TRANSPARENCY TO X-RAYS. 95 



The present sample, which had a high polish, increases uniformly in 

 reflecting power from 40 per cent at 2.5 //, to 60 per cent at 12 fi. It 

 thus appears that the presence of silicon in carborundum lowers the 

 reflecting power up to 9, beyond which it becomes abnormally high. 



The results obtained from the present examination of reflection 

 spectra demonstrate a number of important facts. The first one is the 

 dependence of reflection upon absorption. The second point worthy 

 of notice is that the region of selective reflection begins beyond 7/x, 

 while for the majority of substances studied the region of greatest 

 activity is from 8 to io/x. Whether this coincidence in the grouping of 

 reflection bands (of minerals other than those containing SiO 2 ) is fortu- 

 itous is unknown. A knowledge of their dielectric constants might 

 aid in deciding this point. In considering this question of the selective 

 reflection beyond 7 p., it will be recalled that the "general absorption" 

 of many substances was found 1 to increase beyond this point. 



The results as a whole show that there are not such definite bands, 

 whether found by reflection or by absorption, in the silicates as one 

 would expect. In other words, the silicon radical seems to be differ- 

 ently bonded in each mineral. Possibly there are several radicals SiO 2 , 

 SiO 3 , SiO 4 one or more of which are present in each mineral, or even 

 in different specimens of the same mineral. This would explain the 

 lack of constancy of the occurrence of the bands of quartz at 3 p.. 



The investigation has added one more radical which has definite 

 absorption bands in the infra-red, viz, SO 4 , which has harmonic bands 

 at 4.55 and 9.1 /*. 



TRANSPARENCY TO X-RAYS. 



The majority of these silicates and sulphides were examined under 

 X-rays, and all but graphite were found to be opaque. Of course, the 

 samples were large, which means that in thin sections, no doubt, many 

 of the silicates would be as transparent as glass. 



REFLECTING POWER OE METALS. 



The reflection power of various metals and alloys, which can be 

 easily produced in the form of concave mirrors, has been measured by 

 Hagen and Rubens. 2 The list does not include Co, Zn, Cd, Al, Sn, 

 Pd, and Ir, the reflecting power of which in the form of plane mirrors 

 is herewith presented. 



1 Infra-red Investigations, Washington, 1905. 



2 Hagen and Rubens: Ann. der Phys., 8, p. I, 1902; n, p. 873, 1903. 



