﻿Microscopic Constitution of certain Cornish Rocks. 103 



The fused mixture was treated with dilute hydrochloric acid 

 and subsequently evaporated to dryness. The dried residuum 

 was then moistened with hydrochloric acid, water added, and 

 solution of the chlorides assisted by the application of heat. 

 Silica (a), now separated by filtration, was washed, dried, and 

 ignited. 



After adding a few drops of nitric acid to the filtrate from 

 silica, for the purpose of ensuring the peroxidation of all the iron, 

 it was evaporated to dryness, treated with dilute hydrochloric 

 acid, and filtered, thus separating a small additional quantity of 

 silica (b). 



The filtrate from the second evaporation, after separation of 

 silica b, was rendered nearly neutral by ammonia, and acetate of 

 sodium added. The solution was then briskly boiled, and the 

 precipitated basic acetates of iron and aluminium were collected 

 and washed. This precipitate was dissolved in hydrochloric 

 acid, a few drops of nitric acid added, and, after cooling, re-preci- 

 pitated by ammonia, washed, dried, ignited, and weighed. After 

 weighing, this was dissolved in hydrochloric acid, and a third 

 small portion of silica (c) obtained by filtration. 



The iron in the filtrate from silica c was reduced to the state 

 of ferrous oxide by the introduction of weighed pieces of metallic 

 zinc ; and after making due allowance for the amount of that 

 metal contained in the zinc added, the quantity of iron present 

 was ascertained by Penny's process. This, affcer being trans- 

 formed by calculation into ferric oxide, was subtracted from the 

 united weight of ferric oxide and alumina before found (less 

 silica c), the difference in weight being regarded as alumina*. 



* When phosphoric acid was found in a rock, the amount of that sub- 

 stance as well as that of ferric oxide^ indicated by volumetric estimation, 

 was deducted from the united weights of ferric oxide and alumina, as deter- 

 mined by the balance, and the difference was regarded as alumina. 



For the sake of checking the accuracy of the results obtained in estima- 

 ting alumina by difference, the ferric oxide and alumina were in several in- 

 stances redissolved in hydrochloric acid, and the solution treated with an 

 excess of pure hydrate of sodium in a platinum dish, and, after boiling, the 

 ferric oxide was separated by filtration. The ferric oxide was then re- 

 dissolved, precipitated by ammonia, washed, dried, and weighed. 



The filtrate containing alumina, in solution in caustic soda, was afterwards 

 acidified with hydrochloric acid, chlorate of potassium was added to de- 

 stroy organic matter, the solution boiled, and the alumina precipitated by 

 ammonia and carbonate of ammonium, washed, dried, and weighed. 



In all cases, however, it was found necessary to re-dissolve the oxide of 

 iron twice, and to treat the solution each time with hydrate of sodium in order 

 to obtain satisfactory results. Even when this was done, the quantity of 

 ferric oxide found was invariably slightly in excess of that indicated by vo- 

 lumetric analysis ; and although this difference seldom exceeded two-tenths 

 of one per cent., the volumetric process was finally adopted, as being both 

 more accurate and more expeditious. 



