f. GOL J 
IX. 
THE SECONDARY RADIATION EXCITED BY Y RAYS. 
By F. EK. HACKETT, M.A., Px.D., University College, Dublin. 
[COMMUNICATED BY PROFESSOR J. A. M°CLELLAND, M.A., D.SC. | 
[ Published, January 12, 1909. ] 
Tuer research described in this paper deals with an investigation of the manner in 
which the secondary radiation excited by y rays varies with the atomic weight 
of the substance. It has been shown by McClelland* that the secondary radiation 
excited by 6 rays has a close connexion with the atomic weight of the excited 
substance, and varies in such a way that, from this electrical standpoint, the 
elements are arranged into the same periods as those employed by Mendeleef in 
his chemical classification of them. Since then J. J. Thomson} has shown that 
similar relations exist for the secondary Réntgen radiation; and Barkla has 
found that the absorbability of this radiation depends on the atomic weight of 
the substance which emits it. 
It is therefore interesting to determine how far such a relation holds for 
the secondary radiation excited by y rays; for it may contribute some ight on 
the constitution of matter, and also on the y rays themselves. 
The subject has been touched on in a general manner by Eve and other _ 
observers; and more recently papers have been published by Kleeman,t dealing 
in a detailed manner with the subject, and giving measurements of the intensity 
of radiation from different elements. His method of measurement and the 
numbers he obtained differ considerably from those described in the present 
paper. They will be discussed in the course of the paper. Bragg has also 
published some results which agree in a general way with the present work, 
though his theoretical standpoint is different. It may be stated here that the 
present results can be explained equally well on his theory of the constitution 
of y rays, or on the ether-pulse theory. 
The results of the present vestigation show that the separation of the 
elements into periods is not so marked as in the other cases already mentioned. 
* J. A. McClelland, Trans. Roy. Dub. Soc., vol. ix., 1905. 
t ‘‘ Conduction of Electricity through Gases,” 2nd ed., p. 395, 
+ R. D. Kleeman, Phil. Mag., Noy., 1907; May, 1908. 
TRANS. ROY. DUB, SOC., N.S., VOL. IX., PART IX. 2K 
