3G0 Mr. J. Crosby Chapman on 



electrons themselves colliding with other atoms, and in this 

 way producing the peculiar type of radiation known as 

 fluorescent X-rays. 



This theory, that the secondary homogeneous radiation 

 results from the bombardment of atoms by ejected corpuscles, 

 has been discussed in a research published with Mr. Piper *, 

 as well as in a later paper f. The object of the experiment, 

 suggested by Professor Barkla, and described in the later 

 paper, was to test this theory directly. Allowing X-rays to 

 fall on bromine vapour (C 2 H 5 Br), it was found that the 

 presence of C0 2 in the first case, and H 3 in the second case,, 

 as the gas separating the ethyl bromide molecules, made next 

 to no difference in the amount of secondary radiation emitted 

 by the bromine when excited by X-rays. Whereas on this 

 theory, since the C0 2 , which is a heavy gas, must absorb many 

 of the expelled corpuscles, while a light gas cannot do this, 

 the COo ought to interfere with the conversion of the cathode 

 rays into bromine X-rays, and thus a greater intensity or 

 secondary radiation be obtained in the second case than in 

 the first. This was not found to be so. 



The results obtained were : — 



Secondary radiator .... C 2 H-,Br. 



Intensity of secondary radiation with H 2 separating C 2 H 5 Br molecules _ -. # ~-. 

 Intensity of secondary radiation with C0 2 separating C 2 H 5 Br molecules 



The calculations given in the paper show that, if the theory 

 is correct that the expelled corpuscles from bromine do, by 

 subsequently bombarding fresh bromine atoms, make the 

 latter emit homogeneous radiation, there should be a notice- 

 able difference in the intensity of the radiation in the two 

 cases. From the fact that the radiations were of the same 

 intensity, the conclusion naturally followed that this theory 

 of the indirect action of the X-rays in producing fluorescent 

 X-radiation is untenable. 



This experiment, how r cver, does not seem to Professor 

 Bragg to be decisive. For in a recent paper published with 

 Mr. Porter, he criticises the conclusion stated above, in the 

 following way J: — "This does not seem to me conclusive. 

 If the production of X-rays is a consequence of the encounters 

 of cathode rays with bromine atoms, there will be an ample 

 opportunity for the effect to take place even though the C0 2 

 molecules are scattered among the bromine atoms. If an 

 electron meets a C0 2 molecule first it is not arrested there, 



* Chapman and Piper, Phil. Ma°f. June 1910. 

 t. Chapman, Phil. Mag. April 1911. 

 X Loc. cit. footnote on pp. 1150 & 851. 



