000 times in every second by collisions. If we
could stop the molecules of hydrogen gas, and utilise their energy, as
we utilise the energy of steam or the energy of the water at Niagara, we
should find enough in every gramme of gas (about two-thousandths of a
pound) to raise a third of a ton to a height of forty inches.

I have used for comparison the speed of a rifle bullet, and in an
earlier generation people would have thought it impossible even to
estimate this. It is, of course, easy. We put two screens in the path of
the bullet, one near the rifle and the other some distance away. We
connect them electrically and use a fine time-recording machine, and the
bullet itself registers the time it takes to travel from the first to
the second screen.

Now this is very simple and superficial work in comparison with the
system of exact and minute measurements which the physicist and chemist
use. In one of his interesting works Mr. Charles R. Gibson gives a
photograph of two exactly equal pieces of paper in the opposite pans of
a fine balance. A single word has been written in pencil on one of these
papers, and that little scraping of lead has been enough to bring down
the scale! The spectroscope will detect a quantity of matter four
million times smaller even than this; and the electroscope is a million
times still more sensitive than the spectroscope. We have a
heat-measuring instrument, the bolometer, which makes the best
thermometer seem Early Victorian. It records the millionth of a degree
of temperature. It is such instruments, multiplied by the score,
which enable us to do the fine work recorded in these pages.

[Illustration: _Reproduced from "The Forces of Nature" (Messrs.
Macmillan)._

A SOAP BUBBLE

The iridescent colours sometimes seen on a soap bubble, as in the
illustration, may also be seen in very fine sections of crystals, in
glass blown into extremely fine bulbs, on the wings of dragon-flies and
the surface of oily water. The different colours correspond to different
thicknesses of the surface. Part of the light which strikes these thin
coatings is reflected from the upper surface, but another part of the
light penetrates the transparent coating and is reflected from the lower
surface. It is the mixture of these two reflected rays, their
"interference" as it is called, which produces the colours observed. The
"black spots" on a soap bubble are the places where the soapy film is
thinnest. At the black