156 HUXLEY MEMORIAL LECTURES 



can calculate, as I have done above, the number 

 of rays concerned in forming the halo. In doing 

 so we assume some value for the age of the halo. 

 Let us take the maximum radioactive value. A 

 halo originating in Devonian times may attain a 

 certain central blackening from the effects of, 

 say, io 8 rays. But now suppose we find that we 

 cannot produce the same degree of blackening 

 with this number of rays applied in the laboratory. 

 What are we to conclude ? I think there is only 

 the one conclusion open to us : that some other 

 source of alpha rays, or a faster rate of supply, 

 existed in the past. And this conclusion would 

 explain the absence of haloes from the younger 

 rocks ; which, in view of the vast range of effects 

 possible in the development of haloes, is, other- 

 wise, not easy to account for. It is apparent that 

 the experiment on the biotite has a direct bearing 

 on the validity of the radioactive method of esti- 

 mating the age of the rocks. It is now being 

 carried out by Professor Rutherford under reli- 

 able conditions. 



Finally, there is one very certain and valuable 

 fact to be learned from the halo. The halo has 

 established the extreme rarity of radioactivity as 

 an atomic phenomenon. One and all of the 

 speculations as to the slow breakdown of the 

 commoner elements may be dismissed. The 

 halo shows that the mica of the rocks is radio- 

 actively sensitive. The fundamental criterion of 

 radioactive change is the expulsion of the alpha 



