the Corpuscular IL/pothesis of the y and X Rays. 401 



emerging energy of ft rays is therefore kl/(\~k), which may 

 practically be simplified to M/A, since k is usually so small 

 compared with \. 



The ionization produced in the ionization-chamber may 

 be taken as H(l — <? -A ' D )/A,, where D is the depth of the 

 chamber multiplied by the density of the air. This is not 

 strictly correct, because some of the ft rays will strike 

 against the side walls, which we cannot do without, and will 

 not spend so much energy in the layer of air (of weight D) 

 as they ought to do according to the definition of \'. If we 

 had a material which threw back all ft rays completely, we 

 could avoid this error ; but there is no such material. It can 

 be lessened by having a wide and shallow chamber. There is 

 an error of a different nature in that A/ was defined with 

 reference to rays striking a layer of air normally, whereas 

 the ft rays emerging from the plate will be moving in all 

 directions. But it is not worth while to attempt to avoid 

 such errors just now: it is probably a still greater error to 

 have assumed an exponential law, and our object is to obtain 

 a theoretical result accurate enough to tell us what we should 

 look for. 



We have now to take into account the ionization due to the 

 ft rays produced by the y rays in the air of the chamber. 

 This may be done by direct calculation, or in the following 

 way which seems interesting. 



If the plate which forms the base of the chamber were 

 replaced by a plate of nearly the same atomic weight as 

 the air in the chamber, the y rays would then pass through 

 the same sort of atoms throughout their course. Considering 

 a short path of the course in which there is no great absorption 

 of the y rays, strata of equal weight convert equal quantities 

 of 7-ray energy into /3-ray energy, and will show equal 

 ionization even though the ionization in any stratum is not 

 wholly due to the ft rays made in that stratum. The energv 

 spent on ionization in any stratum is practically equal to the 

 7-ray energy converted in that stratum : thus the ionization 

 in this particular ionization-chamber is measured by k'DI : 

 the A/ does not come in. If we now replace the bottom plate 

 of constants h' and A/ by the plate of constants k and A,, we 

 add a source of ionization amounting to kl(l — e -vl) )/A, but 

 we take away a source of ionization amounting, by the same 

 rule, to k'I(l — e _VD )/A/. We also provide a plate which 

 turns back more effectively some of the ft rays made in the air 

 of the chamber and in the plate at the top, but these are not 

 many and we may neglect them. Thus the ionization in the 

 Phil. Mag. S. 6. Vol. 20. No. 117. Sept. 1910. 2 E 



