Secondary Rays excited by a Rays of Polonium. 40 L 



An expression for o- x in terms of the density of the sub- 

 stance will be obtained in a subsequent paper*. We shall 

 then also obtain some information about the form of the 

 expression for V , using the critical point conditions given 

 in this paper. These conditions will also be used to deter- 

 mine the approximate form of the arbitrary function in the 

 law of molecular attraction given by the writer f . 



London, April 10, 1912. 



XXX \ r . On the Secondary Rays excited by the a Rays of 



Polonium.— 11. By V. E. Found, M.A. 



1. Introduction. 



IN a previous paper by the writer § it has been shown that 

 when the a, rays of polonium strike a carbon or a brass 

 plate secondary rays are produced which are similar to the 

 8 rays of polonium. It has been shown, too, that a very 

 considerable part of this secondary radiation is due to the 

 presence of gas occluded at or deposited on the surface of 

 the brass or carbon plate. The present paper describes some 

 further experiments on the influence of this gaseous layer on 

 the secondary radiation excited in different substances when 

 these are subjected to bombardment by the a rays of 

 polonium. 



2. Description of Apparatus. 



The apparatus used for the investigation was composed of 

 a brass tube A about 3 cm. in diameter and 18 cm. long, 

 one end of which was closed air-tight by an ebonite plug- 

 sealed in with wax. Centrally through this plug was led a 

 brass rod, B, carrying a brass plate, C, upon which a thin 

 circular piece of polonium-coated brass about 1*5 cm. in 

 diameter was fixed. A brass ring, D, passed through the 

 ebonite and connected to earth served as a guard ring. At 

 the other end of the tube A a smaller tube E, 1*5 cm. in 

 diameter, was inserted and soldered in position at the top so as 

 to be co-axial with the former. In this way the space between 

 the tubes A and E was made air-tight. The lower end of 



* Proc. Camb. Phil. Soc. toI. xvi. pt. vii. 

 t Ibid. pt. viii. 



j Communicated by Professor J. C. McLennan. Read before the 

 Canadian Institute, Toronto, January 13, 1912. 



§ Trans. Canadian Institute, 1912 ; Phil. Mag. May 1912. 



