e 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 31,73