oved. By careful tests it has
been determined that a gram of radium emits about twenty-four hundred
gram-calories in twenty-four hours. On this basis a gram of radium in a
year emits enough energy to dissociate about two hundred and twenty-five
grams of water.

What seems most remarkable about this constant emission of heat by the
radium atom is that it does not apparently draw upon external sources
for it, but maintains it by the internal energy of the atom itself. This
latent energy must be enormous, but is only manifested when the atom
is breaking up. In this process of disruption many of the particles are
thrown off; but the greater part seem to be stopped in their flight in
the radium itself, so that their energy of motion is manifested in the
form of heat. Thus, if this explanation is correct, the temperature of
the radium is maintained above that of surrounding substances by the
bombardment of its own particles. Since the earth and the atmosphere
contain appreciable quantities of radio-active matter, this must play
a very important part in determining the temperature of the globe--so
important a part, indeed, that all former estimates as to the probable
length of time during which the earth and sun will continue to radiate
heat are invalidated. Such estimates, for example, as that of Lord
Kelvin as to the probable heat-giving life of the sun must now be
multiplied from fifty to five hundred times.

In like manner the length of time that the earth has been sufficiently
cool to support animal and vegetable life must be re-estimated. Until
the discovery of radium it seemed definitely determined that the earth
was gradually cooling, and would continue to cool, un til, like the
moon, it would become too cold to support any kind of vegetable or
animal life whatever. But recent estimates of the amount of radio-active
matter in the earth and atmosphere, and the amount of heat constantly
given off from this source, seem to indicate that the loss of heat
is (for the moment) about evenly balanced by the heat given out by
radio-active matter. Thus at the beginning of the new century we see
the phenomenon of a single discovery in science completely overturning
certain carefully worked out calculations, although not changing the
great principles involved. It is but the repetition of the revolutionary
changes that occur at intervals in the history of science, a simple
discovery setting at naught some of the most careful calcul