iS2 HUXLEY MEMORIAL LECTURES 



now, geological time since the Devonian 

 is 50 millions of years, then 150 millions 

 of rays built up the halo. If geological 

 time since the Devonian is 400 millions of years, 

 then 1 200 millions of alpha rays are concerned in 

 its genesis. The number of ions involved, of 

 course, greatly exceeds these numbers. A single 

 alpha ray fired from radium C will produce 2*37 x 

 io 5 ions in air. 



But haloes may be found quite clearly defined 

 and fairly dark out to the range of the emanation 

 ray and derived from much less quantities of 

 radioactive materials. Thus a zircon nucleus 

 with a diameter of but 3*4 x icr 4 cms. formed a halo 

 strongly darkened within, and showing radium A 

 and radium C as clear smoky rings. Such a 

 nucleus, on the assumption made above as to its 

 radium content, expels one ray in a year. 

 But, again, haloes are observed with less black- 

 ened pupils and with faint ring due to radium C, 

 formed round nuclei of rather less than 2 x icr 4 

 cms. diameter. Such nuclei would expel one ray 

 in five years. And even lesser nuclei will 

 generate in these old rocks haloes with their 

 earlier characteristic features clearly developed. 

 In the case of the most minute nuclei, if my 

 assumption as to the uranium content is correct, 

 an alpha ray is expelled, probably, no oftener 

 than once in a century; and possibly at still 

 longer intervals. 



The equilibrium amount of radium contained 



