Heat produced by Rontgen Rays in Lead and Zinc* 433 
J. J. Thomson * ; and they are in a measure supported by 
the recent work of Bestelmeyer t, Cooksey J, and Innes§. 
But so little is as yet known about the mechanism of this 
emission of electrons, that it is by no means certain that 
the facts observed by these investigators necessarily involve 
the liberation of atomic energy. And in any event, the 
results of Angerer indicate that this energy, if it is set free, 
forms only a small part of the total produced by the ab- 
sorption of Rontgen rays. 
The source of my own erroneous results was found in the 
greater rate of loss of heat by the zinc, for a given tem- 
perature above its surroundings, than by the lead. In the 
original experiments small strips of the two metals (of different 
thickness so as to produce equal absorption of the rays) 
were held by an ebonite support so that each strip was 
opposite one vane of a radiometer made of thin aluminium- 
foil and suspended by a quartz-fibre. The whole was enclosed 
in a heavy metal case from which the air could be exhausted 
to the point of maximum radiometric sensitiveness. An 
aluminium window, with a movable lead screen outside, 
permitted either or both of the strips to be subjected to the 
action of Rontgen rays ; and through a glass window the 
deflexions of the radiometer could be read by telescope and 
scale. The position of the strips could be reversed and the 
balance of the two vanes tested by a device which is described 
in the former paper. The repulsion of one of the vanes was 
of course primarily dependent on the temperature of the 
surface of the strip to which it was exposed ; to make the 
radiometer deflexions a measure of the quantities of heat 
developed in the two metals, it was necessary that the rate 
at which the two metals lost heat, per degree excess of 
temperature above their surroundings, should be the same. 
If then the steady state was observed when the heat lost 
was equal to the heat generated, the rise in temperature 
of either strip would be proportional to the heat developed 
in it. I sought to realize this condition by covering both 
metals with thin aluminium-foil which was stuck to the 
metal by a very thin layer of wax. It was recognized that,. 
if any considerable part of the total heat were lost over the 
supports to which the ends of the strips were attached, the 
zinc would be at a disadvantage in comparison with the lead 
owing to its greater conductivity and thickness. This 
* 'Conduction of Electricity through Gases/ p. 319. 
f Ann. der Phys. xxii. p. 429 (1907). 
t Am. Jour. Sci. xxiv. p. 285 (1907). 
§ Proc. R. S. A. lxxix. p. 442 (1907). 
