Radioactive Minerals in Common Rocks. 903 



small time taken for half a revolution, a deflexion in the gold 

 leaf electroscope of about ten divisions per minute would 

 have been observed. This experiment shows that of whatever 

 nature the effect inside the atom may be, it certainly is in the 

 case o£ copper and zinc rapidly damped and cannot continue 

 in sufficient strength to give l/250th of the original secondary 

 radiation after a period of 1/3000 second. 



In conclusion, our best thanks are due to Professor Barkla 

 for his valuable advice and his interest in the experiments. 



Wheatstone Laboratory, 

 Kind's College, 

 April, 1910. \ ; 



; 



XCIV. Radioactive Minerals in Common Rocks. 

 By J. W. Waters, B.Sc, A.R.C.S.* 



IN the ' Philosophical Magazine ' for October, 1909, the 

 author described a physical separation of the minerals 

 in some common rocks, according to their radioactivity. 

 It was found that the radioactivity was in each case con- 

 centrated in certain accessory minerals. The rocks were 

 crushed, and the minerals were separated by taking ad- 

 vantage of their different densities and magnetic properties. 

 The same methods have now been applied to a specimen of 

 granite from Dalbeattie, and also to a tertiary granite from 

 the Mourne Mountains. In each case the radioactivity is 

 found to be concentrated in the heavier minerals. 



In the Dalbeattie granite, the most radioactive constituent 

 was a feebly magnetic mineral, which was shown by micro- 

 scopic and spectroscopic examination to be allanite. When 

 the mineral was spread over the tray of a gold-leaf electro- 

 scope, the rate o£ leak of the electroscope was 100 scale- 

 divisions per hour. The other heavy minerals present were 

 magnetite, sphene, and apatite, which were only feebly 

 radioactive. 



In the Mourne granite the most active constituent was 

 very heavy and feebly magnetic. It was found by micro- 

 scopic and spectroscopic examination to consist of zircons 

 mixed with a small amount of a titanium mineral. The rate 

 of leak of the electroscope in this case was 600 scale-divisions 

 per hour. The mineral was radioactive enough to produce 

 frequent scintillations on a zinc sulphide screen. 



The object of these experiments was to accumulate material 

 for finding the helium ratio and hence the age of ordinary 



* Communicated by the Hon. R. J. Strutt, F.R.S. 



