PRESENT PROBLEMS IN RADIOACTIVITY. 17 



Since most of the heating effect of radium is due to the a rays, 

 it is to be expected that all radioactive substances which emit them 

 will also emit heat at a rate proportional to their a 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 a 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 con- 

 sequence 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 a particles independently 

 of the substance in which they are produced. Experiments of this 

 character would be difficult, but they would throw light on the im- 

 portant question of the division of the radiated energy between the 

 expelled a 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 Eamsay 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 10 7 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 10 9 gram calories. As we have seen, the 

 heat emission of radium is about equally divided between the radium 

 itself and the three other a ray products which come from it. The 

 heat emitted from each of the other radioactive substances, while their 



VOL. LXVII. — 2 



