26 REPORT — 1898. 



or it may linger for hours, as in the case of some of the yttria earths, and 

 especially with the earthy sulphides, where the glow lasts bright enough 

 to be commercially useful. Excessively phosphorescent bodies can be 

 excited by light waves, but most of them require the stimulus of electrical 

 excitement. 



It now appears that some bodies, even without special stimulation, 

 are capable of giving out rays closely allied, if not in some cases identical, 

 with those of Professor Rontgen. Uranium and thorium compounds are 

 of this character, and it would almost seem from the important researches 

 of Dr. Piussell, that this ray-emitting power may be a general property of 

 matter, for he has shown that nearly every substance is capable of 

 affecting the photographic plate if exposed in darkness for sufficient 

 time. 



No other source for Rontgen rays but the Crookes tube has yet been 

 discovered, but rays of kindred sorts are recognised. The Becquerel rays, 

 emitted by uranium and its compounds, have now found their companions 

 in rays — discovered almost simultaneously by Curie and Schmidt — emitted 

 by thorium and its compounds. The thorium rays affect photographic 

 plates through screens of paper or aluminium, and are absorbed by metals 

 and other dense bodies. They ionise the air, making it an electrical 

 conductor; and they can be refracted and probably reflected, at least 

 diffusively. Unlike uranium rays, they are not polarised by transmission 

 through tourmaline, therefore resembling in this respect the Rontgen rays. 

 I Quite recently M. and Mme. Curie have announced a discovery 

 which, if confirmed, cannot fail to assist the investigation of this obscure 

 branch of physics. They have brought to notice a new constituent of 

 the uranium mineral pitchblende, which in a 400-fold degree possesses 

 uranium's mysterious power of emitting a form of energy capable of im- 

 pressing a photographic plate and of discharging electricity by rendering 

 air a conductor. It also appears that the radiant activity of the new 

 body, to which the discoverers have given the name of Polonium, needs 

 neither the excitation of light nor the stimulus of electricity ; like 

 uranium, it draws its energy from some constantly regenerating and 

 hitherto unsuspected store, exhaustless in amount. 



It has long been to me a haunting problem how to reconcile this 

 apparently boundless outpour of energy with accepted canons. But as 

 Dr. Johnstone Stoney reminds me, the resources of molecular movements 

 are far from exhausted. There are many stores of energy in nature that 

 may be drawn on by properly constituted bodies without very obvious 

 cause. Some time since I di'ew attention to the enormous amount of 

 locked up energy in the ether ; nearer our experimental gi-asp are the 

 motions of the atoms and molecules, and it is not difficult mentally so to 

 modify Maxwell's demons as to reduce them to the level of an inflexible 

 law and thus bring them within the ken of a philosopher in search of a 

 new tool. It is possible to conceive a target capable of mechanically 



