680 PROCEEDINGS OF THE AMERICAN ACADEMY. 



any measurement of this quantity is of little significance unless accom- 

 panied by complete specification of the conditions under which the rays 

 are produced. But as a confirmation of the qualitative result of the 

 earlier investigations, it serves the purpose for which it was under- 

 taken ; and, together with the recently published work of Wien,^^ of 

 Bumstead,^"^ of Angerer,!"* and of Carter ^^ establishes beyond reason- 

 able doubt the fact that the absorption of Rontgen rays in metals is 

 accompanied by the appearance of a measurable amount of heat. It 

 may be of interest, however, to remark that the result reached above 

 is of the same order of magnitude as those found by the other in- 

 vestigators : Dorn's tube yielded an output of 0.00151 to 0.00168 gm. 

 cal. per sec, Rutherford and McClung found 0.011 gm. cal. per sec, 

 "VVien has obtained 0.0015 gm. cal. per sec, Angerer gives a maximum 

 value of 0.013 gm. cal. per sec, and Carter finds 0.00514 gm. cal. per 

 sec. In this connection it may be added that in the latter part of the 

 present research rays were used the energy of which was undoubtedly 

 of the order of 0.01 gm. cal. per sec. 



IV. Experiments on Transmission. 



1. Change in Transmission accompany itig Change in Thickness 



of Screen. 



The change in transmission which accompanies a change in the 

 thickness of a metallic screen has been examined by many observers, 

 and has been found in every case to follow the same qualitative law, 

 namely, that each succeeding equal increment of thickness is less ef- 

 fective as an absorbing medium than the one preceding it. Experiments 

 made in the course of the present research confirm this law for all the 

 metals examined, and it seems necessary here only to indicate the mode 

 of procedure followed in these experiments, and to give a specimen of 

 the curves obtained. 



A series of screens, of the metal to be examined was prepared. Each 

 screen was built up to the desired thickness from thin sheets of 

 the metal, — a method which is justified by experiments described 

 below. The screens were then interposed, one by one, in the patli of 

 the rays, and the corresponding deflections of the instrument observed. 

 Just before and just after taking each deflection the screens were with- 



12 W. Wien, Ann. d. Tliys., 18. OiU {\Wb). 



13 II. A. lUinistoad, AnitM". Jour. Sci., 171. 1 (lOOG). 

 " E. An.uaTor, Ann. d. Phys.. 21, 87 (lUOt;). 



" E. Carter, Ann. d. I'liys., 21, 955 (190(5). 



