produced in a gas by y rays. 175 



The ratio of this part of the ionisation in a chamber to that 

 produced by the radiation from its walls, will depend on the ratio 

 of the total inside surface of the walls of the chamber to its 

 volume, and other conditions. The numbers obtained in the 

 experiments just described suggest, however, that in most cases 

 this ratio will probaVjly be greater than one half, or the ionisation 

 produced directly by the primary 7 rays is in most cases greater 

 than 50°/^ of the total ionisation. 



The connection of this result with other quantities will now 

 be considered. 



Eve* has made a determination of the total number of ions 

 produced per second by the 7 rays from a gram of radium bromide 

 in a volume of air which completely absorbs the rays. He 

 measured the ionisation in a cylindrical chamber 51 cm. high 

 and 23 cm. in diameter, the chamber being made of sheet 

 aluminium '4 mm. thick. Assuming the absorption of the 7 rays 

 by air to be the same as an equal mass of aluminium — whose 

 coefficient of absorption is known — and knowing the volume of the 

 chamber and its distance from the radium, etc., the number of 

 ions produced per second by the 7 rays of a gram of radium could 

 be calculated. In this way Eve obtained 8*9 x 10^^ for the number 

 of ions produced. 



The results obtained in this paper suggest that the ionisation 

 in his chamber due to the 8 rays produced directly by the 7 rays, 

 was roughly about half of the total ionisation ; or, the number of 

 B rays produced per second directly by a gram of radium is equal 

 to 4'4 X 10". This number should be as near (if not nearer) to the 

 true value of this quantity, as that given by Eve for the total 

 number of ions produced by the 7 rays of a gram of radium, in 

 which case the penetrating /3 rays from the gas are supposed 

 to spend all their energy in ionisation. 



Crowtherf has shown that the amount of ionisation produced 

 by the secondary radiation from the molecules of a gas exposed to 

 X rays is small in comparison with the total amount of ionisation 

 produced, and that the ionisation in a gas is therefore principally 

 due to the direct action of the X rays. 



This can be shown to be true also in the case of /3 rays. 

 The ionisation of the gas in a chamber exposed to /3 rays may 

 be divided into two parts. One part consists of slow-moving ions 

 or 8 rays produced directly by the primary /9 rays and the 

 secondary /S rays from the walls of the chamber. The other part 

 consists of the ionisation produced by the /3 rays of high velocity 

 ejected from the molecules of the gas. The first part is propor- 

 tional to the pressure, while the second part, from what has gone 



* Fhil. Mag. p. 192, Sep. 1906. 



t Proc. Cavib. Phil. Soc. p. 34, Vol. xv. Pt. 1, 1908. 



