﻿106 On the Chemical Composition of certain Cornish Rocks, 



The oxide of manganese, or oxide of nickel,, was obtained from 

 the filtrate from oxide of iron and alumina and excess of car- 

 bonate of barium, after the removal of baryta in the form of sul- 

 phate of barium. 



Oxide of chromium was estimated by fusing in a platinum cru- 

 cible about 100 grains of pulverized rock with four times its 

 weight of carbonate of sodium, and half that quantity of nitrate 

 of potassium. 



The fused mass was then treated by boiling water, a certain 

 amount of alcohol added, and the whole heated gently for several 

 hours. The insoluble matter was subsequently separated by 

 nitration, acetic acid in excess added to the filtrate, and the 

 chromium precipitated as chromate of lead by the addition of a 

 solution of the acetate of that metal. 



The estimation of sulphur was effected by attacking a weighed 

 portion of the pulverized rock with nitrohydrochloric acid and, 

 after removing the insoluble matter by nitration, precipitating 

 the sulphuric acid as sulphate of barium ; from the weight of 

 this the amount of sulphur originally present was calculated. 



In the earlier analyses the alkalies were estimated by fusion 

 with baryta in the usual way; but after having made several 

 estimations of potassa and soda in the same rock by the two pro- 

 cesses, the following was ultimately adopted as being more ex- 

 peditious and at the same time equally accurate. 



A convenient weight (from 30 to 35 grains) of the finely pow- 

 dered rock was thoroughly attacked in a platinum dish by strong 

 hydrofluoric acid, and, after the addition of sulphuric acid, evapo- 

 rated to dryness and ignited over a gas-flame ; the residue was 

 now treated with distilled water which was well boiled, taking 

 care to break down all lumps by means of a glass rod. 



The whole was transferred without filtration to a large beaker, 

 and excess of ammonia and carbonate of ammonium added. 



After the separation by filtration of insoluble matter, and of 

 the precipitate by ammonia and carbonate of ammonium, the fil- 

 trate was evaporated to dryness and the ammoniacal salts expelled 

 by ignition. The residue was treated with sulphuric acid, and 

 again evaporated to dryness and ignited to drive off excess of 

 sulphuric acid, dissolved in water, baryta-water added in excess, 

 and filtered. The filtrate was acidified with acetic acid, evapo- 

 rated to dryness, ignited, and the carbonate of barium and char- 

 coal separated by filtration ; the alkaline carbonates in the filtrate 

 were now decomposed by hydrochloric acid, and the whole. eva- 

 porated to dryness. A few drops of solution of carbonate of 

 ammonium were then added, and the platinum dish strongly 

 heated to drive off ammoniacal salts. The residue was treated 

 with water, any insoluble matter separated by filtration, and the 



