﻿102 Mr. J. A. Phillips on the Chemical Composition and 



versed in all directions by veins of milky quartz, some of which 

 afford good specimens of tourmaline and of well- crystallized 

 cassiterite, which is usually associated with the former ; crystals 

 of tourmaline are found in the felspar, as well as in the veins of 

 quartz by which it is intersected. 



These workings are locally known as the " Glass Mine ; " and 

 the felspar, after being separated from quartz and fragments 

 stained by oxide of iron, by careful hand-picking, is shipped for 

 use in the manufacture of certain descriptions of pottery. 



Professor W. H. Miller, P. U.S., of Cambridge, who kindly 

 made for me a crystallographic examination of this mineral, 

 found it difficult to obtain very accurate measurements, on ac- 

 count of the imperfection of the faces, but pronounced it to be a 

 monoclinic felspar. 



Its chemical composition was found to be as follows : — 





I. 



II. 



Oxygen ratio 









of mean. 



Water* . . , 



. -83 



•50 





Silica . , . , 



65-00 



65-33 



34-84 



Alumina . . , 



19-00 



19-16 



9-10 



Ferric oxide . . 



•50 



•50 



•15 



Lime . . . 



, 1*57 



1-68 



•48 



Magnesia . . 



, trace 



trace 





Potassa • • , 



10-37 



10-37 



1-76 



Soda . . . 



. 2-40 



2-40 



•62 



99-67 99-94 



From the above ratio we obtain the following : — 



Silica 12-18 



Sesquioxides . . . 3*19 

 Protoxides . . .TOO 

 This is manifestly in the proportion of 1 : 3 : 12 ; and the fel- 

 spar is consequently orthoclase. 



In making the foregoing analyses the following general rou- 

 tine was adopted. 



The amount of moisture present was ascertained by heating a 

 weighed quantity of the finely pulverized rock in a water-bath 

 until it ceased to lose weight. Another weighed portion was 

 moderately ignited, in a platinum crucible, in one of Griffin's 

 gas-furnaces, and the loss of weight noted. 



About thirty grains, more or less, of the finely powdered rock 

 was intimately mixed, in a platinum crucible, with from five to 

 six times its weight of pure carbonate of sodium, and fused at a 

 high temperature in a gas-furnace. 



* Of which '33 was lost in the water-bath. 



