THE ADVANCE OF SCIENCE 71 



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 elec- 

 troscope. But the most remarkable feature about them 

 is their power of penetrating substances opaque to ordi- 

 nary 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 phos- 

 phorescent 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-rays, 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 be- 

 sides the glowing vacuum-tubes of the electricians' labo- 

 ratory 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 ' phos- 

 phorescent paint,' which contains sulphide of calcium, a 



