380 UNSUSPECTED RADIATIONS. 



the explanation that used to be j^iven some time ago/ Now, it Avas in 

 this rather neglected domain that Henri Bccquerel discovered the won- 

 derful radiations Avhieh have received his name, and which, owing to 

 the speculations they provoked as regards the th(H)ry of matter, liave 

 engrossed for the last four years the attention of ])hysicists, cA'en more 

 than the Kontgen rays themselves. 



It will be remembered that a phosphorescent screen wliicii began to 

 glow in the proximity of a vacuum tube upon which Kontgen was 

 experimenting led lum to his memorable discovery. It was only nat- 

 ural, therefore, to see wh(»ther phosphorescent screens would not 

 reinforce th(» X-rays; and in the cours<M)f such experiments M. Henry 

 noticed that a phosphorescent sulphide of zinc gave up radiations 

 which, like the Rontgen rays, would pass through black paper and 

 affect after that the photographic plate." M. Nic^wenglowski, also at 

 Paris, made the same remark concerning a sulphide of lime previous!}^ 

 exposed to light.'' Then, at the next sitting of the Paris Academy of 

 Sciences Henri Becquerel came forward Avith a work on the radiations 

 emitted by ])hosphorescent substances,' and this lirst work was fol- 

 lowed })y (juite a iuun))er (d" papers in which the new radiations were 

 studied under all possible aspects. Becqum-el was joined in !us 

 researches by many othei-s, and especially b}- Mme. Sklodowska-Cairie 

 and her husband, M. Pierre Curie, Avho soon disi'overed. with the 

 aid of the new radiations, two new elements, and l)y this time the 

 "Becquerel rays" have already a bulky literature. During the past 

 year nearl}' every Aveek brought with it the discovery of some new and 

 puzzling property of these radiations.' 



The main point of the discovery was that phosphorescent l)odies 

 emit not only the well-known glow, which is \-isihle to our eye, l)ut 

 also invisible radiations, similar to the Rontgen ray. Some salts of 

 the metal uranium, and the metal itself, need not be exposed to light 



^The terms "phosphorescence" and "fluorescence" are rather indiscrhiiinately 

 used to describe i^lowing after an exposure to light, as the distinction between the 

 two, jiroposed by Wiedemann, can not ]je maintained any longer. Other cauKes may 

 also provoke "himinescence: " the diamond glows after having been sHghtly Iieated, 

 quartz after some rublnng, and gases when they are electrifieil. As to the many 

 luminescent animals, such as the glowworm, various marine animals and baciteria- 

 we are not concerned with them now. 



^Comptes Rendus of the Parij Academy of Hciences, the 10th of February, 1896, 

 Vol. CXXII, p. 312. 



='Ibid.,CXXII,p.386. 



^Ibid., the 24th of February, 1896, Vol. C^XXII, p. 420. Further <-ommuiiications in 

 the same and subsequent volumes. 



•'•The literature of the subject is already immense. The main t^ontributiouH to 

 it will be found in Comptes Rendus, Philosophical Magazine, and Annalen der 

 Physik. Excellent articles for the general reader appeared in Nature, the 14th of 

 June, 1900, and in Revue Generale des Sciences, the 30th of January, 1899, by Mme. 

 Sklodowska-Curie. 



