664 Mr. J. A. Crowther on the Secondary 



depend upon their action either in ionizing a gas, or upon a 

 sensitized plate, or in lighting up a luminescent screen ; 

 all o£ which actions depend, in all probability, not only upon 

 the energy of the rays but also upon their hardness. Until 

 some method can be devised which is independent of the 

 character of the rays, the comparison of the intensities of 

 rays of different penetrating power must remain a matter of 

 some doubt. 



Using X-rays of the same type, but different gases, 

 Rutherford* found that the ionization was proportional to 

 the amount of absorption. If we can assume that this 

 result holds true for the same gas, but with rays of different 

 penetrating power, it will be possible to correct for the effect 

 of varying hardness upon the ionization, and thus to obtain 

 for the gases giving off soft radiation results directly com- 

 parable with those from air and other gases. Although, as 

 mentioned above, there seems to be no direct experimental 

 evidence upon the point, it seems probable that with a gas 

 like air, and with rays of not too widely differing character, 

 the assumption may represent at any rate a first approxima- 

 tion to the truth. Accordingly, while retaining in the first 

 column of the table the values obtained directly from the 

 ionization in the ionization-chambers, it was thought worth 

 while to add a further column in which values for the 

 secondary radiation, corrected on this assumption, were given. 



The coefficient of absorption of the secondary rays was 

 determined by placing a sheet of tinfoil over the widow d, of 

 the chamber containing the gas under observation, a balance 

 having previously been obtained between the chambers in 

 the usual way. This absorbed a certain proportion of the 

 rays (the absorption for air amounted to 25 per cent, of the 

 radiation) and the balance was destroyed. The pressure in 

 the standard air-chamber was then reduced until a balance 

 was again obtained, and the experiments were repeated with 

 air in place of the gas. Great care was taken to keep the 

 bulb as constant as possible in hardness, as the values obtained 

 varied with the penetrating power of the rays. Some little 

 time, however, is always occupied in making an accurate 

 balance, and hence it was impossible to ensure that the rays 

 were of quite the same hardness in any two cases. Thus the 

 results are hardly as accurate as those obtained in other 

 portions of the experiment. It is possible, however, to test 

 with very great accuracy whether the radiation from two 

 different gases has the same penetrating power or not. For 

 this two screens cut from the same piece of tinfoil are taken, 

 * Phil. Mag. xliii. p. 254 (1897). 



