18 PNEUMATO CHEMICAL APPARATUS. 



air, smoke, &c. They are sometimes supplied with 

 air from the natural action of the fire, which rare- 

 fies the air about the ignited fuel ; and the rarefied 

 air becoming specifically lighter, ascends into the 

 chimney, whilst the colder, and consequently hea- 

 vier air, is forced by the atmosphere to enter at the 

 lower part of the furnace. Some furnaces are sup- 

 plied with air by means of bellows ; and those are 

 applied for forging iron, or for reducing metals 

 from the ore, which is called smelting. Hence the 

 furnaces derive their various names, and are called 

 simple, or open furnaces, reverberator]/ furnaces, 

 wind, or air furnaces, blast furnaces, forges^ smelt- 

 ing furnaces, 8c. 



A very useful kind of furnace, for many pur- 

 poses, is that invented by Dr. BLACK, of Edinburgh, 

 represented in Fig. 6. It consists of a cylindrical 

 or elliptical body of sheet-iron, coated within with 

 a mixture of loam and clay. The aperture A at 

 top is closed occasionally with an iron saucer full 

 of sand, which forms a sand-bath ; B is the door 

 of the fire-place, and C is the ash-pit register, which 

 slides so as to admit more or less air. D is an iron 

 tube which goes into the chimney of the room, to 

 carry off the smoke. 



Blow-pipes are used for directing the flame of a 

 candle or lamp against any bit of ore or other sub- 

 stance required to be examined. They ought to 

 have a bulb upon the middle of their stem, to 

 contain the moisture that is formed from the breath. 

 See Fig. 7- 



The blow-pipe contrived by Dr. Black, of a 

 conical form, represented in Fig. 8., is very con- 

 venient; a, is the nozzle. 



When a solid substance, in powder or otherwise, 



