350 



EECEEATIVE SCIENCE. 



trial portions of the ore, some- cast-iron 

 crucibles, and a few test-tubes. Leaving the 

 assay-office, we find our way into the build- 

 ing where the principal operation, that of 

 smelting the ore, is performed. Its central 

 portion is entirely occupied by a huge re- 

 verberatory furnace of peculiar construction ; 

 the chief points of difierence between a lead- 

 smelting and an ordinary furnace being, that 

 the hearth or sole, upon which the ore is 

 placed, inclines gently from all sides towards 

 the centre like a saucer. Above the farnace 

 is a hopper, or quadrangular funnel, through 

 which the ore is introduced ; and separated 

 from the hearth by a bridge of refractory 

 brick is the fire, which, all being arranged, 

 comes' sweeping as a sheet of flame over the 

 galena. It is very important that a lead 

 furnace should be provided with a consider- 

 able extent of horizontal passages or flues, 

 before terminating in a chimney. Kot only 

 is this precaution based upon economical 

 motives, seeing that part of the lead vola- 

 tilizes in the form of oxide, but the sulphur- 

 ous acid which is generated is extremely 

 hurtful to both animal and vegetable life. 

 In the present instance all the furnaces open 

 into a flue of a mile in length, which, running 

 up the side of the hill, terminates eighty feet 

 above its summit. 



In order to understand how it is that by 

 merely heating galena to dull redness, we can 

 obtain metallic lead, we must consider, in the 

 first place, its chemical composition. This 

 may be thus represented : — 



Lead, one combining proportion 104 

 Sulphur „ „ 16 



120 



Galena, therefore, is a protosulphide of 

 lead. Now, when it is heated in contact with 

 atmosphiric air, it unites with four atoms of 

 oxygen, and from being a sulphide becomes a 

 sulphate. We will write the composition of 

 the sulphate also- 



Lead, one combining proportion 104 

 Oxygen four „ „ 32 



Sulphur one „ „ 16 



152 



Now oxygen enters into several combina- 

 tions with sulphur in different proportions, 

 and the result of one union is sulphurous 

 acid, or 



Sulphur, one combining proportion 16 

 Oxygen two „ „ 16 



32' 



With these data, it will not be difficult to 

 understand how it is that when 120 parts of 

 galena (which may be either grains, pounds, 

 or tons) are heated with 152 parts of sul- 

 phate of lead, obtained by roasting another 

 820 parts of galena in free contact with the 

 air, the result will be that the 32 parts of 

 sulphur contained in both compounds will 

 unite with the 32 parts of oxygen contained 

 in the sulphate to form sulphurous acid, 

 while 208 parts of lead wiU be set free in the 

 melted state. 



In practical smelting, the oxidation of one- 

 half of the galena is very ingeniously brought 

 about. A charge of ore, usually amounting 

 to two tons, is introduced into the furnace and 

 the fire lighted. When the entire mass has 

 arrived at a dull red heat, it is stirred from 

 time to time with a long iron rake, through 

 apertures provided for the purpose at dif- 

 ferent parts of the furnace. By this means 

 the surface of the ore is constantly renewed, 

 and a fresh portion exposed to the action of 

 the atmospheric oxygen. Upon the proper 

 management of this stage of the operation 

 its ultimate success depends, and it is here 

 that the practical knowledge of the smelter 

 is most fully shown. After the lapse of a 

 certain time, which for a charge of two tons 

 of ore is about eight hours, the whole of 

 the galena has become reduced to the me- 

 tallic state. The furnace is then tapped, 

 and the lead allowed to run ofi* into an iron 



