Dr. S. P. Thompson on Hyperphosphorescence. 105 



vations of M. Becquerel with respect to uranium salts, ob- 

 servations which have since been so remarkably extended. 

 While agreeing with the Rontgen rays in the property of 

 penetrating aluminium, zinc, and other opaque materials, and 

 in exercising photographic actions, the Becquerel rays differ 

 in the circumstance that they can be refracted and polarized. 

 Whatever the Rontgen rays may eventually prove to be, the 

 Becquerel rays consist of transverse waves of an exceedingly 

 high ultra-violet order. 



The circumstance that the strongest fluorescent effects are 

 found in the compounds of two metals having such heavy 

 atomic weights as platinum and uranium, when correlated 

 with the other circumstance that the absorbing power towards 

 .T-rays is greatest in elements of the greatest atomic weights, 

 naturally suggests a new application of the law of reciprocity 

 between emission and absorption. If that law can hold good 

 in the phenomena of the Rontgen rays, or of the closely- 

 related Becquerel rays, one would argue that the best sub- 

 stances to employ as emitters of such radiations would be 

 those substances which absorb them most freely. Now the 

 property of emitting Rontgen rays has been observed in many 

 substances, but always under the stimulation of the kathodic 

 discharge. In Rontgen's original research glass was the 

 radiator. Porter and Jackson independently found platinum- 

 foil to be superior. Roiti has found porcelain and mica also 

 to serve. The writer has observed Rontgen rays to be 

 emitted from the following substances exposed to kathode 

 discharges : — calc-spar, apatite, rubies, sapphires, diamonds, 

 uranium glass, scheelite, tourmaline, a phosphorescent enamel 

 containing 60 per cent, of sulphide of calcium, sulphide of 

 zinc (hexagonal blende), zinc, aluminium, copper, iron, mag- 

 nesium, and platinum. Of the metals in the above list, iron 

 and platinum appeared to work better than copper, aluminium, 

 or magnesium. The low melting-points of the last two 

 render them unsuitable. Metallic uranium would have been 

 tried had it been possible* to obtain a specimen ; but all 

 inquiries in London proved fruitless. Of the other substances 

 named, the phosphorescent materials seemed to have some 

 advantages over ordinary glass, but they are not so convenient 

 to manage as the metals. Apatite was tried because, consist- 

 ing as it does chiefly of phosphate of lime, it was thought that 

 the A'-rays emitted from its surface could be more certainly 



* [While these lines have been going through the press, a specimen of 

 metallic uranium has been given me by Mr. C. Vautin. It emits .r-rays 

 freely under the kathode discharge.— 3. P. T.] 



