of Rontgen Radiation. 403 



possible, the slabs were made so thick that the time of expo- 

 sure was never less than half an hour, and in some instances 

 more than an hour. In no case was there an over exposure, 

 and in all cases the images or shadows as formed beneath the 

 compound and its constituents appeared simultaneously and 

 remained of equal intensity during the process of developing. 

 The substances teste 1 in this way were : — 



Zinc blende ZnS. 



Stibnite Sb 2 S 3 . 



(Jhalcocite CuS. 



Galena PbS. 



Speculum metal 68 parts copper, 32 parts tin. 



Regulus of Venus ... 58 parts copper, 42 parts antimony. 



Aluminium bronze ... 92| parts copper, 1\ parts aluminium. 



Brass 65 parts copper, 35 parts zinc. 



It would be very desirable of course to work with many 

 more substances ; but, apart from alloys, those compounds, 

 all of whose constituents in the uncombined form are solid 

 and suitable to work with, are by no means numerous. How- 

 ever, from this small number of substances examined I judged 

 that possibly the phenomenon of Rontgen-ray absorption was 

 an atomic one, and concluded that if so all compounds none 

 of whose elements were highly absorptive of Rontgen rays 

 would be transparent to them, while those containing one or 

 more elements which in the free state were opaque to this 

 radiation would themselves be opaque to it. This supposition 

 was fully borne out in every test made — some forty or more 

 compounds of the most varied nature being examined. 



Owing to the complex nature of Rontgen radiation, quanti- 

 tative measurements of the coefficients of absorption of different 

 elements may not be very reliable ; but it seems that the ab- 

 sorption is a function, though probably not a linear one, of 

 atomic weight. About this and certain other points I hope 

 to have something to say in a future communication. 



In closing I wish to thank Professor Rowland for allowing 

 me the use of the physical laboratory of the Johns Hopkins 

 University, where this work was done during July of the 

 present year. I wish also to thank Dr. Chandler of Baltimore 

 for kindly lending me his splendid Rontgen-ray outfit, and 

 Dr. Mallet of the University of Virginia for a number of 

 alloys. 



University of Virginia, 

 September (!, 1897. 



2 G2 



