42 Mr. A. L. Fletcher on the Radioactivity of 



modification adopted by Professor Joly, of boiling — for 

 thirty minutes — with talc, in vacuo. A slight departure 

 from the method described by Professor Joly was made. 

 At the conclusion of the boiling, the cooling water was cut 

 off, and the ebullition accelerated until steam began to 

 condense in the receiver bulb, when the gas was cut off, and 

 the pinch-cock closed simultaneously. In addition to the 

 possible advantage of a brief violent ebullition, this had the 

 effect of removing any emanation from the flask and con- 

 denser, without the addition of any water to the solution ; 

 which was then ready for the estimation of contained thorium. 

 A slow transference of the emanation to the electroscopes 

 was then effected through the capillaries — the process 

 occupying about 10 minutes. A glass tube, containing a 

 water-bubble, served to indicate the moment when the electro- 

 scope was filled to atmospheric pressure. 



The Mocks. 



The following representative specimens of the rocks 

 dealt with, were selected, and submitted to microscopic 

 examination : — 



Specimen 1. — Large phenocrysts of both soda-lime and 

 lime-soda felspars with rare sanidine, in a turbid ferruginous 

 ground-mass with small tabular felspars, and brownish 

 microliths. Magnetite and red oxide of iron abundant. 

 Many indeterminate iron-stained crystals. No glass. Much 

 augite in yellowish-green phenocrysts. Borders often ferru- 

 ginous. General appearance of rock highly altered. External 

 colour light grey. Structure trachytic. 



An altered Trachyte, with some Andesitic characteristics. 



Specimen 4. — Ground-mass consisting of grains of felspar, 

 and turbid glass with indeterminate ferruginous particles. 

 The whole stained red-brown with iron oxide. Many frag- 

 ments of andesitic lava. External colour brown. 



A Felspathic Tuff, much altered. 



Specimen 7. — A fine-grained homogeneous ground-mass, 

 consisting mostly of tabular felspar — sometimes banded — 

 with some turbid glass. Considerable calcitic matter de- 

 veloped. Iron-stained particles common. External colour 

 red. 



A fine-grained, altered, Andesitic Tuff. 



Specimen 8. — Many twinned phenocrysts of soda-lime and 



