560 Dr. A. S. Eve on the Number of Ions produced by 



in ionizing, the radii of the molecules would have to be as 

 small as 8 x 10 ~ 10 cm. 



If the supposition were made that a y ray is an entity 

 with a linear path, and if it were further assumed that 

 3*4 x 10 10 such entities were ejected from a curie of radium 

 every second, it follows that one such entity, on an average 

 path of 250 metres, makes 10*6 x 10 14 /3-4 x 10 10 , or 30,000 

 ions, or 1*2 ions per cm. of path. In this case only one 

 collision in every 7000 is effective in producing an ion. 



In Townsend's work on ' Ionization of Gases by Collision,' 

 it appears to be proved that at low pressures practically every 

 encounter of a negative ion is effective in ionizing, provided 

 the velocity exceeds a certain limit. In seems certain, there- 

 fore, that the (3 particle pierces the molecule in its flight, 

 and that ionization results only in about 2 per cent, of the 

 collisions. 



Heating Effect. 



It has been shown by Rutherford and Geiger * that the 

 a particles from a gramme of radium and from its three 

 succeeding a-ray products, produce 244 x 10 14 ions per second. 

 The heating effect is about 110 calories per hour, the greater 

 part of which is due to the a particles. It appears from 

 recent work by Geiger f that the a. particles expend nearly 

 the whole of their energy in ionization. It is thus possible 

 to calculate from my determinations the heating effects due 

 to the j3 and y rays from a curie of radium C, making the 

 assumption that the energy expended in ionization is a 

 measure of their heating effects when absorbed. 



Rays. 



Total ions per second. 



Calories/hour. 





244 X10 11 



5xlO u 



10-2 X10 14 



103-5 



2-0 

 4-5 



8 







110 



Thus, the /3 and y rays together contribute 6 per cent, of 

 the total heat, and the 7 rays alone 4 per cent. J This result 



* Peoc. Roy. Soc. A. lxxxi. p. 141. 

 •f- Proc. Roy. Soc. A. lxxxiii. p. 505. 



% Rutherford, ' Radioactivity,' p. 200, has calculated the energy 

 ratio for a and j3 particles as 83 for a /3 particle with velocity PoxlO 9 . 



