elements, and at last the chemist found that he had seventy or
eighty of these "ultimate realities," each having its own very
definite and very different characters. As it is the experience
of science to find unity underlying variety, this was profoundly
unsatisfactory, and the search began for the great unity which
underlay the atoms of matter. The difficulty of the search may be
illustrated by a few figures. Very delicate methods were invented
for calculating the size of the atoms. Laymen are apt to
smile--it is a very foolish smile--at these figures, but it is
enough to say that the independent and even more delicate methods
suggested by recent progress in physics have quite confirmed
them.

Take a cubic millimetre of hydrogen. As a millimetre is less than
1/25th of an inch, the reader must imagine a tiny bubble of gas
that would fit comfortably inside the letter "o" as it is printed
here. The various refined methods of the modern physicist show
that there are 40,000 billion molecules (each consisting of two
atoms of the gas) in this tiny bubble. It is a little universe,
repeating on an infinitesimal scale the numbers and energies of
the stellar universe. These molecules are not packed together,
moreover, but are separated from each other by spaces which are
enormous in proportion to the size of the atoms. Through these
empty spaces the atoms dash at an average speed of more than a
thousand miles an hour, each passing something like 6,000,000,000
of its neighbours in the course of every second. Yet this
particle of gas is a thinly populated world in comparison with a
particle of metal. Take a cubic centimetre of copper. In that
very small square of solid matter (each side of the cube
measuring a little more than a third of an inch) there are about