v] RATE OF EMISSION OF ENERGY 153 
are the values for the 8 rays corresponding to #, and 2, for the 
arays. It thus follows that 
i, Ny W, : 
Ea Waa 
dX, and dA, are difficult to determine directly for the radio-active 
substance itself, but it 1s probable that the ratio \,/A, 1s not very 
different from the ratio for the absorption coefficients for another 
substance like aluminium. This follows from the general result 
that the absorption of both a and @ rays is proportional to the 
density of the substance; for it has already been shown in the 
case of the 8 rays from uranium that the absorption of the rays in 
the radio-active material is about the same as for non-radio-active 
matter of the same density. 

With a thick layer of uranium oxide spread over an area of 
22 sq. cms., it was found that the saturation current between 
parallel plates 6°1 cms. apart, due to the a rays, was 12:7 times 
as great as the current due to the 8 rays. Since the @ rays were 
entirely absorbed between the plates and the total ionization 
produced by the @ rays is 154 times the value at the surface of the 
plates, 
W, _ total number of ions due to a rays 
W., total number of ions due to B rays 
AT xs Gal 
aR F 0:5 approximately, 
Now the value of A, for aluminium is 2740 and of 2, for the 
same metal 14, thus 
i, lies ru W, 
Es ea re W, 
This shows that the energy radiated from a thick layer of 
material by the 8 rays is only about 1 per cent. of the energy 
radiated in the form of a rays. 
This estimate is confirmed by calculations based on indepen- 
dent data. Let m,, m, be the masses of the a and 8 particles 
respectively. Let v,, v, be their velocities. 



= 100 approximately. 
Energy of one a particle mv? e 


Energy of one 8 particle m.v,2 ms 
pia 
2 
