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XLII. On the Heating Effects produced by Rontgen Rays in 
Lead and Zinc. By H. A. Bumstead, Ph.D., Professor 
of Physics, Yale University *. 
IN an earlier number of this Journal f the writer described 
a series of experiments, from which it appeared that, 
when Rontgen rays were equally absorbed in lead and in 
zinc, approximately twice as much heat was generated in the 
lead as in the zinc. These experiments were carried out 
in the Cavendish Laboratory of the University of Cambridge, 
and the further prosecution of the investigation was in- 
terrupted by the writer's return to America. An unusual 
pressure of other duties prevented the resumption of the 
work until ]ast summer and autumn, when a considerable 
series of observations was made with such variations of the 
conditions as might be expected to reveal certain possible 
errors in the original experiments. It soon became apparent 
that errors had been present, and that the difference in the 
quantities of heat generated in the two metals was much less 
than had appeared from the earlier experiments. The source 
of the original mistake was inherent in the apparatus used 
(a special form of radiometer), and although it could be 
diminished it was not easy to eliminate it altogether : there 
still remained an uncertainty of from 5 to 10 per cent, as to 
the equality of the heat in the two metals. I had accordingly 
planned, before publishing this correction, to attempt to 
bring the result within narrower limits, by substituting for 
the radiometer, a thermopile, with which the principal 
source of difficulty could be easily avoided. In the mean- 
while, however, a paper by E. Anger er has appeared J, in 
which a series of very careful experiments of this kind are 
recorded. Anger er's results leave no doubt, I think, that 
the heating effects in lead and zinc are equal to within a few 
per cent. ; the total effect is so small and the experimental 
difficulties are so considerable, that it does not seem practical 
at present to seek for a possible small difference within these 
limits. Certain facts in connexion with the emission of 
electrons by metals make it not improbable that there may 
be some liberation of atomic energy when ultra-violet light 
or Rontgen rays fall on a heavy metal. Such considerations 
have been advanced by Lenard §, by W. Wien || , and by 
* Communicated by the Author, 
t Phil. Mag. February 1906, p. 292. 
% Ann. der Phys. xxiv. p. 370 (1907). 
§ Ann. der Phys. viii. p. 169 (1902). 
|| Ibid, xviii. p. 991 (1905). 
