67(; Paths of Particles Ejected from Atoms [March 7, 



chamber in the path of a narrow beam of X-rays, which was made to 

 traverse the supersaturated air of the cloud chamber immediately 

 after its expansion. 



The absorption of X-rays by the copper is evident at a glance 

 (Fig. 12) from the diiference of the density of the clouds condensed 

 on the incident and transmitted beams. 



In passing through the copper the X-rays produce immense 

 numbers of cathode-rays which form dense clouds immediately in 

 front of and behind the copper plate. The clouds are not quite in 

 contact with the copper, the clear space next the plate being due to 

 the air becoming warmed by contact with the copper before the 

 passage of the rays, so that the ions fail to find the supersaturation 

 necessary for their growth into water drops. 



From the researches of Barkla and others we know that when 

 exposed to X-rays the copper plate will emit secondary rays — the 

 homogeneous or characteristic or fluorescent rays of copper. These 

 will in turn cause the air to emit secondary cathode or beta-rays 

 The visible cloud trails left by these are seen in the photograph 

 (Fig. 12). A photograph of this kind shows at once the distribution 

 of the secondary radiation from a substance as well as the nature of 

 the cathode-rays produced by this radiation in the surrounding gas. 

 The cathode- or beta-rays produced in air by the copper-rays are all 

 much alike in length (about 1 mm.) ; this is in striking contrast to 

 the very varying length, ranging up to 2 or 3 cm., of those produced 

 by the primary X-rays. 



A photograph taken under similar circumstances with a silver 

 plate in place of the copper one shows similar effects, but the 

 cathode-rays produced in air by the silver-rays are many times as 

 long. 



Some photographs were also taken with X-rays incident upon the 

 copper plate after their intensity had been reduced by interposing a 

 considerable thickness of aluminium. This cuts out especially the less 

 penetrating radiation. The individual cathode-rays which start 

 from the copper are now readily seen (Fig. 13) ; they were before 

 too closely interlaced to be separately visible. The surprising 

 feature of this photograph is the great length of some of the cathode- 

 rays emitted by both copper and air exposed to the X-rays. Some of 

 the tracks are about 3 cm. in length when the air is at atmospheric 

 pressure. 



[C. T. R. W.] 



