen gases, the
water is indeed destroyed, but the matter of which it consisted remains
unchanged in weight. If the water weighed 252·5 grains, the oxygen gas
will weigh 224·45 grains and the hydrogen gas will weigh 28·05 grains.
And nothing that man has been able to do has affected the weight of a
given quantity of either of these gases. So far as we know, elementary
bodies retain their weight under all circumstances, and can be traced by
it whatever shape they may take. If this is true it follows that, in the
order of nature, matter is =indestructible=: the quantity of it neither
increases nor diminishes.
Hence it follows that natural things and artificial things resemble one
another in one respect. It is true of both that the matter of which they
are composed is never destroyed and never increased; and therefore the
order of events in nature as much consists in the joining together and
putting apart of natural bodies by natural agencies, as the order of
events in the artificial world consists in the joining together and the
putting apart of natural bodies by human agencies.