When our means of observation of any natural fact fail to carry us
beyond a certain point, it is perfectly legitimate, and often extremely
useful, to make a supposition as to what we should see, if we could
carry direct observation a step further. A supposition of this kind is
what is called a =hypothesis=, and the value of any hypothesis depends
upon the extent to which reasoning upon the assumption that it is true,
enables us to explain or account for the phenomena with which it is
concerned.
Thus, if a person is standing close behind you, and you suddenly feel a
blow on your back, you have no direct evidence of the cause of the blow;
and if you two were alone, you could not possibly obtain any; but you
immediately suppose that this person has struck you. Now that is a
hypothesis, and it is a legitimate hypothesis, first, because it
explains the fact; and, secondly, because no other explanation is
probable; probable meaning in accordance with the ordinary course of
nature. If your companion declared that you fancied you felt a blow, or
that some invisible spirit struck you, you would probably decline to
accept his explanation of the fact. You would say that both the
hypotheses by which he professed to explain the phenomenon were
extremely improbable; or in other words, that in the ordinary course of
nature fancies of this kind do not occur, nor spirits strike blows. In
fact, his hypotheses would be illegitimate, and yours would be
legitimate; and, in all probability, you would act upon your own. In
daily life, nine-tenths of our actions are based upon suppositions or
hypotheses, and our success or failure in practical affairs depends upon
the legitimacy of these hypotheses. You believe a man on the hypothesis
that he is always truthful; you give him pecuniary credit on the
hypothesis that he is solvent.
Thus, everybody invents, and, indeed, is compelled to invent, hypotheses
in order to account for phenomena of the cause of which he has no direct
evidence; and they are just as legitimate and necessary in science as in
common life. Only the scientific reasoner must be careful to remember
that which is sometimes forgotten in daily life, that a hypothesis must
be regarded as a means and not as an end; that we may cherish it so long
as it helps us to explain the order of nature; but that we are bound to
throw it away without hesitation as soon as it is shown to be
inconsistent with any part of that order.
48. =The Hypothesis that Water is composed of Separate Particles
(Molecules).=
It has been pointed out that we cannot see, and indeed that there is not
much hope of our ever being able to see, the separate particles of
water, even if water is composed of such particles. But it is perfectly
legitimate to suppose that water is made up of such particles, if that
hypothesis will enable us to explain the properties of water.
Let us suppose then that any portion of fluid water is really composed
of a prodigious number of particles less (and probably much less) than a
millionth of an inch in diameter. We may call these particles
=molecules=.[4]
Footnote 4:
Diminutive of _moles_, a mass.
We are justified, in accordance with the general properties of matter (§
18), in supposing that these molecules tend to approach one another. But
the fact that water is slightly compressible justifies the supposition
that its molecules are not in actual contact, but that they are
separated by interspaces, just as the motes in the air of a dusty room
are so separated.
What is it that keeps the molecules apart? We have seen that great
mechanical pressure brings them but slightly nearer to one another;
hence there is an equivalent resistance of some kind which keeps them
apart. This resistance must have the same origin as the sensation which
we know as heat, for it has been seen that diminution of heat diminishes
the bulk of water; that is, allows the molecules to come closer
together; that is, diminishes their tendency to keep asunder. Increase
of heat, on the other hand, increases the volume of water; that is to
say, drives the molecules further apart, or increases their tendency to
keep asunder.
Suppose we call the cause of the tendency of the molecules of water to
come together an =attractive force=; and the cause of their keeping
apart, which manifests itself to us as the sensation of heat and is, as
we have seen, in all probability, a rapid vibratory or whirling motion
of the molecules, a =repulsive force=; then, in the liquid state, these
forces are so adjusted that the molecules are quite free to move, and
yet hold together.
By adding heat the repulsive force is increased, until the molecules are
about twelve times as far apart as they were in each direction; while
the attractive force is overcome, and the molecules fly off in all
directions as soon as they are unconfined. On the other hand, by taking
heat away, the repulsive force is diminished, until the molecules become
inseparable and the water assumes the solid form.
It is probable that the expansion of fluid water, at a temperature below
39°, depends upon the molecules taking up a peculiar arrangement as they
approach one another. If sixteen men are formed into a column, four
deep, and each man a foot from the other, the same men may stand closer
together and yet form a hollow square, which occupies a larger space.
That the molecules of water do take up a particular order in assuming
the solid condition, is shown by the crystalline form of ice. Each
crystal of hoar-frost owes its shape to the arrangement of its
molecules, according to a definite geometrical pattern.
Thus the hypothesis that water is composed of separate molecules, is
useful, for it helps us to some extent to explain the properties of
water. And, when you study physics and learn the laws of motion, you
will find that there is no end to the number of the truths established
by observation and experiment, which can be explained by this
hypothesis. Hence it may fairly be adopted and employed as a means of
picturing to ourselves the order of nature, so long as no facts are
discovered which are inconsistent with it.