552 Mr. C. G. Barkla on Energy of 
between the secondary and primary radiations when pene- 
trating rays constituted the primary beam than when it 
consisted of more absorbable rays. It is not due to a greater 
fraction of the energy going as secondary radiation. 
We are thus led to the conclusion that— } 
The energy of secondary radiation from a definite mass of air 
through which a primary beam of uniform intensity passes Is a 
definite fraction of the energy of the primary beam, whatever be 
the penetrative power of the primary radiation ; or the intensity 
of secondary radiation depends on the intensity of the primary 
radiation, but not on the character of that radiation. This 
Seta Sis ior etical result given by J. J. Thomson (‘ Con- 
duction of Electricity through Gases,’ p. 271). 
Energy of Secondary Radiation. 
In the passage of Rontgen radiation through gases, part of 
the energy of this radiation is spent in the pr roduction of ions 
and subsequently appears as heat in the gas when the ions 
recombine, part appears more directly as heat, and part is 
productive of secondary radiation. The approximate pro- 
portionality between the absorption and the ionization led to 
the assumption that practically all the energy was spent in 
the work of ionization ; but the experiments of H. A. Wilson * 
and of Townsend together with those of Rutherford and 
McClung +, giving the energy required to ionize a molecule 
and that absorbed per molecule ionized respectively, lead 
to the conclusion that only a small fraction of the energy 
absorbed is spent in the work of ionization. The energy of 
secondary radiation has been regarded as negligible. The 
experiments described show that the secondary radiation from 
gases differs little in absorbability from the primary radiation. 
It is hence an easy matter to compare the intensities of the 
secondary and primary radiations, for these are proportional 
to the ionizations produced by them in equal volumes of the 
same gas through which they pass, and hence the total 
energy of secondary radiation may be compared with that of 
the primary. 
The electroscope used to measure the intensity of the 
secondary radiation was placed behind a square aper ture 5 em. 
by 5 cm. in a lead screen, in such a position as to receive the 
secondary rays proceeding from the air in a direction ap- 
pr oximately at right angles to the direction of propagation of 
the primary rays (fig. p. 544). 
* Phil. Trans. A. 197, p. 415 (1901). 
+ Phil. Trans. 196, p. 25 (1901). 
