H. A. Bumstead— Rontgen Mays in Lead and Zinc. 303 



with aluminium paint, instead of foil, to make the coefficient of 

 absorption for light less uncertain. 



Two series of observations with this arrangement were made 

 (one with Rontgen rays, the other with light), during which 

 the balance of the radiometer vanes was tested, the position of 

 the metals reversed and various corrections applied as detailed 

 in the former paper. The ratio of the lead effect to the zinc 

 effect was : 



With Rontgen rays 14Y ± '04 



With light ._"..___ 1-43 ± '06 



These results show that the heating effects of Rontgen rays 

 in the two metals are equal, with an uncertainty of between 5 

 and 10 per cent. 



Some time before the above results were obtained, two 

 experiments of another kind were made with the view of test- 

 ing certain aspects of the hypothesis that Rontgen rays caused 

 atomic disintegration. The first was an attempt to find out 

 whether any rays similar to a-rays existed among the secondary 

 radiations given off when a heavy metal is exposed to Rontgen 

 rays. An iron tube was provided with an aluminium window 

 at the side, through which a beam of Rontgen rays could be 

 sent ; this beam fell upon a lead plate at an angle of 45°. To 

 the upper end of the tube, 3 cm above the center of the lead 

 plate, was cemented a glass plate, the inner side of which was 

 coated with powdered zincblende. The tube was exhausted to 

 Q.^mm an( j ^ e z incblende screen examined by means of a lens 

 in the ordinary manner, while the rays fell on the lead plate. 

 No scintillations were seen ; the sensitiveness of the eye was 

 tested by alternate observations of another, similar screen placed 

 above a weak radium preparation which gave a few scattered 

 scintillations; this was placed near the Rontgen tube and 

 observed while the tube was excited. A large Muller water- 

 cooled tube was used (20 cm in diameter) and a heavy discharge 

 sent through it ; the focus was only 28 cm from the lead plate, 

 so that the latter was exposed to very intense rays. The air 

 between the lead and the screen would have formed, at atmos- 

 pheric pressure, a layer only 0'004 mm thick. 



I also tried to find out whether Rontgen rays had an accel- 

 erating effect upon the disintegration of a radio-active substance. 

 The active deposit of thorium was used in preference to one of 

 the more permanent radio-active substances. An aluminium 

 plate, one side of which had been exposed to thorium emana- 

 tion for some hours, was placed over a hole in the wall of an 

 electroscope with the exposed side inward. A number of 

 measurements of the activity were made and then the plate 



