272 Dr Col(|uIioun cm the Assay 



instantaneously, in consequence of the lowness of the tempc*- 

 rature which sufficed to produce Hquefaction, and its colour 

 also passes with more than the usual quickness to a dull red. 



Every metallic oxide which is reducible in the temperature 

 of the assay furnace, should be carefully rejected from the 

 composition of the flux, as the reduced metal would inevitably 

 form an alloy with the metallic button. The oxides against 

 which it is chiefly necessary to be guarded, are those of man- 

 ganese, copper, lead, and arsenic. In the assaying of iron- 

 ores it is no uncommon practice to substitute pounded glass 

 for silica ; but, whenever this is done, it is necessary to avoid 

 every species of glass whose composition is tainted with the 

 oxides of manganese, arsenic, or lead. 



With these precautions we are not aware that it is necessary 

 to add any thing farther on the subject of the dry assay. 



§ % On the Humid Assay of Ironstones, 

 We have now considered at some length the details of what 

 seem to be the most advantageous methods of conducting the 

 examination of an iron-ore by the dry assay. It has already 

 been remarked that it is to this process that the metallurgist 

 has by far the most frequent recourse when he wishes to prove 

 an unknown ore. In convenience and dispatch it is so emi- 

 nently preferable to the humid assay, that, although its results 

 never can be relied on with the same degree of confidence, yet, 

 as they are generally possessed of tolerable accuracy, it is very 

 widely adopted in practice. By the dry method, if it has been 

 judiciously followed out, a single operation enables the smelter 

 to separate his ore into its two grand classes of constituent 

 bodies, the earths on the one hand, and the metals on the other. 

 He thereby obtains in miniature a pretty correct representa- 

 tion of the relative proportions in which the amount of metal 

 and the amount of earth exist in the stratum of ore, and also 

 of the general properties which the ore will be found to possess 

 when treated in the smelting furnace. 



In many cases, therefore, the dry assay is found to serve all 

 the purposes of the practical metallurgist : but there are also 

 many in which the humid assay or chemical analysis of the ore 

 will be found highly useful, and some in which it is indispen- 



