16 POPULAR SCIENCE MONTHLY. 



of the atom, after the a particle is expelled, is the atom of the emana- 

 tion, so that each atom of radium gives rise to one atom of the emana- 

 tion. Let q be the number of atems 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 = q/X 

 where A is the constant of change of the emanation. Now Kamsay 

 and Soddy deduced from experiment that the volume of the emana- 

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

 at atmospheric pressure and temperature. It has been experimentally 

 deduced that there are 3.6 X 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 

 in consequence deduce, since N = 3.6 X 10 16 and A = 2.0 X 10 -6 , the 

 value q = 7.2 X 10 10 - Now the particles expelled from radium in a 

 state of radioactive equilibrium are about equally divided between four 

 substances, viz., the radium itself, the emanation, radium A and 

 radium C. We may thus conclude that the number of a particles 

 expelled per second from 1 gram of radium in radioactive equilibrium 

 is 2.9 X 10 11 . The value deduced by this method is intermediate 

 between the values previously obtained (see previous table), on the 

 assumption that the heating effect is entirely due to the a particles. 



I think we may conclude from the agreement of these two methods 

 of calculation that the greater portion of the heating effect of radium 

 is a direct result of the bombardment of the expelled a particles, and 

 that, in all probability, about 5 X 10 10 atoms of radium break up 

 per second.* 



The energy carried off in the form of /? and y rays is small com- 

 pared with that emitted in the form of a rays. By calculation it can 

 be shown that the average kinetic energy of the /? particle is small in 

 comparison with that of the a particle. This is confirmed by com- 

 parative measurements of the total ionization produced by the a and (3 

 rays, when the energy of the rays is all used up in ionizing the gas, 

 for the total ionization produced by the (3 rays is small compared with 

 that due to the y rays. The total ionization produced by the y rays 

 is about the same as that produced by the /3 rays, showing that, in 

 all probability, the energy emitted in the form of these two types of 

 radiation is about the same. From the point of view of the energy 

 radiated and of the changes which occur in the radioactive bodies, the 

 a rays thus play a far more important role in radioactivity than the 

 f3 or y rays. Most of the products which arise from radium and 

 thorium emit only a rays, while the f3 and y rays appear only in the 

 last of the series of rapid changes which take place in these bodies. 



* By measuring the charge carried by the a rays, the writer {Nature, 

 March 2, 1905) has recently deduced that 6.4 X 10 10 atoms of radium break up 

 per second. 



