$68 Dr Colquhoun on the Assay 



be observed to be studded with small globules or particles of 

 iron. In order to separate these, let the whole be pulverized, 

 then draw a small magnet repeatedly through the powder, 

 which will gather up all the minute fragments of iron. The 

 weight of these must of course be deducted from the total 

 weight of the scoria, and added to that of the metallic button. 



The assayist is now in a condition to fix some of the gene- 

 ral results of his experiments. In the first place, the weight 

 of the metallic button is in each case a direct measure of the 

 amount of metal contained in the ore. In the second place, 

 the weight of the flux, as originally introduced into the cruci- 

 ble deducted from the weight of the scoria, gives the amount 

 of earthy matter contained in the portion of ore submitted to 

 assay ; aijd, in the third place, the sum of the weights of the 

 metallic button and the scoria, deducted from the weight of 

 the original mixture of ore and flux, will leave a quantity 

 which measures the amount of volatilizable matter which has 

 escaped during the operation of assaying. If the ore has been 

 submitted to assay in its calcined state, this loss of weight in 

 volatilizable matter will consist entirely of oxygen, which has 

 been disengaged from the oxide of iron in the process of its 

 reduction. But if the ore was not calcined previously to the 

 assaying, this volatilized matter will, of course, consist of a 

 mixture of oxygen and carbonic acid. It is obvious, however, 

 that even in this instance the amount of oxygen which the ox- 

 ide of iron loses during its reduction may be easily determin- 

 ed, by calcining a quantity of the crude ore per se, and de- 

 ducting the loss of weight which it sustains in the calcination, 

 from the gross weight of volatilizable matter which is separat- 

 ed from the ore in the process of assaying. 



An examination of the scoria may now be made with much 

 advantage, as its appearances will often be found to indicate 

 in a pretty accurate manney both the nature and the relative 

 proportions of the earthy substances contained by the ore. 



If the scoria be found uncommonly transparent, and posses- 

 sing most of the characters of a glass, such as brittleness, 

 breaking into sharp-edged fragments and with a vitreous, con- 

 choidal fracture, &c., it is certain that the assay has contained 

 a superabundance of silica. Scoriae thus constituted are al- 



