90 Dr Colquhoun on the Argillaceous Ore of Iron. 



grating it, by small portions at a time, with a sufficient quan- 

 tity of nitre in a platinum crucible. By this process it was 

 evident that the whole of the earthy ingredients of the coal 

 would be retained by the alkali of the nitre. In order to se- 

 parate these, after the deflagration of the coal had been com- 

 pleted, the alkaline residuum was supersaturated with nitric 

 acid, the solution was evaporated to dryness, and the dry sa- 

 line mass was then digested with water. The silica was by 

 this means extricated in an insoluble state, and the alumina 

 was afterwards precipitated from the remaining liquid by boil- 

 ing it with an excess of carbonate of soda. The results which 

 were furnished by this analysis corresponded, as nearly as 

 could have been expected, with those obtained from the pre- 

 ceding ones, in respect both to the quantity, and to the rela- 

 tive proportions of the earthy ingredients of the coal. 

 2. On Limestone. 

 We have already explained the purpose for which a flux is 

 resorted to by the ironsmelter. Almost the only flux he ever 

 employs is limestone, a mineral which is, fortunately for him, 

 found in the utmost profusion as a component rock of the in- 

 dependent coal basins. It has the property of promoting the 

 fusibility of almost all the earthy mixtures which occur in the 

 ironstone. But there are certain mixtures of these earthy in- 

 gredients on which limestone acts with much more efficacy, as 

 a flux, than it does upon others, and it is therefore of import- 

 ance to attend, not only to the quantity of iron-ore and of 

 limestone which are thrown together into the furnace, but also 

 to the earthy ingredients which may exist in either of these 

 minerals. The extraneous substances contained by the lime- 

 stones are generally of the same nature v/ith those which we 

 find in the ores. They are carbonate of protoxide of iron, 

 carbonate of magnesia, clay, sand or siliceous clay, and car- 

 bonaceous or bituminous matter. And it is fortunate for the 

 ironsmelter that the limestone associates with these, in at least 

 as great a vaiiety of proportion as the ironstone does. From 

 this circumstance, it only requires that the smelter who does 

 not wish to make his business a mere mechanical routine, 

 should study the nature of all the limestones which are situa- 

 ted conveniently within his reach, as well as of his iron ores. 



