ON THE RADIO-ACTIVITY OF MATTP:R.' 



By Henki BKCguEKKi., D. C. L.. Pli. I). 



The property })()s.s('ssed by certuiu Ixxlics of ciiiittiiiy- invisible iind 

 penetrating rays was univnown six years a<i(). The speculations 

 l)ronoht alxHit l»y tlie experiments of M. lioentii'en led to tlu^ exam- 

 ination of material bodies to see if any of them had (lie power of emit- 

 ting simihir radiations; tlic phenomenon of phos])h()rescence naturally 

 was first thought of, being a known method for the transformation 

 and emission of energy. This idea, however, could not be applied to 

 the phenomena with which we ai'e occu|)ied, but it was xcry fruitful. 

 It led to the choice, among phosphorescent bodies, of the salts of 

 uranium, of which the optical constitution is remarkable on account 

 of the harmonic series of the bands of their absoi'btion and phosj)hor- 

 escent spectra. It was vvdiile exj)erimenting with these bodies in iSiH) 

 that I first o})served the new phenomena which 1 am about to bring 

 before 3'()U this e\'ening. I have here the ])lates of the double sulphate 

 of uranium and potassium, obtained by Li])manirs metliod. wliicli I 

 used for my iirst experiments. 



After ha\ing placed one of these plates on the black paper which 

 covered a })hotographic plate, and leading it for sexcral hour.>. I 

 o])served on de\eh)ping the plate tliat the uranium salt had emitted 

 certain active rays, traversing the black jjaper. as well as \arious 

 screens interposed between the plate and the actixc body, such as thin 

 sheets of glass, aluminum, copper, etc. I soon saw that this phe 

 nomenon had nothing to do with ph()sphoresenc(>, oi- with any known 

 method of excitation, such as luminous or electi'ic rays, or any a])pr(^- 

 cia])h» variation of temperature. 



1 liad to deal, therefore, with a spontaneous ])lienomenor, of a new 

 order. The absence of any known exciting cause on a body prepared 



" A discourse delivered in Frencli licl'mc llic iiiciiihcrs <il' (lie Udval Iiistitntion, 

 London, Friday, March 7, 1902. Hcjiriiitcd, Ky pciinis-ion, after icvisiim ol text liy 

 Professor Becqnerel, from translation in Scientific Ainciiran SiipiilcnH'nt, No. l.'>7'.l, 

 June 7, 1902. Illustrations from original article in I'roceedinjrs of tlic Knya! Institu- 

 tion of (ireat Britain. 



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