344 Messrs. McLeiuiMii and Burton on the ' 



which, traversing the air in the room, passed throiigli the 

 walls of the receiver. It was also shown that the conductivity 

 of fresh atmospheric air, after being confined in a metallic 

 cylinder, rapidly fell to a minimum value, that a gradual rise 

 then took place, and that a steady state was reached after some 

 hours. 



On account of the known decay of the constituent in 

 atmospheric air responsible for excited radioactivity, the fall 

 in the initial conductivity w^as attributed to its presence in 

 the cylinder, and the subsequent rise to an emanation of a 

 similar character given off by the walls of the containing 

 vessel ; the steady state representing a condition of equilibrium 

 where the rate of decay of this emanation was equal to the 

 rate at which it was produced. 



It was also pointed out that the limiting value of the con- 

 ductivity thus reached varied with the metals forming the 

 walls of the receiver. In experiments with lead, tin, and 

 zinc the conductivity was highest with lead and lowest with 

 zinc. In view of these results and on account of the known 

 complex character of the radiations from such highly radio- 

 active substances as radium and thorium, a series of expe- 

 riments was made to investigate still further the radioactivity 

 of ordinary metals. 



As a result of these experiments we find that when a 

 cylinder of any metal is inclosed within a second of the same 

 material, insulated from it, and surrounded by air or other 

 gases, it gradually acquires a negative charge, and after a 

 short time reaches a state of equilibrium at a definite potential 

 below that of the inclosing cylinder. 



So far the experiments made indicate that the negative 

 charge acquired by the cylinder results from a process in 

 which an excess of positively charged corpuscles is expelled 

 from its surface. This process is probably identical with 

 the a radiation observed by Rutherford * and others in the 

 highly radioactive substances radium, thorium, and uranium, 

 and experiments are now being made by the writers to deter- 

 mine its relation to the efiect observed by Guthrie f, and 

 developed by Elster and Geitel J, J.J. Thomson §, Richardson ||, 

 StruttH, and others, that a metal — platinum, for example, — 

 when heated to a dull red, will discharge a negatively charged 

 body placed near it, but not one positively charged. 



* Phil. Mag. Feb. 1903, p. 177. 

 t Phil. Mag. [4] vol. xlvi. p. 257 (1873). 

 t Wied. Ann. vol. xxxviii. p. 27 (1889). 

 '^ Phil. Ma^. vol. xliv. p. 203 (1897). 

 II Proc. Camb. Phil. Soc. vol. xi. p. 286. 

 H Phil. Mag. [6] vol. iv. p. 98 (1902). 



