294 Transactions of the 



strikes through the perforations into the ore and the gaseous products 

 of the process pass through the opposite wall into a series of condens- 

 ers. The condensers arc large chambers of brick, wood, iron or glass, 

 in which there are cross-walls, and these cross-walls are broken through 

 alternately at the bottom and the top, to admit of the passage-of the 

 vapor, but at the same time to compel it to take a circuitous course. 

 This at once lengthens the distance over which it must pass, giving it 

 time to cool, and, by producing gentle eddies in the current, favors the 

 fall of minute particles of solid or liquid matter. The floors of the 

 chambers are curved and slope towards the outside wall, so that the 

 metal, as it liquifies, may collect at one point in each chamber, whence 

 it may either be allowed to run out continually through a small opening, 

 or may be tapped when it has accumulated. As the vapor must be cooled 

 down as far as practicable, the natural draft is very slight, and a tall 

 chimney, generally with a small fire in it, is needful to draw in the re- 

 quisite amount of air through the grate. 



The working is very simple; the fire is gradually increased from the 

 start, until, after from three to four days, the mass of ore has come to a 

 moderately bright red heat, plenty of air being admitted all the time. 

 When this point has been reached, the distillation is considered complete, 

 and the furnace is closed up and allowed to cool. As soon as it is cold, 

 the exhausted ore is removed through suitable openings and the furnace 

 is recharged. 



OTHER DIFFICULTIES. 



The necessity of allowing the furnaces I have mentioned to cool down 

 before recharging, involves great loss of time and fuel. The first fur- 

 nace devised to allow of a continuous working was the Haehner furnace. 

 In this the ore and fuel are charged together into a shaft, the bottom of 

 which consists of a grate with movable bars. By moving one or more 

 of these bars, the lumps of exhausted ore can be allowed to drop from 

 the furnace, while fresh material is added at the top. The condensation 

 chambers are arranged as in the Almaden furnace. The continuity of 

 the Haehner furnace is a great advantage, but not the mixing of fuel 

 and ore. It has consequently been modified so as to resemble furnaces 

 long in use for ores of other metals, by allowing the flame from one or 

 more outlying fireplaces to strike into the shaft filled with ore alone. 

 An important advantage is gained, as in the exceedingly similar Swedish 

 furnace, by placing the openings for the entrance of the flame some dis- 

 tance from the bottom of the shaft, and allowing the air for roasting to 

 enter at the opening intended for withdrawing exhausted ore. In this 

 way the air is heated by the hot worthless material, and the heat thus 

 returned to the upper portion of the furnace. 



The "Knox & Osborne" furnace of California, and the latest furnace 

 of the works at Idria, Southern Austria, correspond to this general de- 

 scription. The former is provided with a single fire, which draws straight 

 across the column of ore in the shaft, and is built of heavy brick walls. 

 The Idria furnace, of which a copy is now putting up at New Almaden, 

 is provided with three fireplaces, which draw through nearly three 

 quarters of the height of the column, the gases passing out at two open- 

 ings close to the top of the furnace. The walls are of a construction first 

 adopted in England for iron-blast furnaces, and since applied with great 

 success to a variety of other furnaces — a comparatively thin lining of 

 fire-brick, surrounded by an annular space filled with air, answers ad- 

 mirably as a non-conductor of heat, outside of which is another thin 



