The exploration of this wonder-world of atoms and molecules by the
physicists and chemists of to-day is one of the most impressive triumphs
of modern science. Quite apart from radium and electrons and other
sensational discoveries of recent years, the study of ordinary matter is
hardly inferior, either in interest or audacity, to the work of the
astronomer. And there is the same foundation in both cases--marvellous
apparatus, and trains of mathematical reasoning that would have
astonished Euclid or Archimedes. Extraordinary, therefore, as are some
of the facts and figures we are now going to give in connection with the
minuteness of atoms and molecules, let us bear in mind that we owe them
to the most solid and severe processes of human thought.
Yet the principle can in most cases be made so clear that the reader
will not be asked to take much on trust. It is, for instance, a matter
of common knowledge that gold is soft enough to be beaten into gold
leaf. It is a matter of common sense, one hopes, that if you beat a
measured cube of gold into a leaf six inches square, the mathematician
can tell the thickness of that leaf without measuring it. As a matter of
fact, a single grain of gold has been beaten into a leaf seventy-five
inches square. Now the mathematician can easily find that when a single
grain of gold is beaten out to that size, the leaf must be 1/367,000 of
an inch thick, or about a thousand times thinner than the paper on
which these words are printed; yet the leaf must be several molecules
thick.
The finest gold leaf is, in fact, too thick for our purpose, and we turn
with a new interest to that toy of our boyhood the soap-bubble. If you
carefully examine one of these delicate films of soapy water, you notice
certain dark spots or patches on them. These are their thinnest parts,
and by two quite independent methods--one using electricity and the
other light--we have found that at these spots the bubble is less than
the three-millionth of an inch thick! But the molecules in the film
cling together so firmly that they must be at least twenty or thirty
deep in the thinnest part. A molecule, therefore, must be far less than
the three-millionth of an inch thick.
We found next that a film of oil on the surface of water may be even
thinner than a soap-bubble. Professor Perrin, the great French authority
on atoms, got films of oil down to the fifty-millionth of an inch in
thickness! He poured a measured drop of oil upon water. Then he found
the exact limits of the area of the oil-sheet by blowing upon the water
a fine powder which spread to the edge of the film and clearly outlined
it. The rest is safe and simple calculation, as in the case of the
beaten grain of gold. Now this film of oil must have been at least two
molecules deep, so a single molecule of oil is considerably less than a
hundred-millionth of an inch in diameter.
Innumerable methods have been tried, and the result is always the same.
A single grain of indigo, for instance, will colour a ton of water. This
obviously means that the grain contains billions of molecules which
spread through the water. A grain of musk will scent a room--pour
molecules into every part of it--for several years, yet not lose
one-millionth of its mass in a year. There are a hundred ways of showing
the minuteness of the ultimate particles of matter, and some of these
enable us to give definite figures. On a careful comparison of the best
methods we can say that the average molecule of matter is less than
the 1/125,000,000 of an inch in diameter. In a single cubic centimetre
of air--a globule about the size of a small marble--there are thirty
million trillion molecules. And since the molecule is, as we saw, a
group or cluster of atoms, the atom itself is smaller. Atoms, for
reasons which we shall see later, differ very greatly from each other in
size and weight. It is enough to say that some of them are so small that
it would take 400,000,000 of them, in a line, to cover an inch of space;
and that it takes at least a quintillion atoms of gold to weigh a single
gramme. Five million atoms of helium could be placed in a line across
the diameter of a full stop.
[Illustration: An atom is the smallest particle of a chemical element.
Two or more atoms come together to form a molecule: thus molecules form
the mass of matter. A molecule of water is made up of two atoms of
hydrogen and one atom of oxygen. Molecules of different substances,
therefore, are of different sizes according to the number and kind of
the particular atoms of which they are composed. A starch molecule
contains no less than 25,000 atoms.
Molecules, of course, are invisible. The above diagram illustrates the
_comparative_ sizes of molecules.]
[Illustration: INCONCEIVABLE NUMBERS AND INCONCEIVABLY SMALL PARTICLES
The molecules, which are inconceivably small, are, on the other hand, so
numerous that if one was able to place, end to end, all those contained
in, for example, a cubic centimetre of gas (less than a fifteenth of a
cubic inch), one would obtain a line capable of passing two hundred
times round the earth.]
[Illustration: WHAT IS A MILLION?
In dealing with the infinitely small, it is difficult to apprehend the
vast figures with which scientists confront us. A million is one
thousand thousand. We may realise what this implies if we consider that
a clock, beating seconds, takes approximately 278 hours (i.e. one week
four days fourteen hours) to tick one million times. A billion is one
million million. To tick a billion the clock would tick for over 31,735
years.
(In France and America a thousand millions is called a billion.)]
[Illustration: THE BROWNIAN MOVEMENT
A diagram, constructed from actual observations, showing the erratic
paths pursued by very fine particles suspended in a liquid, when
bombarded by the molecules of the liquid. This movement is called the
Brownian movement, and it furnishes a striking illustration of the truth
of the theory that the molecules of a body are in a state of continual
motion.]