in the luminiferous aether, though the
possibility of explaining them as flights of non-electrified particles
is before the minds of some physicists.

Still another kind of radiation has been discovered more recently by
Thomson, who has found that in high vacua, rays become apparent which
are absorbed at once by air at any ordinary pressure.

The emission of all these different types of radiation involves a
continual drain of energy from the radio-active body. When M. and Mme
Curie had prepared as much as a gramme of radium chloride, the energy of
the radiation became apparent as an evolution of heat. The radium salt
itself, and the case containing it, absorbed the major part of the
radiation, and were thus maintained at a temperature measurably higher
than that of the surroundings. The rate of thermal evolution was such
that it appeared that one gramme of pure radium must emit about 100
gramme-calories of heat in an hour. This observation, naturally as it
follows from the phenomena previously discovered, first called attention
to the question of the source of the energy which maintains indefinitely
and without apparent diminution the wonderful stream of radiation
proceeding from a radio-active substance. In the solution of this
problem lies the point of the present essay.

In order to appreciate the evidence which bears on the question we must
now describe two other series of phenomena.

It is a remarkable fact that the intensity of the radiation from
a radio-active body is independent of the external conditions of
temperature, pressure, etc. which modify so profoundly almost all other
physical and chemical processes. Exposure to the extreme cold of liquid
air, or to the great heat of a furnace, leaves the radio-activity of a
substance unchanged, apparent exceptions to this statement having been
traced to secondary causes.

Then, it is found that radio-activity is always accompanied by some
chemical change; a new substance always appears as the parent substance
emits these radiations. Thus by chemical reactions it is possible to
separate from uranium and thorium minute quantities of radio-active
materials to which the names of uranium-X and thorium-X have been given.
These bodies behave differently from their parents uranium and thorium,
and show all the signs of distinct chemical individuality. They are
strongly radio-active, while, after the separation, the parents uranium
and thorium are found to have lost some