638 Prof. J. Joly and Mr. A. L. Fletcher : 



less mica. The diverging arrangement of the darkened 

 cracks from large areas of blackened biotite situated near 

 the margin of the crystal strongly suggests the idea that a 

 radioactive solution has at some time penetrated the biotite. 

 An occasional yellowish staining of the muscovite may pos- 

 sibly indicate the presence of this solution.. Zircons, not 

 present in the muscovite, often act as radioactive nuclei in the 

 biotite, but some of the smallest nuclei are, apparently, not 

 zircon. They are, possibly, uraninite or some allied mineral. 

 The following analysis of this mica shows it to be of the 

 variety called haughtonite. The iron is nearly all in the 

 ferrous state and is calculated as such. 



SiO, 34-12 



Fed '24,-12 



A1 2 3 25-62 



MuO (V48 



KoO 13-40 



1)7-74 



The small quantity of magnesia is remarkable. The mica is 

 of a deep reddish brown colour with small optic-axial angle. 

 A careful experiment gave the density as 2*934. From these 

 data we find the average square root of the molecular weight 



to be 4*84, and the quotient » to be 1*65. Hence r' has 



o 



the value 0*000473; i. e. the range in this mica corresponding 

 to one centimetre in air. 



A determination of the radioactivity of the mica was made 

 at a sacrifice of one gram of material. The experiment was 

 made in the new Physical Laboratory (by kind permission 

 of Professor W. E. Thrift) under circumstances precluding 

 any probability of contamination. The result arrived at, on tv\ o 

 identical determinations, was 11*87 X 10~ 12 gram per gram. 

 This result gives, on the face of it, an idea of the extremely 

 small quantity of radioactive material sufficing to originate 

 a halo. It is not too much to fay that some thousands of 

 halos, or equivalent rad oactive staining, occur in a gram 

 of this mica. An endeavour to observe any local radioactive 

 intensity by placing halo-rich parts of the mica beneath and 

 in contact with a screen of sensitive zincblende gave a 

 negative result. 



Using the value obtained above for the range-equivalent 

 of 1 cm. in air, and the measurements given in the table 

 p. 632, the following are the limiting ranges and the ranges 



