PRESIDENTS ADDRESS. 7 



jusLifiwl. One element is actually under our eyes converted into anotlier ; 

 the element radium decays into a gas which changes into another 

 element, namely helium. 



Radium, this wonder of wonders, was discovered owing to the study 

 of the remarkable phosphorescence, as it is called — the glowing without 

 heat — of glass vacuum-tubes through which electric currents are made to 

 pass. Crookes, Lenard, and Rontgen each played an important part in 

 this study, showing that peculiar rays or linear streams of at least three 

 distinct kinds are set up in such tubes — rays which are themselves 

 invisible, but have the property of making glass or ether bodies which 

 they strike glow with phosphorescent light. The celebrated Rontgen 

 rays make ordinary glass give out a bright green light ; but they pass 

 through it, and cause phosphorescence outside in various substances, such 

 as barium platino-cyanide, calcium tungstate, and many other such salts ; 

 they also act on a photographic plate and discharge an electrified body 

 such as an electroscope. But the most remarkable feature about them is 

 their power of penetrating substances opaque to ordinary light. They 

 will pass through thin metal plates or black paper or wood, but are 

 stopped by more or less dense material. Hence it has been possible to 

 obtain ' shadow pictures ' or skiagraphs by allowing the invisible Rontgen 

 rays to pass through a limb or even a whole animal, the denser bone 

 stopping the rays, whilst the skin, flesh, and blood let them through. 

 They are allowed to fall (still invisible) on to a photographic plate, when 

 a picture like an ordinary permanent photograph is obtained by their 

 chemical action, or they may be made to exert their phosphorescence- 

 producing power on a glass plate covered with a thin coating of a 

 phosphorescent salt such as barium platino-cyanide, when a temporary 

 picture in light and shade is seen. 



The rays discovered by Rontgen were known as the X-i'ays, because 

 their exact nature was unknown. Other rays studied in the electrified 

 vacuum-tubes are known as cathode rays or radiant corpuscles, and 

 others, again, as the Lenard rays. 



It occurred to M. Henri Becquerel, as he himself tells us, to inquire 

 whether other phosphorescent bodies besides the glowing vacuum-tubes 

 of the electrician's laboratory can emit penetrating rays like the X-rays. 

 I say ' other phosphorescent bodies,' for this power of glowing without 

 heat — of giving out, so to speak, cold light — is known to be possessed by 

 many mineral substances. It has become familiar to the public in the 

 form of 'phosphorescent paint,' which contains sulphide of calcium, a 

 substance which shines in the dark after exposure to sunlight — that is to 

 say, is phosphorescent. Other sulphides and the minerals fluor-spar, 

 apatite, some gems, and, in fact, a whole list of substances have, under 

 different conditions of treatment, this power of phosphorescence or shining 

 in the dark without combustion or chemical change. All, however, 

 require some special treatment, such as exposure to sunlight or heat or 

 pressure, to elicit the phosphorescence, which is of short duration only. 



