, is indestructible, and just as matter exists in
various forms so does energy. And we may add, just as we are ignorant of
what the negative and positive particles of electricity which constitute
matter really are, so we are ignorant of the true nature of energy. At
the same time, energy is not so completely mysterious as it once was. It
is another of nature's mysteries which the advance of modern science has
in some measure unveiled. It was only during the nineteenth century that
energy came to be known as something as distinct and permanent as matter
itself.

Forms of Energy

The existence of various forms of energy had been known, of course, for
ages; there was the energy of a falling stone, the energy produced by
burning wood or coal or any other substance, but the essential
_identity_ of all these forms of energy had not been suspected. The
conception of energy as something which, like matter, was constant in
amount, which could not be created nor destroyed, was one of the great
scientific acquisitions of the past century.

[Illustration: WAVE SHAPES

Wave-motions are often complex. The above illustration shows some fairly
complicated wave shapes. All such wave-motions can be produced by
superposing a number of simple wave forms.]

[Illustration: THE POWER OF A MAGNET

The illustration is that of a "Phoenix" electric magnet lifting scrap
from railway trucks. The magnet is 52 inches in diameter and lifts a
weight of 26 tons. The same type of magnet, 62 inches in diameter, lifts
a weight of 40 tons.]

[Illustration: _Photo: The Locomotive Publishing Co., Ltd._

THE SPEED OF LIGHT

A train travelling at the rate of sixty miles per hour would take rather
more than seventeen and a quarter days to go round the earth at the
equator, i.e. a distance of 25,000 miles. Light, which travels at the
rate of 186,000 miles per second, would take between one-seventh and
one-eighth of a second to go the same distance.]

[Illustration: ROTATING DISC OF SIR ISAAC NEWTON FOR MIXING COLOURS

The Spectroscope sorts out the above seven colours from sunlight (which
is compounded of these seven colours). If painted in proper proportions
on a wheel, as shown in the coloured illustration, and the wheel turned
rapidly on a pivot through its centre, only a dull white will be
perceived. If one colour be omitted, the result will be one colour--the
result of the union of the remaining six.]

It is not possible to enter deeply into