i 4 6 HUXLEY MEMORIAL LECTURES 



development as a spherical shell or envelope. 

 But here we learn that there is probably a certain 

 difference in the course of events attending the 

 immediate passage of the ray in the gas and in 

 the solid. In the former initial recombination 

 may obscure the intense ionisation near the end 

 of the range. We can only detect the true end 

 effects by artificially separating the ions by a 

 strong electric force. If this recombination hap- 

 pened in the mineral we should not have the 

 concentric spheres so well defined as we see them 

 to be. What, then, hinders the initial recombina- 

 tion in the solid? The answer probably is that 

 the newly formed ion is instantly used up in a 

 fresh chemical combination. Nor is it free to 

 change its place as in the gas. There is simply a 

 new equilibrium brought about by its sudden pro- 

 duction. In this manner the conditions in the 

 complex molecule of biotite, tourmaline, etc., may 

 be quite as effective in preventing initial recom- 

 bination as the most effective electric force we 

 could apply. The final result is that we find the 

 Bragg curve reproduced most accurately in the 

 delicate shading of the rings making up the per- 

 fectly exposed halo. 



That the shading of the rings reproduces the 

 form of the Bragg curve, projected, as it were, 

 upon the line of advance of the ray and repro- 

 duced in depth of shading, shows that in yet 

 another particular the alpha ray behaves much 

 the same in the solid as in the gas. A careful 



