436 Prof. H. A. Bumstead on the Heating Effects 
A more careful consideration of the conditions of the 
original experiments showed that it was not impossible that 
the result obtained was due to escape of heat by conduction 
through the ebonite disk which supported the strips, the 
aluminium leaf which covered the disk (to prevent electrical 
effects), and the copper wires by which the strips were 
earthed. The lead strips had also between them and the disk 
pieces of cardboard to bring their front surfaces into the 
same plane with those of the thicker zinc strips. I accordingly 
repeated the experiments with the following modifications : — 
the cardboard was left out and the zinc strips sunk into 
recesses cut in the disk ; the aluminium-foil was scraped 
away from the vicinity of the ends of the strips ; and the 
strips were earthed by manganin wires O05 mm. in diameter 
and 3 cm. long ; also the strips were covered 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 1*47 + '04, 
With light l*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 
testing 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 centre of the lead plate, was cemented a glass 
plate, the inner side of which was coated with powdered zinc- 
blende. The tube was exhausted to 0*1 mm. and the zinc- 
blende screen examined by means of a lens in the ordinary 
manner, while the rays fell on the lead plate. No scintil- 
lations were seen ; the sensitiveness of the eye was tested by 
alternate observations of another similar screen placed above 
