IS 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 ligliter, 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^ reverheratory furnaces, 

 tvindy or air furnaces, blast furnaces, forges^ smelt' 

 ing furnaces, <§t. 



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. 



