420 On the Smelting of Tin Ores . 
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 
yaluable metal, it is of course treated with every precaution to 
guard against waste. 
Some metallic substances will be found, however, which, from 
their specific gravity approaching nearly to that of tin ore, or 
rather exceeding it, cannot be removed by any process of wash- 
ing; these are mostly decomposable by a red heat, which the 
oxide of tia will bear without alteration, Therefore, after as 
much has been done as possible to render the ores clean on the 
dressing-floors, they are taken to the lurning-house, which is 
furnished with small reverberatory furnaces, on the floor of which 
the ores are spread, and submitted to the action of a moderate 
and regular fire: they are frequently turned over by an iron rake 
to expose fresh surfaces, and a considerable volatilization of sul- 
phur and arsenic takes place; the former seems principally to be 
consumed, and the latter is condensed by long horizontal flues 
constructed for this purpose. After the ores come from the 
burning-house, the process of dressing is completed by further 
washing, which is rendered easy by the alteration which has been 
produced in the relative weight of the substances. 
Copper ore is not unfrequently present in these cases, and, as 
it is in part converted into sulphate of copper, the water which 
is first used is preserved, and a portion of copper obtained from 
it by means of iron, 
The great specific gravity of the tin ore, as I have before re- 
_ marked, renders it possible with care to subject it to many ope- 
rations in dressing without much waste; and they are, therefore, 
applied until the whole is generally so clean, as to yield a pro- 
duce of metal equal to from 50 to 75 per cent, In this state 
they are sold by the miner to the smelter, who determines their 
yalue 
