H. A. Bumstead — Rontgen Rays in Lead and Zinc. 299 



Art. XXXI. — On the Heating Effects produced by Rontgen 

 Rays in Lead and Zinc ; by JEL A. Bumstead. 



[Contributions from the Sloane Physical Laboratory of Yale University.] 



In an earlier number of this Journal* the writer described a 

 series of experiments from which it appeared that when 

 Rontgen rays were equally absorbed in lead and in zinc, ap- 

 proximately twice as much heat was generated in the lead as 

 in the zinc. These experiments were carried out in the Caven- 

 dish Laboratory of the University of Cambridge and the fur- 

 ther prosecution of the investigation was interrupted by the 

 writer's return to America. An unusual pressure of other 

 duties prevented the resumption of the work until last summer 

 and autumn, when a considerable series of observations was 

 made with such variation of the conditions as might be ex- 

 pected to reveal certain possible errors in the original experi- 

 ments. 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 elimi- 

 nate 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 substitu- 

 ting for the radiometer a thermopile, with which the principal 

 source of difficulty could be easily avoided, In the meanwhile, 

 however, a paper by E. Angerer has appeared, f in which a series 

 of very careful experiments of this kind are recorded. Ang- 

 erer'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 con- 

 siderable, that it does not seem practicable at present to seek for 

 a possible small difference within these limits. Certain facts 

 in connection 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 J. J. Thomson ;|| and they are in a meas- 

 ure supported by the recent work of Bestelmeyer,^ Cooksey,** 

 and Innes.ff 



* Vol. xxi, p. 1, 1900. f Ann. der Phys., xxiv, p. 370, 1907. 



{Ibid., viii, p. 169, 1902. £Ibid., xviii, p. 991, 1905. 



I Conduction of Electricity through Gases, p. 319. 



m \ Ann. der Physik., xxii. p. 429, 1907. 



** This Journal, xxiv. p. 285, 1907. 



ffProc. Boy. Soc, A. lxxix, p. 442,1907. 



