Radio-activity and the Electron Theory. 419 



Thoria prepared by the ignition of Dr. Knofler's highly purified 

 thorium nitrate* was now tested in a similar apparatus to the one last 

 described, the only alteration being the removal of the covering 

 aluminium plates, thoria not being self-luminous. The experiment 

 was continued for 108 hours, 1125 litres of air being drawn through 

 the cells during the time. 



Measurements of the developed images gave the following results : 



Circle over cell A Opacity log. = 0-306 ; Opacity = 2'02. 

 Circle over cell B Opacity log. = 0-260 ; Opacity = 1-82. 

 Circle over cell C Opacity log. = 0*046 ; Opacity = O'lO. 



Here also the results agree with those tried with radium and actinium 

 compounds, that corpuscles are carried by a current of air from cell B, 

 through the connecting tube to cell C. They also confirm those of 

 Dr. Rutherford who finds that thorium emanations travel in a current 

 of air while retaining their activity and of P. Curie and A. Debierne, 

 who show that induced radio-activity can be transmitted through capil- 

 lary tubes of an internal diameter of O'l mm. and 75 cm. in length, 

 bent once at right angles. 



I have not obtained, however, a similar result with the emanations 

 from hydrogen peroxide. As shown by Dr. Russell, this substance has 

 a strong action on a sensitive photographic plate. The emanation 

 from a bottle half full of hydrogen peroxide acts strongly on a sensi- 

 tive film laid over the open mouth of the bottle for 24 hours, while 

 there is no action in 72 hours if a U-shaped tube passed through the 

 cork of the bottle and the sensitive film is put close to the open end of 

 the tube. Dr. Russell tells me his observations confirm my experi- 

 ments. 



A highly active self-luminous radium compound loses some of its 

 power on long exposure to the ordinary air of the laboratory. Igni- 

 tion to red-heat restores, however, its self-luminosity, and when sealed 

 in a vacuum its activity remains as great as at first. When enclosed 

 in glass the glass soon assumes a pink colour, which is not superficial, 

 as already observed by M. and Mdme. Curie. By immersing a section 

 of the glass in a liquid of the same refracting power as itself, the 

 colour is seen to penetrate below the surface. If, however, the radium 

 compound is sealed in vacua in a quartz tube no coloration takes place, 

 and I can detect no diminution of energy even in 12 months. 



Electrons from radium will pass through aluminium and a considerable 

 length of air and affect a sensitive film.t Experiments on this point 

 were tried with polonium, and it was found that air offered great 

 obstruction. 



* ' Roy. Soc. Proc.,' vol. 66, p. 421, May, 1900. 



f Using an active compound of radium, I have obtained an impression on a 

 sensitive film through a penny-piece. 



