H. A. Bumstead — Rontgen Rays in Lead and Zinc. 301 



to the more rapid loss of heat by the zinc. My recent exper- 

 iments, however, force me to the conclusion that the zinc did 

 lose heat more rapidly (for a given temperature) mainly over 

 the supports, and that, in the control experiments, this was 

 accidentally compensated by a greater absorption of the inci- 

 dent light by the zinc, possibly owing to a thinner layer of 

 wax between it and its covering of aluminium foil. The experi- 

 ments with light were made at the very end of my stay in 

 Cambridge and could not be repeated on account of lack of 

 time ; if they had been repeated under slightly varied condi- 

 tions the error would doubtless have been discovered. The 

 agreement between the two metals, however, was so good, and 

 the improbability of two unrelated large errors which exactly 

 compensated each other was so great, that I was led to put 

 more confidence in this result than it deserved. 



When the experiments were again taken up, however, I did 

 consider the possibility that the emissivity of a surface and its 

 absorption of light might vary together — thus destroying the 

 force of the control experiment. It was not likely that the 

 emissive power of a surface for low temperature radiations 

 would be proportional to its absorption of the high temperature 

 radiation from an incandescent filament, but I nevertheless 

 made some experiments to test the matter. For this purpose 

 two lead strips were used, one of which was left with its orig- 

 inal dull surface and the other covered with aluminium foil. 

 Rontgen rays and light were both used ; the rays gave deflec- 

 tions agreeing to about 5 per cent ; with light on the other 

 hand, the dull lead strip gave four times the deflection of the 

 other. It thus appeared (as was expected) that the rate at 

 which heat was lost varied very little with the state of the sur- 

 face, while the absorption of light varied greatly. Thus there 

 appeared to be no reason for distrusting the control experiment. 



A series of experiments was next made, by means of an 

 electroscope, on the amount of secondary radiation from lead 

 and zinc, in order to test the possibility that a considerable 

 fraction of the energy in the case of the zinc might escape in 

 this form. It appeared from these that the total intensity of 

 the secondary rays escaping from both surfaces of the zinc strip 

 (as measured by the ionization produced) was less than 1/15 of 

 the primary rays absorbed. 



The energy measurements were again taken up and a change 

 was made in the method which would render it independent of 

 the rate of loss of heat from the metals. The strips of lead 

 and zinc were held at the ends by massive brass clamps, con- 

 nected to binding screws outside the case by means of rods 

 insulated from the case. In this way a known current of elec- 

 tricity could be sent through either strip; the resistances of the 



Am. Jour. Sci.— Fourth Series, Vol. XXV, No. 148.— April, 1908. 

 21 



