1822.] Tin Ores in Cornwall and Devonshire. 453 



the surface, while the tin is flowing out by unstopping the tap- 

 hole. 



They are thus divided, and the tin is laded into moulds, so as 

 to form plates of a moderate size, and put by for a further refin- 

 ing. The slag, which rapidly hardens into a mass, is removed 

 to a dressing-floor, where, being broken up and stamped, it is 

 washed, and a quantity of tin taken from it, which is called 

 Prillion, and which is afterwards smelted again. 



No operation in smelting is more easy than that practised for 

 tin ores, nor is there any one in which the reasons for the mode 

 of treatment are so obvious. There are but two things to accom- 

 plish in this first process ; to obtain perfect fusion of the earths 

 so as to suffer the metal to separate easily from them, and to 

 decompose the oxide of which the ore uniformly consists. 



The addition of lime contributes to effect the former, and that 

 of carbonaceous matter or coal completes the reduction of the 

 ore. The separation of the metal from the earths then takes 

 place in the usual way during fusion, by the difference in their 

 specific gravities, the one precipitating to the bottom of the 

 furnace, from whence it is drawn off" by the tap-hole, and the 

 other, floating on the surface, is removed in the manner I have 

 described. 



The plates of tin, which are the produce of this smelting, are 

 somewhat impure, and are more or less so according to the qua- 

 lity of the ore which has been used ; they are reserved until a 

 sufficient quantity of them are obtained to proceed with the 

 refining, which is performed either in the same furnace, after 

 ore-smelting is finished, or in a similar one, which may be 

 reserved for the purpose. 



All the processes for refining metals in the fire must be per- 

 formed by taking advantage of some property in which the metai 

 operated on may differ from those with which it is alloyed, and 

 which it is intended to separate from it. These differences may 

 consist in the facility or difficulty of oxidation, in their tendency 

 to volatilize, in the temperature required for fusion, or in their 

 relative specific gravities. 



Upon an attention to ihe two latter circumstances are founded 

 the operation for refining tin. The substances which are most 

 to be suspected in the produce of the first melting, and which it 

 is desirable to separate, will probably be iron, copper, arsenic^ 

 tungsten from the wolfram, which the miners call mock-lead, 

 and a portion of undecomposed oxides, sulphurets, or arseniates, 

 and of some earthy matter or slag. 



The furnace for refining is raised but to a very moderate 

 degree of heat, and the plates of tin being placed in it are suf- 

 fered to melt very gradually, and the metal flows from the fur- 

 nace at once into the kettle, which is now kept hot by a small 

 fire placed beneath it. The more infusible substances will now 

 be left in the furnace, and a further purification of the tin is 



