232 Dr Colquhoun o?i the Argillaceous Ore of Iron. 



the remaining four, the quantity varied from 0.62 to 0.02 per 

 cent. All these specimens had been taken from different strata 

 of working ores, and the pyrites existed interspersed through 

 them in a state of so minute division, that its presence could 

 not be detected by the aid of a magnifying glass. 



Phosphorus appears never to exist in the ore but in the state 

 of phosphoric acid. It is of much rarer occurrence than the 

 sulphur, and is a still more injurious ingredient. The acid 

 is reduced in the operation of smelting, and the phosphorus 

 thereby developed unites with the metallic product, and ren- 

 ders it so excessively brittle that its most valuable properties 

 are thereby destroyed. But it is fortunately so seldom seen, 

 that, with the exception of three specimens found in different 

 parts of France, and described by M. Berthier,* containing re- 

 spectively 0.3, 0.8, and 6.1 per cent, of phosphoric acid, there 

 seems to be no notice of any phosphorised ore upon record. 

 The last specimen, in which so large an amount of the acid 

 existed, was brought from St Etienne, (departement de la 

 Loire,) where it was found in the very remarkable state of 

 large continuous beds that were several feet in thickness. 



There are still some occasional ingredients of the ore, whichy 

 on account of their closer analogy with the carbonate of iron, 

 we have delayed mentioning, until all the other extraneous 

 substances were discussed. These are the carbonates of lime, 

 of magnesia, and of oxide of manganese. In upwards of a 

 dozen analyses of different ores, the results indicated an amount 

 of carbonate of lime, varying from 1 to 15.4 per cent., and of 

 carbonate of magnesia from 3.75 to 14 per cent. But in those 

 localities where the principal strata are of a calcareous nature, 

 the proportion of lime is frequently so superabundant as to 

 make the mineral rank rather among the ferruginous lime- 

 stones than among the ores of Iron. The carbonate of man- 

 ganese is also occasionally met with in the ores, but for the 

 most part only in very small quantity. We have never seen 

 it exceed 0.32 per cent., though the analyses of Descostils and 

 Berthier show it to exist sometimes to an amount that varies 

 from 4 to 7 per cent. 



From this very general view of the constitution of the ar- 



• Annates des Mines, iv. 376, 381, 383. 



