Rontgen Radiation from Gases and Vapours. 671 



might have been obtained as large as those given by bromine 

 and arsenic. 



Taking the atom of bromine, for example, though only 5 

 times as heavy as that of oxygen, it gives off under the same 

 conditions nearly 400 times as much secondary radiation. 

 These figures are calculated from the corrected values given 

 in column III. of Table I. If the directly observed results 

 given in column I. had been employed, the ratio would have 

 been greater still. It is impossible to regard the largeness of 

 the amounts of secondary rays given off by bromine and 

 arsenic as due solely, or mainly, to want of proper correction 

 for the softness of the rays ; for the values for nickel, which 

 gives off radiation considerably softer than that from either 

 bromine or arsenic, are not particularly high, while the rays 

 from tin and iodine, which have high values for the secondary 

 radiation, have the same hardness as the rays from air and, 

 therefore, do not involve this correction. We must therefore 

 believe that the energy radiated from a bromine atom, for 

 example, is some hundreds of times greater than that radiated 

 under the same conditions from an atom of oxygen. 



In the last column of Table III. I have given the values of 

 the ratio of the intensity of secondary radiation to the atomic 

 weight, for the elements in the first column of the table. It 

 will be seen that this ratio is constant for the elements from 

 helium to oxygen. Hydrogen, as usual in X-ray measure- 

 ments, forms an exception among the lighter elements. The 

 ratio then begins to increase, sulphur giving l'l, chlorine 1*6, 

 and nickel 2*0 times the atomic weight. Apparently the ratio 

 then increases rapidly as we pass up the table of atomic 

 weights until it attains values for arsenic and bromine of 75 

 and 78 respectively. Passing along to tin and iodine, it 

 apparently diminishes, although, as mentioned before_, the 

 values for these elements being variable harder rays migh'. 

 give a considerably higher ratio. 



As mentioned above, there seems to be no connexion 

 between the amount of secondary radiation and the hardness 

 of the secondary -rays. It is interesting to note, however, 

 that the hardness of the secondary rays from the various 

 gases is almost exactly the same as that found for the second- 

 ary rays from the corresponding elements in the solid state. 

 The two sets of figures are given in the following table for 

 comparison ; the values for the solid elements being taken 

 from a paper by Barkla *. 



* Phil. Mag. xi. p. 825 (1006). 



