Bays of Radioactive Substances. 327 



Now X is difficult to measure experimentally for air, but 

 we can get an approximate estimate of its value from 

 Lenard's results that the absorption of cathode-rays is 

 proportional to the density of any given substance and inde- 

 pendent of its chemical nature. 



Xow A. for aluminium for /3-rays from uranium is about 

 14, and \ divided by the density is 54. Xow taking density 

 of air as -0012, we find that 



X for air = '0065. 



The total number of ions produced in air is thus 154 q 

 when rays are completely absorbed. 



Now, from the above table, we see that the ionization of 

 deviable rays is '0074 of the ions produced by «-rays when 

 /5-rays passed over a distance of 5*7 cms. of air. 



We thus have approximately 



Total number of ions produced bv /3-ravs -0074 ^ „ , 

 j " * ■■' — y i "■)_]- ■ / 



Total number of ions produced by a-rays 5*7 — 



Thus about \ of energy radiated into air b}^ a thin layer of 

 uranium is carried by the electrons. The ratio for thoria is 

 about ^27 an d f° r radium about j^, assuming the rays to 

 have about the same average value of X. 



This calculation only takes into account the energy which 

 is radiated out into the surrounding gas : but on account of 

 the ease with which the «-rays are absorbed, even with a thin 

 layer, the greater proportion of the radiation is absorbed by 

 the radioactive substance" itself. This is seen to be the case 

 when we recall that the a-radiation of thorium or radium is 

 reduced to half value after passing through a thickness of 

 about '0005 cm. of aluminium. Taking into consideration 

 the great density of the radioactive substances, it is probable 

 that most of the radiation which escapes into the air is due 

 to a thin skin of the powder not much more than "0001 cm. 

 in thickness. 



TVe can, however, form an estimate of the relative rate of 

 emission of energy by the a.- and /3-rays in the f olio win o- 

 way : — 



The total energy V>\ radiated to the surface per sec. bv a 

 thickness d, is given by 



if \ x d is large. 



t\ x 



