1822.] Tin Ores iti Cornwall and Devonshire. 451 



tioned, only the indestructible metals, and the oxide of tin, are 

 now discovered existing in deposits of this nature. 



The operations of dressing Stream Tin are simpler than those 

 for Mine Tin. It is smelted also in a different manner, and pro- 

 duces a superior metal known by the name of Grain Tin, which 

 is principally used by the dyers, and for the finer purposes. 



The processes for dressing Mine Tin are in many respects the 

 same as are used for all other ores, but are subject to some varia- 

 tion, which are attributable to the following peculiarities. 



1 . Being for the most part found intimately dispersed through- 

 out the matrix, the whole is necessarily pounded down to a very 

 fine state to admit of the perfect separation of the ores. 



2. That being unalterable by moderate degrees of heat, it 

 admits of calcination, by which the specific gravity of the sul- 

 phurets or arseniats with which it is mixed, may be lessened, 

 and a mode obtained of rendering them more separable. 



3. That the weight of Tin Ore being greater than most others, 

 it is less liable to waste in the processes of washing, and, there- 

 fore, may be dressed so as to be nearly clean from all substances 

 not actually adhering to it. 



From the first of these peculiarities it follows, that all tin 

 mines must be furnished with stamping-mills of sufficient power 

 to bruise down the ores raised, which is generally done so as to 

 produce a minute division of the whole, and on this account, 

 formerly, the quantity and fall of water that could be applied to 

 this purpose usually limited the quantity of ore that could be 

 returned from a mine, or the whole was frequently carried to' 

 some spot favourable to the erection of water-wheels to be 

 applied to this purpose. Within a few years steam-power has 

 been applied to stamping-mills, and has tended to increase the 

 supply of tin ores. Engines for this purpose, of considerable 

 power, are working with great effect at two of the largest tin 

 mines in Cornwall, Wheal Vor and Great Huas ; from which are 

 now arising abundant returns of the metal, and where formerly 

 it would have been impossible to have produced it. 



The state of division, or the size, as the tin dressers call it, is 

 regulated by a plate of iron pierced with small holes, through 

 which the whole passes from the stamping-mill, being washed 

 through by a rapid stream of water conducted upon it for the 

 purpose. This is a point of great importance, and is regulated 

 by the state of dissemination in which every ore is found. 



It is not the intention of this memoir to detail the processes of 

 dressing which are common to most ores, and, therefore, it may 

 be sufficient to remark that, after being stamped, the tin ores are 

 washed according to the usual mode, so as to separate the earthy 

 mixture and as much of that of a metallic nature as is possible. 

 All these operations are conducted with more than common care 

 and accuracy; for as tin ore holds such a large proportion of 



2g2 



