294 Dr. C. G. Barkla on 
Thus:— 
Using the rays which are emitted by an X-ray tube in 
which the pressure is as high as possible when producing 
X-rays of appreciable ionizing power, the absorbability of 
the secondary rays from carbon differs exceedingly little if 
at all from the primary. Direct experiments did not indicate 
the slightest difference. 
Using such a beam as the primary to be tested for polari- 
zation by study of the intensities of secondary radiation from 
carbon, the maximum amount of polarization of a primary 
was observed — about 20 per cent, variation. When such a 
primary was used to produce a secondary beam from carbon, 
the secondary beam was found by a study of the tertiary 
rays to be fairly completely polarized. 
When such a primary beam was incident on carbon, the 
ratio of the intensities of secondary radiation in the direction 
opposite to that of propagation of the primary and in a 
direction at right angles was very close to the theoretical 
value for perfect scattering, i. e. 2 : 1 (within 5 per cent.). 
As the primary rays became more penetrating, the dif- 
ference in the ionizing powers of the primary and secondary 
beam from carbon became more marked; the difference in 
their penetrating powers increased ; the evidence of polariza- 
tion of the primary became less pronounced*; and the ratio 
of intensities of secondary radiation opposite to and perpen- 
dicular to the direction of primary propagation decreased. 
According to this theory, then, all those substances of low 
atomic weight — up to and including sulphur — when subject 
to " soft " X-rays should emit secondary rays varying in 
intensity in the two directions approximately in the ratio 
2:1. It is possible, I think probable, that the behaviour of 
sulphur varies much more rapidly with a variation of the 
penetrating power of the primary than any of the others, 
and that for some primary rays this ratio of intensities may 
be very different from that found by experiments on the 
lower part of this group. 
Paper, when tested in the same way as carbon for " soft " 
rays, gave off radiation whose intensity was found to vary 
in the ratio 2*01 : 1, which again after correction shows the 
true ratio of intensities not appreciably different from that 
for the " softest " rays from carbon. 
* This might possibly he due to the penetrating primaries actually 
being less polarized, but when considered with other changes it appears 
probable that much of the decrease of observed polarization is due to the 
secondary beams not consisting of purely scattered rajs, and so not 
shoicing "the polarization. The best results showing polarization in the 
secondary beam weie always ohtained by using easily absorbed rays. 
