RADIOACTIVITY VISUALISED 489 



cathode or beta-rays. The visible cloud trails left by these 

 are seen in the photograph (fig. 11). 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. 12); they were before too closely interlaced to be separ- 

 ately 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. 



