Skcttion III., 1899. [ 9 ] Trans. R. S. C. 



II. — Thorium and Uranium Radiation. 



(Preliminary note) 



By E. EuTHERFORD, Ma.B.Sc, Macdonald Professor of Physics, 

 McGrill University, Montreal. 



AND 



E. B. Owens, E.E., Tyndall Fellow Columbia University, Macdonald 

 Professor of Electrical Engineering, McGill University, Montreal. 



(Presented by Professor Cox, read May 26th, 1899.) 



In 1896 Becquerel discovered that the compounds of the metal 

 uranium continuously emitted a radiation similar in character to Eont- 

 gen rays. These rays have the property of acting on a photographic 

 plate in the dark, and of making the gas through which they pass a 

 partial conductor of electricity. They also have the power of passing 

 through considerable thicknesses of metal and in general behave very 

 similarly to X-rays emitted from a so-called " soft " tube. 



In 1897 Schmidt {Wi6,d. Annal, May, 1898) found that the com- 

 pounds of thorium had the same property as uranium. In a 

 paper by one of us (Eutherford, Phil. Mag., Jan., 1899) the radiation 

 emitted by uranium has been considered in detail. The present paper 

 is an extension of the investigation to the radiation emitted by thorium 

 and its compounds, and a comparison of the two types of radiation. 



The methods of investigation are similar to those used in. the 

 previous work. The intensities of the radiations were compared elec- 

 trically by measuring the rate of discharge of electrification produced 

 by the rays. A layer of the radio-active substance was uniformly spread 

 on a small platinum plate which rested on the top of a larger brass 

 one. An insulated parallel brass plate about 4 cms. distant was con- 

 nected to one pair of quadrants of a delicate electrometer, the other 

 pair of which was earthed. The lower plate was connected to one pole 

 of a battery of 95 volts, the otheT pole of which was also to earth. When 

 the quadrants of the electrometer were separated the top plate gradu- 

 ally acquired the potential of the lower plate, and the rate of movement 

 of the electrometer needle was taken as a measure of the current through 

 the gas ; fhe gradual charging of the top plate being due to the move- 

 ment of the charged particles or ions produced by the radiation through- 

 out the volume of the gas. It has been shown {lac. cit.) that the 

 intensity of the radiation is proportional to the current through the 



