Radioactivity of the Leinster Granite, 



107 



of 11"87 X 10 -12 gr. per gr. was from a large hexagonal 

 crystal of biotite, with a maximum width of 2*5 cms., enclosed 

 in a crystal of muscovite. It revealed countless pleochroic 

 halos under the microscope, and showed considerable radio- 

 active darkening along cracks, and in the neighbourhood of 

 the junction. 



.Professor Joly obtained for a specimen of the Ballyknockan 

 granite, using a very limpid solution, a radioactivity of 

 5'5 x 10~ 12 gr. per gr. on two closely agreeing experiments 

 on the same hand-specimen ( f Radioactivity and Geology,' 

 p. 43). In another experiment on granite from this locality, 

 but using a different method of investigation, the result 

 obtained by Prof. Joly was 2'6x 10~ 12 gr. per gr. In this 

 case the melt from the crucible was broken up and closed with 

 some distilled water in a flask for 21 days, when hydrochloric 

 acid was carefully run in, the evolved gases passed over 

 soda-lime, and finally the contents of the flask briskly boiled 

 for fifteen minutes. The steam was returned by a condenser 

 attached to the flask. The gases unabsorbed were collected 

 over a potash solution and finally admitted into the electro- 

 scope. In this procedure, undissolved or precipitated matter 

 remained in the flask (see fig. 1). 



Fig. 1. 



A third method was tried upon the same rock. The gases 

 evolved during the decomposition of the rock in presence of 

 the fusion mixture were, after absorption by soda-lime &c, 

 admitted into the electroscope. In this case the charge, 

 consisting of the powdered rock and the fusicn mixture of 

 carbonates, is melted in an air-tight platinum still which can 

 be heated to a high temperature over the blowpipe. A 

 platinum tube leading from the top of the still conveys the 



