Dr Colquhouu on the Argillaceous Ore of' Iron. 91 



and by doing so, he will find himself enabled in almost all 

 cases, by a judicious selection, to charge his furnace with a 

 compound mass of ironstones and limestones, the respective 

 earthy ingredients of which shall be mutually adapted to flux 

 each other under the heat of the blast furnace. 



The efficacy of limestone is peculiarly conspicuous in the 

 smelting of siliceous ores. Silica and the protoxide of iron 

 unite, on being ignited together, in the proportion of four parts 

 of silica to 4.5 parts of the oxide, forming a very fusible com- 

 pound ; and in this state of union, their mutual affinity is so 

 strong, as to be undisturbed by the exhibition of any excess 

 of carbonaceous matter. It is impossible thereby to reduce 

 the oxide to the state of metal. But lime has a yet stronger 

 affinity for silica than the protoxide of iron possesses. Ac- 

 cordingly, when it is joined in the furnace with a siliceous ore, 

 it unites with the silica, and the oxide of iron being now no 

 longer protected by a more energetic affinity, is easily reduced 

 by the exhibition of a due dose of carbon. In this way a 

 quantity of metal is set at liberty and obtained in a state of 

 purity, which under other circumstances would have been ab- 

 sorbed by the scoria and have gone to waste. 



Those limestones which have been for a long series of years 

 employed at the Clyde Iron Works afford a practical illustra- 

 tion of the fact that a limestone is not by any means to be 

 always valued according to its purity, but, on the contrary, 

 that a certain admixture of some earthy matters may aid its 

 power as a flux, in acting upon the earths of the ore with 

 which it is to be associated in the furnace. They are only 

 of two kinds, and are brought from two different strata at 

 Crossbasket already mentioned in the vicinity of Glasgow. 

 These strata are found overlying a coal formation, and between 

 the uppermost of them and the cultivated soil, there is inter- 

 posed only a thin stratum of sandstone. The upper stratum, 

 distinguished at the works by the name of the upper post 

 limestone, is about eighteen inches in thickness. The lower 

 stratum is situated about eighteen inches under this, the inter- 

 vening space being occupied by a bed of indurated clay, which 

 is full of the remains of shells. It is termed the lower post 

 limestone, and is between three and four feet in thickness. 



