PRESENT PROBLEMS OF RADIOACTIVITY 167 



cult to distinguish between the heating effect of the emanation and 

 that of radium A. The curve of variation with time of the heating 

 effect. of the emanation tube after removal of the emanation is very 

 nearly the same as the corresponding curve for the activity measured 

 by the a rays. These results show that each of the products of radium 

 supplies an amount of heat roughly proportional to its activity meas- 

 ured by the a rays. Each product loses its heating effect at the same 

 rate as it loses its activity, showing that the heating effect is directly 

 connected with the radioactive changes. The results indicated that 

 the product, radium B, which does not emit rays does not supply an 

 amount of heat comparable with the other products. This point is 

 important, and requires more direct verification. 



Since the heat emission is in all cases nearly proportional to the 

 number of a particles expelled, the question arises whether the bom- 

 bardment of these particles is sufficient to account for the heating 

 effects observed. The kinetic energy of the a particle % mv 2 can be at 

 once determined since ^ and V are known. The following table 

 shows the kinetic energy of the a particle deduced from the measure- 

 ments of Rutherford and Des Coudres. The third column shows the 

 number of a particles expelled from 1 gram of radium per second on 

 the assumption that the heating effect of radium (100 gram-calories 

 per gram per hour) is entirely due to the energy given out by the 

 expelled a particles. 



Number of a particles 



Observer Kinetic energy expelled per second 



from 1 gram of radium. 



Rutherford 5.9 X lO" 8 ergs. 2 X lO" 11 



Des Coudres 2.5 X 10~ 6 ergs. 5 X 10~ u 



This hypothesis that the heating effect of radium is due to bombard- 

 ment of the a particle can be indirectly put to the test in the follow- 

 ing way. It seems probable that each atom of radium in breaking up 

 emits one a particle. On the disintegration theory, the residue of the 

 atom, after the a particle is expelled, is the atom of the emanation, so 

 that each atom of radium gives rise to one atom of the emanation. 

 Let q be the number of atoms in each gram of radium breaking up 

 per second. When a state of radioactive equilibrium is reached, the 

 number N of emanation particles present is given by N %, where 

 ^ is the constant of change of the emanation. Now Ramsay and 

 Soddy deduced from experiment that the volume of the emanation 

 released from 1 gram of radium was about one cubic millimeter at 

 atmospheric pressure and temperature. It has been experimentally 

 deduced that there are 3.6X 10 19 molecules in one cubic centimeter 

 of gas at ordinary pressure and temperature. The emanation obeys 

 Boyle's law and behaves, in all respects, like a heavy gas, and we may 



