672 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the means used to detect the heat developed in the metal. Dorn im- 

 mersed the metal in a body of gas ; the rise of temperature of the gas, 

 due chiefly to the heat produced in the metal, caused a corresponding 

 rise of pressure, which was made evident upon a delicate pressure- 

 balance. Rutherford and McClung and Schups, and Leininger following 

 them, made the metal the sensitive arm of a bolometer. The methods 

 differed also in the correction for the incomplete absorption of the rays. 

 Dorn arranged his apparatus, so that the rays had to pass through sev- 

 eral layers of the metal, and thus were almost completely absorbed. 

 The others observed the diminution of heating effect which occurred 

 when a piece of metal, of the same kind and thickness as the absorbing 

 sheet, was interposed between the latter and the tube, and hence esti- 

 mated what fraction of the rays was effective in producing heat. 



The method followed throughout the present research was a modifica- 

 tion of this general plan. A Boys' radiomicrometer 7 was the instrument 

 used, the thin piece of metal which partially absorbed the rays being 

 placed at one of the junctions of the thermal couple. With this in- 

 strument, which is described in detail below, a heating effect was readily 

 perceived ; and some measurements were made of the whole energy of 

 the Riintgen rays emitted per second by a tube, which measurements 

 are included in this paper as a confirmation of the results obtained by 

 the earlier investigators. 



In the course of this work it became plain that there is at least one 

 possible source of error which may affect, more or less, all measurements 

 of the energy of Rontgen rays based upon observations of the heat de- 

 veloped by the absorption of the rays in thin metallic sheets. The 

 well-known fact that the character of Rontgen radiation is changed by 

 passage through solid substances shows that the methods thus far used 

 to correct for the imperfect absorption of such sheets may yield only a 

 first approximation to the truth. This uncertainty as to the exact effect 

 produced upon a beam of Rontgen rays by passage through metallic 

 sheets suggested the subjects experimentally investigated in this paper. 

 These subjects are : 



(1) The effect of varying the thickness of a metallic sheet upon 

 transmission through it. 



(2) The effect of varying the intensity of the incident radiation 

 upon transmission through a metallic sheet. 



(3) The effect of the surfaces of the sheet upon transmission through 

 it, as distinguished from the eff'ect occurring within its substance. 



(4) The effect of transmission through a sheet of one metal upon the 

 penetrating power of the rays for a sheet of another metal. 



' C. V. Boys, riiil. Trans., 180, 159 (1888-1889). 



