9f!© Dr Colquhouti on the Assay 



tion of the scoria indicates the nature of the earthy ingredients. 

 The appearance and properties of the metalHc product may 

 be so much modified by a variety of circumstances, which may 

 not only be extrinsic to the composition of the ore, but which 

 cannot themselves be subjected to any certain rule, that it is 

 always difficult to deduce from an examination of it any cer- 

 tain inference regarding the quality of the iron. Thus, the 

 intensity and duration of the ignition, some apparently trivial 

 modification of the nature of the fluxing material, or a change 

 in the mode of cooling the crucible, will often produce re- 

 markable differences in the nature and qualities of the metal- 

 lic product. Indeed, in as far as the metallic ingredient of 

 any ore is concerned, the amount of it seems almost the only 

 property which can be certainly gathered from even the most 

 careful assay. We may notice, however, a few of the most 

 striking variations in the quality of the metallic button which 

 will encounter the assayist in his experiments, and also, so far 

 as possible, state the causes which produce them. 



If the temperature of the assay furnace has been low, the 

 jmetallic product never can present the appearance of black 

 cast-iron, which is never produced without the application of 

 fan intense heat. Again, if the crucible be rapidly cooled, no 

 matter from what intensity of heat, black cast-iron will not be 

 formed ; for even the most perfect specimen of this variety 

 of carburetted iron may at once be converted into a body pos- 

 sessing all the external and physical characters of white cast- 

 iron, by merely melting it, and cooling it suddenly. It is a 

 'Singular fact, that by this simple variation in the mode of 

 cooling, the same piece of metal, although it must obviously 

 contain in either case the same quantity of carbon, will never- 

 theless have the most opposite properties comtnunicated to it 

 in colour, texture, and hardness. The white cast-iron is of 

 9- white colour, with a silvery aspect ; it is excessively hard, 

 find is composed of large crystalline laminae irregularly su- 

 perimposed upon each other. On the contrary, the black cast- 

 iron is of a dark grey colour ; it is soft, and exhibits a small 

 crystalline or granular texture. 



The flux may also exercise an equally remarkable influence 

 over the properties of the metalHc product of the assay. 



