i!^ Dr Colquhoun ofi the Assay 



troduce this into a large platinum crucible, after which let it be 

 ignited in a low red heat during the first five minutes, and 

 then in a pretty strong red heat, in which it should be kept till 

 the operation is finished. If the ore happens to be overcharged 

 with carbonaceous matter, the calcination will be more com- 

 plete if it be conducted in a shallower vessel than a common 

 crucible ; and it will be necessary to stir the powder frequent- 

 ly, so as to expose the whole of it during the course of the ig- 

 nition, as much as possible, to the free and direct access of the 

 atmospheric air. After complete calcination, the loss of weight 

 sustained during the process will be found to indicate, with 

 considerable accuracy, the proportion of carbonic acid and other 

 Yolatilizable matter which had been contained by the ore. 



The intermixture of charcoal powder with the flux has been 

 frequently recommended as a useful auxiliary to it for the pur- 

 pose of reducing the oxide of iron. But this practice is not 

 only quite unnecessary, but it may often prove extremely pre- 

 judicial to the accuracy of the assay. A perfect reduction of 

 the oxide may be always insured by simply keeping the whole 

 materials of the assay surrounded externally with carbonaceous 

 matter, as for example, by coating the inside of the assay cru- 

 cible with a lining of charcoal powder. The presence of the 

 carbonaceous particles necessarily adds also to the refractori- 

 ness of the mixture, and constitutes a strong mechanical bar- 

 rier obstructing the aggregation of the globules of reduced 

 iron. Besides this, it is well known to every one at all versant 

 in the assaying of iron-ores, that iron, if imbedded in a state 

 of minute division among charcoal powder and exposed to an 

 intense heat, is liable to be converted into plumbago. This is 

 a substance unattractible by the magnet ; and as the assayist 

 is always in the habit of searching his scoria with a magnet for 

 the purpose of gathering up any small particles of iron which 

 may not have made their way, during fusion, to the metallic 

 button, he is deprived of all benefit from this resource in so 

 far as such iron may have been formed into plumbago, and a 

 proportionate disappearance of the metallic product is of course 

 inevitably produced. 



The practice of incorporating the ore and flux into a paste 

 with oil is worthy of censure for a similar reason : the oil^ when 



