132 TECHNOLOGY. 
takes place in the mines; and consists in separating those pieces of rock which 
apparently contain no ore, from those which contain more or less of it. 
The richest portions are to be subjected to the dry stamping; the next in 
grade, which are too rich to be subjected immediately to wet stamping, are 
first sifted, and thus are made to yield much pure ore. There are, then, two 
other qualities of ore distinguished, which are subjected to wet stamping and 
sifting. 
Sifting serves to separate the rich ore from the fragments of sterile rock, 
the whole having first been subjected to stamping, either wet or dry, and 
to distribute and separate the ores in the order of the coarseness of the 
grain. The sieves are plunged into vessels of water, and violently agi- 
tated by a series of up and down motions, and thus the mineral substances 
are raised up and fall nearly in the order of their specific gravity, the 
metallic portions sinking to the bottom ; those particles which pass through 
the meshes of the riddles settle at the bottom of the vessel, and are after- 
wards exposed to washing, when they are worth the trouble. : 
The powdering of the ores is performed in stamping-mills. The stamps are 
raised by wipers or cams on a revolving-shaft, and are permitted to fall 
upon the material in troughs; the stamps are shod with iron at their lower 
ends, and weigh from two to three hundred pounds (pl. 27, jig. 21). A 
stream of water passes constantly through the trough, and the pounded ore 
passes with the water immediately to a series of shallow receivers united by 
channels; the richest portions of the ore, being heaviest, settle nearest to the 
stamping-trough, and the lighter particles next, until the water arrives at the 
last receiver, where the lightest particles are thrown down. 
2. RoastinG. 
The ore prepared as above is submitted to another operation, called roast- 
ing, before coming to the furnaces. 
Iron ore, which requires only to be pulverized to assist its melting, is 
roasted to render it friable, and disengage its water and carbonic acid. 
Sulphur, antimony, and arsenic are also volatilized by the process of roast- 
ing, and by their union with the atmosphere various products are formed. 
At alow temperature sulphates are formed, which, as the heat is elevated, 
yield sulphuric acid gas; the metallic oxides remain behind. 
3. FURNACES. 
The different furnaces made use of in metallurgic operations may be 
divided, according to their construction, into open furnaces, stack furnaces, 
reverberatory furnaces, and crucible furnaces. In the two first classes the 
fuel is mixed with the ore; in the third, only the flame operates upon it ; and 
in the fourth, the material to be heated is inclosed in crucibles, which are 
exposed either to immediate contact with the fuel or to its flame. In some 
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