{ 043 | 
LXI. Energy of Secondary Réutgen Radiation. By CHARLES 
G. Barxua, VW.Sc. (Vict.), B.A. (Cantab.), King’s College, 
Cambridge; Oliver Lodge Fellow, University of Liverpool *. 
: a paper on ‘Secondary Radiation from Gases subject 
to X-rays ” f experiments were described which led to the 
following conclusions :— 
All gases subject to X-rays are a source of secondary radia- 
tion, the nature of which is similar to that of the primary 
radiation. The absorbability of the secondary radiation is 
(within the limits of possible error—about 10 per cent. of the 
absorption coefficient for aluminium) the same as that of the 
primary radiation producing it. 
For a given primary radiation the intensity of secondary 
radiation from different gases at the same pressure and tem- 
perature is proportional to the density of the gas from which 
it proceeds. : 
The opinion was expressed that the secondary radiation is 
due to a kind of scattering of the primary by the corpuscles 
constituting the molecules of the gas. 
Results similar to those which led Sagnact to conclude 
that the secondary radiation from air was more absorbable 
than the primary radiation producing it had been obtained, 
but the evidence was then considered insufficient to lead to a 
definite conclusion as to the difference in character of the 
two radiations. A direct method of comparison did not 
indicate the slightest difference in the absorption of the 
primary and secondary radiations by similar plates of 
aluminium. 
As the experiments of Townsend§ and Sagnac]| on secondary 
radiation from metals showed that this radiation was more ab- 
sorbable than the primary radiation producing it—the change 
in penetrating power, however, depending on the metal—and 
as the results referred to led to the probability of a transfor- 
mation of the radiation by air, further and more careful 
experiments were made on the subject. 
The following method was employed :-— 
A beam of X-rays passed through rectangular apertures in 
two parallel lead screens A and B (see figure). Two screens, 
C and D, were placed in planes perpendicular to the others, 

* Communicated by the Physical Society: read March 25, 1904. 
+ C. G. Barkla, Phil. Mag. | 6] v. p. 685 (1903). 
} Comptes Rendus, cxxvi. pp. 321-523 (1898). 
§ Proc. Camb. Phil. Soc. x. p. 217 (1899). 
|| Comptes Rendus, cxxy. p. 942 (1897). 
