394 Prof. A. S. Eve on the Number of Ions produced 



negligible. The first supposition is the one which is always 

 made, although it may on occasion lead to difficulty, as, for 

 example, in the case of the flow of viscous fluid between two 

 parallel planes which, as is well known, is mathematically 

 stable for small disturbances, but is experimentally unstable 

 if the velocity exceeds a certain value. If the real part of 

 k had been negative, the wave would necessarily have been 

 unstable ; but, although it is actually zero, it is not therefore 

 absolutely certain that the wave is stable. 



The second supposition, that a 4 is negligible, may be 

 justified by the considerations that a 2 does not exceed one- 

 tenth, and that in the final equation which is found for k the 

 coefficient of a 2 (a does not occur to any odd power in this 

 equation) is of the same order of magnitude as the term 

 independent of a. 



XLII. On the Number of Ions produced by the Gamma Radia- 

 tion from Radium. By A. S. Eve, D.Sc, Macdonald 

 Professor of Physics, McGill University, Montreal*. 



IF q ions are produced, directly or indirectlv, in a cubic 

 centimetre of air, at standard temperature and pressure, 

 at a distance of r cm. from a source of Q gm. of radium, 

 then 



pKQ/rV, 



where K is a constant, and fju is the coefficient of absorption 

 of the 7 rays in air. 



Also the total number of ions produced in air by the 

 7 rays from Q gm. will be found by integration to be 

 N = 4 7 rKQ/ A t. 



The first determination f of K was made in 1906 with an 

 aluminium vessel, 0'4 mm. thick, and for radium bromide, 

 assuming <? = 3"4 x 10~ 10 , the value was 3'1 x 10 9 . This 

 is equivalent to K = 3'8 x 10 9 for a gramme of radium with 

 6 = 4-7 xlO" 10 . 



The second and third determinations % , made in 1911, with 

 very thin- walled testing vessels, gave values K = 3*74xl0 9 

 and 3*81 x 10~ 9 respectively. Accepting Ohadwick's value 

 for ft as '000060 the corresponding value of N is 7'8 x 10 14 . 



In Rutherford's 'Radioactive Substances ' (1913), p. 295, 

 it is stated that Moseley and Robinson have found 

 N = 13xl0 14 . This corresponds to K=6'2xl0 9 , a value 



* Communicated by the Author. 

 f Phil. Mag. September 1906. 

 % Phil. Mag. October 1911. 



