300 II. A. Bu instead — Rontgen Hays in Lead and Zinc. 



But so little'is as yet known about the mechanism of this emis- 

 sion of electrons, that it is \>y no means certain that the facts 

 observed by these investigators necessarily involve the libera- 

 tion of atomic energy. And in any event, the results of Ang- 

 erer indicate that this energy, if it is set free, forms only a 

 small part of the total produced by the absorption 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 temperature 

 above its surroundings, than by the lead. In the original 

 experiments small strips of the two metals (of different thick- 

 ness 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 fiber. 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 deflections 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 tem- 

 perature of the surface of the strip to which it was exposed ; to 

 make the radiometer deflections a measure of the quantities of 

 heat developed in the metals, it was necessary that the rate at 

 which the two metals lost heat, per degree excess of tempera- 

 ture 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 alumin- 

 ium 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 com- 

 parison with the lead owing to its greater conductivity and 

 thickness. This possibility appeared to be excluded (as well 

 as any sensible difference in the emissivity of the surfaces) by a 

 control experiment in which the strips were heated by exposure 

 to the light of an incandescent lamp, instead of Rontgen rays. 

 The deflections of the radiometer were almost exactly equal in 

 this case, and the whole behavior was such as to indicate that 

 the conditions specified above were fulfilled ; the question was 

 discussed in the previous paper and the experiments with light 

 were taken as excluding the possibility of the result being due 



