Since most of the heating effect of radium is due to the α rays, it is to be expected that all radioactive substances which emit them will also emit heat at a rate proportional to their α ray activity. On this view, both uranium and thorium should emit heat at about one millionth the rate of radium. It is of importance to determine directly the heating effect for these substances and also for actinium and radio-tellurium.
According to the disintegration theory, the α particle is expelled as a result of the disintegration of the atom of radioactive matter. While it is to be expected that a greater portion of the energy emitted will be carried off in the form of kinetic energy by the expelled particles, it is also to be expected that some energy will be radiated in consequence of the rearrangement of the components of the system after the violent ejection of one of its parts. No direct measurements have yet been made of the heating effect of the α particles independently of the substance in which they are produced. Experiments of this character would be difficult, but they would throw light on the important question of the division of the radiated energy between the expelled α ray particle and the system from which it arises.
The enormous emission of energy by the radioactive substances is very well illustrated by the case of the radium emanation. The emanation released from 1 gram of radium in radioactive equilibrium emits during its changes an amount of energy corresponding to about 10,000 gram calories. Now Ramsay and Soddy have shown that the volume of this emanation is about 1 nubic millimeter at standard pressure and temperature. One cubic millimeter of the emanation and its product thus emits about 107 gram calories. Since 1 c.c. of hydrogen, in uniting with the proportion of oxygen required to form water, emits 3.1 gram calories, it is seen that the emanation emits about three million times as much energy as an equal volume of hydrogen.
It can readily be calculated on the assumption that the atom of the emanation has a mass 100 times that of hydrogen, that I pound of the emanation some time after removal could emit energy at the rate of about 8,000 horse-power. This would fall off in a geometrical progression with the time, but, on an average, the amount of energy emitted during its life corresponds to 50,000 horse-power days. Since the radium is being continuously transformed into emanation, and three-quarters of the total heat emission is due to the emanation and its products, a simple calculation shows that 1 gram of radium must emit during its life about 109 gram calories. As we have seen, the heat emission of radium is about equally divided between the radium itself and the three other α ray products which come from it. The heat emitted from each of the other radioactive substances, while their
