J. P. Cooke, Jes on Cryophyllite. 219 
Determination of Fluorid of Silicon —A weighed amount of 
finely pulverized mineral is first decomposed in a tared plati- 
num dish with dilute sulphuric acid, and the greater part of the 
water having been removed on a steam bath and the excess of 
sulphuric acid having been driven off by gradual heating ona 
sand bath, the whole mass is ignited during at least an r 
the dish meanwhile being covered with a piece of platinum foil 
forming part of the tare. The dish with its contents having 
been weighed, the dried mass is next treated with hydrochloric 
acid and the residual silica determined in the usual way, and in 
the filtrate from the silica the sulphuric acid is determined and 
Weighed as sulphate of baryta. We thus obtain four weights: A, 
the weight of the mineral employed in the analysis; B, the weight 
of the ignited residue; C, the weight of the residual silica; and 
D, the weight of the residual sulphuric acid. It is now evident 
that A+D—B equals the weight of the fluorid of silicon, and that 
—C—D equals the weight of the sum of all the bases present. 
The iron is of course present in the residue as sesquioxyd, but 
Test of our analysis, but also enables us to test the accuracy of 
our method. In the analysis given below it appears that the 
