Different Types of y Rays jrom Radioactive Substances. 537 



in aluminium is 0'36, fju 2 works out to 0*00050 at 0° and 

 normal pressure. 

 Then 



Nx : N 2 = n 1 /A 2 : n 2 fii : =l : 0*54, 



i. e. as much as 35 per cent, of the total y energy from 

 radium D would then be present in the harder type. 



This, of course, is a maximum estimate. If the density 

 law breaks down for radiation of this penetrating power — as 

 previously described ionization measurements seem to indi- 

 cate — the value for /z 2 would be about twice as high ; 

 this would lower the percentage of the hard rays to about 

 17 per cent. 



<y Rays from Radium B and Radium C. — A thin-walled tube 

 containing from 1 to 50 millicuries of emanation was used as 

 source of radiation and placed about 16 cm. from the ioniza- 

 tion chamber, so that the a rays could not reach it. The 

 /3 rays were turned away by a strong magnetic field. When 

 the ionization chamber contained air the effect of the soft 

 rays from radium B (fi = 4oO cm. -1 Al) was small compared 

 with that due to the more penetrating rays, so that it was 

 difficult to determine the amount of the soft rays with much 

 accuracy. The ionization-chamber was therefore filled with 

 sulphur dioxide ; the curve, represented in fig. 2, showed a 

 much more pronounced initial drop. The ratio OB : BA: AO 

 gives an estimate of rti : n 2 : // 3 , the number of ions produced 

 separately by the soft rays of radium B, the hard rays 

 from radium B(^ = 0'51 cm. -1 Al), and the still harder rays 

 from radium C ( yLt = * 1 1 5 cm. -1 Al), in a layer of unit 

 thickness close to the source. A slight correction has to be 

 made for the absorption in the air between source and 

 electroscope. 



The value of jjl y for S0 2 can be deduced from Owen's 

 values for X rays on the assumption that the radiation cor- 

 responds to a characteristic radiation of the " K" series from 

 an element of atomic weight ]>(214 — 18) =83. It is found 

 equal to 0*0321. 



The value of /x 3 — by direct measurement in liquid S0 2 — 

 was found equal to 0*000124. 



The value of yu, 2 is probably about 4*4 times larger and 

 equal to 0*000540. since the absorption coefficient- in 

 aluminium are in this ratio. 



Then 



XT XT XT 24*08 18*27 58*91 , ir , on 

 N x : N 2 : N 3 = -—j:^-^ s -g—- =1 s 4o : 639. 



