ken and cemented
together again (to use a crude but effective image) without accurately
fitting the parts, so that there is a residual strain all round the
place." (Larmor, loc. cit.) Thus he explains in quasi-mechanical terms
the properties of an electron. But whether we remain content for the
time with our identification of matter and electricity, or attempt to
express both of them in terms of hypothetical aether, we have made a
great step in advance on the view that matter is made up of chemical
atoms fundamentally distinct and eternally isolated.

Such was the position when the phenomena of radio-activity threw a new
light on the problem, and, for the first time in the history of science,
gave definite experimental evidence of the transmutation of matter from
one chemical element to another.

In 1896 H. Becquerel discovered that compounds of the metal uranium
continually emitted rays capable of penetrating opaque screens and
affecting photographic plates. Like cathode and Rontgen rays, the
rays from uranium make the air through which they pass a conductor
of electricity, and this property gives the most convenient method of
detecting the rays and of measuring their intensity. An electroscope may
be made of a strip of gold-leaf attached to an insulated brass plate
and confined in a brass vessel with glass windows. When the gold-leaf is
electrified, it is repelled from the similarly electrified brass plate,
and the angle at which it stands out measures the electrification. Such
a system, if well insulated, holds its charge for hours, the leakage
of electricity through the air being very slow. But, if radio-active
radiation reach the air within, the gold-leaf falls, and the rate of
its fall, as watched through a microscope with a scale in the eye-piece,
measures the intensity of the radiation. With some form of this simple
instrument, or with the more complicated quadrant electrometer, most
radio-active measurements have been made.

It was soon discovered that the activity of uranium compounds
was proportional to the amount of uranium present in them. Thus
radio-activity is an atomic property dependent on the amount of an
element and independent of its state of chemical combination.

In a search for radio-activity in different minerals, M. and Mme Curie
found a greater effect in pitch-blende than its contents of uranium
warranted, and, led by the radio-active property alone, they succeeded,
by a long series of chem