Selective Absorption of Rontgen Rays. 



147 





number w, say taken such that the leakage from the charged disk 

 was approximately the same as when the glass was used. The rate 

 of leak was then measured accurately. The ratio of the rate of leak 

 with the glass to that with the n sheets of tinfoil gives a measure of 

 their relative transparency to Rontgen rays. 



A number of tinfoil sheets is now placed over the hole ; the glass 

 plate is placed on the top, and the rate of leak measured. The glass 

 is removed and the same n sheets of tinfoil as were formerly used 

 put in its place, and the leakage measured. The ratio of the rate -of 

 leak in the latter two cases is a measure of the relative transparency 

 of -the glass and the n tinfoil sheets to the Rontgen rays after they 

 have been already screened by passing through several layers of 

 tinfoil. 



The two ratios thus obtained should be equal if the Rontgen rays 

 are all of one kind, but if the glass is relatively less transparent in 

 the .second case it can only be explained by assuming that the 

 Rontgen rays are not homogeneous, and that some of them are more 

 readily absorbed by the glass and others by the tinfoil. 



Various substances were tested against tinfoil in the manner 

 described. With some there was no selective absorption, with others 

 it was very marked. Glass gave none, with mica and paraffin the 

 effect was small, with fuchsine, eosine, fluorescine, aesculin, and 

 barium sulphide the effect was very marked. With several fluores- 

 cent screens the effect was also marked. Pure water also gave a 

 distinct though smaller effect. 



The table below sets forth the results obtained with these sub- 

 stances. 



Column B gives the quotient of the rate of leak through the sub- 

 stance in column A to that through a number of tinfoil layers which 

 gave approximately the same leak. Column C gives the quotient .of 

 the rate of leak through the substance to that through the same 

 tinfoil layers after the rays have already passed through four layers 

 of tinfoil. 



Calcium tungstale 



Calcium platinocyanide . . 



Luminous paint 



Potassium platinocyanide 



Fuchsine 



Eosine 



Aesculin 



Fluorescine 



Barium sulphide 



B. 



07 



80 



o 

 10 



15 



ai 



3:3 



32 



'30 



0, 



0-85 

 0-86 

 0-71 

 0-87 

 0-77 

 1-00 

 0-90 

 1-08 

 0-97 



Difference. 



0-22 

 0-44 

 0-29 

 0-23 

 0-38 

 0-31 

 0-43 

 0-21 

 0-33 



M 2 



