AN EXPERIMENTAL STUDY OF RADIO-ACTIVE SUBSTANCES. 159 



be made of two papers hy German chemi.sts. Fritz Gei.sel obtained 

 radimu from uranium ores other than pitchblende, and remarks: 



"Freshly crystallized barium salts containing- radium are only 

 slig'htly active, but in a few days or weeks they reach a maxinuun. 

 They are strong^est when anhydrous; moisture stops activity and heat- 

 ing restores it." (Ann. Phys. Chem. 69, 91, 1899.) 



Becquerel rays have the same intensity in partial vacuum as at ordi- 

 nary air pressure. This was proved by electrical and photographic 

 experiments made by J. Elster and H. Geitel. (Wiedemann's Ann. 66, 

 135, 1898.) 



Through the enterprise and liberality of the Smithsonian Institu- 

 tion, and by the courtesy of Secretary Langley, 1 have enjoyed the 

 opportunit}" of studying small specimens of these rare and costly sub- 

 stances. They comprised 10 grams of ' ' radio-active substances " in two 

 portions, prepared b}^ E. de Haen, manufacturing chemist, of Hano- 

 ver, Germany, and -l grams of "'chlorure de barium radifere '■ and 4 

 grams of "polonium subnitrate" from the Societe Centrale de Pro- 

 duits Chimiques (ancienne Maison Rousseau), Paris, said to be pre- 

 pared according to the instructions of Madame and Monsieur Curie. 



The samples from Hanover were marked "A'" and " B," respec- 

 tively, and a memorandum accompanying them stated that "B " excites 

 liuorescence in barium-platino cyanid more energetically than "A," 

 whereas the latter is self-luminous. As a matter of fact, I found l)oth 

 luminous in the dark, and " B" the brighter of the two. 



The specimens were inclosed in hermetically sealed bottles and pro- 

 tected from light by strawboard cylinders. On removing the wrap- 

 pings in a dark room both were seen to emit greenish-white light that 

 gave to the enveloping papers a peculiar glow, similar to the fluores- 

 cence produced by Rontgen rays. I here call special attention to the 

 fact that during all the time that I have had the substances under 

 examination they have been kept in the dark, no light reaching them 

 stronger than that of the yellow and orange-red of a photographic 

 dark room, so that insolation has played no part in renewing their 

 energy. 



The grayish-white powders proved to be wholly soluble in water, and 

 the solution gave the usual reactions for barium chlorid. 



Moistening the radium chlorid with cold water does not immediately 

 stop emission of light, but on heating to boiling the luminosity ceases. 

 On expelling the water and heating in a platinum dish to dull redness, 

 the material resumed its luminosity after a few days "in the dark. 

 The fact that radium compounds resume their power of emitting light 

 slowly has been noted by Geisel, but he fails to state whethei- the salt 

 regains its property without exposure to sunlight. 



The substances "A" and "B" wei'e examined with a fluoroscope at 

 Jirst without success, but in a perfectly dark room, after the eye 



