430 Proceedings of Royal Society of Edinburgh. [sess. 
The Energy of the Rontgen Rays. By Rev. Alexander 
Moffat, M.A., B.Sc. ( Communicated by Dr C. G. Knott.) 
(Read January 9, 1899.) 
The number of experiments that have been made on the Rontgen 
rays is already very large, and much has been learned from them 
regarding the properties of these rays. There has been, however, as 
yet, but little done to determine their energy. Indeed, the only 
accurate work which has been carried out in this direction is that 
of E. Dorn (Wied. Ann., Bd. 63, p. 160), who measured the heat 
produced by Rontgen rays falling on a metal plate. This investiga- 
tion is, however, not complete, as Dorn did not determine the 
number per second or the duration of the discharges of the 
Rontgen rays. 
On the suggestion of Professor E. Wiedemann (Erlangen), in 
whose laboratory I have been working, I have tried to investigate 
this subject; and, although I have not had time to complete my 
experiments, as I now find I have to return to India much earlier 
than I expected, I shall describe the methods adopted and the re- 
sults obtained, in the hope that they may not be without interest. 
I first sought to determine in absolute measure the quantity of 
light produced on a screen of barium platino-cyanide by Rontgen 
rays. For this purpose I used an optical bank with the barium 
platino-cyanide screen and the Rontgen lamp at one end, the 
source of light which I used as a standard of comparison at the 
other end, and between them a movable photometer. The ar- 
rangement of the apparatus is shown in the following diagram. 
The Rontgen lamp which I used was one supplied by the 
Yoltohm-Elektricitats-Gesellschaft, A. G., Munich. It was as 
shown in the second diagram, the anti-cathode being a copper 
hemisphere with a platinum disc for the plane face, set at an angle 
of 45° to the axis of the lamp, and the vacuum being one suitable 
for a short spark. The radius of the bulb was 4*2 cm. The 
current was obtained from a twenty-plate Topler machine, which 
