Rays of Radioactive Substances. 319 



In the case of radium a capacity of '005 microfarad was 

 in parallel with the electrometer. 



The results for uranium are in agreement with the view 

 that each portion of the mass is sending out electrons 

 uniformly. The number of electrons which escape is at first 

 proportional to the thickness, but tends towards a maximum 

 as the electrons from the lower layers are absorbed before 

 reaching the surface. 



The table for radium shows, as far as it goes, a similar 

 action, only in that case we did not have a sufficient amount 

 of substance to investigate the effect of thicker layers. In 

 this sample of radium (from P. de Haen, Hannover) the 

 de viable rays were 250 times as intense as from an equal 

 weight of uranium oxide. 



§ 4. Deviable Rays from Thorium Compounds. 



Thorium oxide is much weaker in deviable rays than an 

 equal weight of uranium oxide, although the non-deviable 

 rays are of about the same intensity. For this reason the 

 presence of the deviable rays in thorium is more difficult to 

 detect and measure than for uranium. In measurements on 

 thorium compounds precautions must be taken that the 

 presence of the radioactive emanation and the exciied radio- 

 activity produced by it are not responsible for the deviable 

 rays observed. 



About 5 gr. of the thorium compound was spread uniformly 

 in a rectangular lead vessel 3 cms. long, 2 cms. wide, and 

 1 cm. deep., and a very thin plate of mica waxed down over 

 the top. This allowed most of the deviable rays to pass 

 through, but absorbed most of the non-deviable rays, and was 

 impervious to the emanation. Observations were taken as 

 soon as possible after the thorium was placed in the vessel, 

 and the difference of current observed with the magnetic 

 field off and on. By means of side-tubes in the lead vessel 

 a slow current of air was then passed over the thorium, 

 carrying away the emanation. The amount of deviable rays 

 was found to be unchanged, showing that the deviable rays 

 (if any) from the emanation did not appreciably affect the 

 result. If the side-tubes of the lead vessel were closed, 

 and the thorium left undisturbed for 24 hours, the amount of 

 deviable rays was considerably increased. This increase was 

 found to be due to the deviable rays given out by the excited 

 radioactivity produced by the emanation on the whole interior 

 of the containing vessel. This wa- directly tested by re- 

 moving the mica plate from the thorium vessel and placing 

 it between the poles of the electromagnet. It was found to 



