AN EXPERIMENTAL STUDY OF UADIO-ACTIVE SUBSTANCES. 157 



electroscope inclosed in a metallic box with glass sides and communicat- 

 ing with a metal disk exterior to the box. This disk lies in a horizon- 

 tal plane a few centimeters above another parallel disk, which serves 

 as a support for the substances under examination. When the elec- 

 troscope is charged by rubbing the upper disk with a piece of ebonite 

 the gold leaf diverges from the perpendicular and will remain so for 

 some time if undisturbed; on placing a layer of a radio-active body on 

 the insulated lower disk the air between the two disks becomes a con- 

 ductor and the gold leaf at once resumes its normal position. To 

 estimate the rapidity of the displacement of the gold leaf, a microscope 

 fitted with a micrometer eyepiece is attached to the apparatus at right 

 angles to the axis, and with the aid of a watch beating seconds the time 

 is noted which the gold leaf takes to reach a certain point on the scale 

 of the micrometer. 



Becquerel first announced that the rays given out by uranium 

 exhibited the phenomena of polarization, reflection, and refraction, 

 but this was not confirmed by other observers, and on repeating his 

 experiments with radium and with polonium Becquerel got contra- 

 dictory and negative results. The French chemist observed that the 

 rays emitted by difl'erent bodies are very unequally absorbed; the rays 

 of radium and of uranium freely penetrate plates of quartz, fluorite, 

 and mica, but those of polonium are absorbed by these minerals and 

 scarcely penetrate paper. On the other hand, rays of polonium pass 

 through aluminum more freely than those of uranium. 



The rays of divers origin are also influenced in different ways by a 

 magnetic field; in an irregular magnetic field formed by a powerful 

 electro-magnet, the rays emitted by radium are deflected and concen- 

 trated on the poles. To show this Becquerel devised ingenious experi- 

 ments giving photographic records. (C. R. 130, 996, Deceml)er 11, 

 1899.) On examining the rays of polonium compounds (furnished by 

 Madame Curie) he found that polonium acted differently from radium 

 (C. R., December 26, 1899), and his results failed to confirm the obser- 

 vations of Geisel previously announced. Later Madame Curie also 

 published a note on the dissimilar behavior of the rays of polonium 

 and of radium in a magnetic field. (C. R. 130, 73, Januar}' 8, 1900.) 

 The subject has also been studied at Vienna by Stefan Meyer and Egon 

 R. von Schweidler. (Phys. Ztschr. 10. 113.) 



Becquerel rays excite phosphorescence in gems, minerals, barium 

 sulfid, calcium sulfid, etc.; in fluorite the phosphorescence remains 

 twenty -four hours after the influence of the radium has been removed, 

 exactly as when exposed to the light of the electric arc. 



In studying the power that these rays have of communicating energy 

 to inactive bodies, Madame Curie worked with substances so well puri- 

 fied that they were 50,000 times more powerful than uranium, and the 



