PHYSICAL PROPERTIES OF MINERALS. 127 



is provided with a joint at 5, and the side of the barrel or cylinder oppo- 

 site to that from which the heak issues, comes off by unscrewing, for 

 the purpose of enabling us to wipe it dry occasionally. This barrel 

 serves to contain the little appendages of the beak, when the instrument 

 is not in use. This instrument is also made of brass with an ivory mouth 

 piece. 



The flame employed in using the blowpipe may be either that of a 

 candle or lamp; though considerable choice exists among candles and 

 lamps for this purpose. A tallow or wax candle in order to answer best, 

 should be made with its wick somewhat larger than ordinary ; and of 

 lamps, one furnished with a single wick and fed by olive oil has received 

 the preference. In using the candle, the wiok is previously bent in the 

 direction to which we wish to direct the flame. 



The keeping up a continual stream of air through the blowpipe is at 

 first attended with some difficulty. This, however, is best overcome 

 by an attention to the following directions. Closing the. mouth, keep 

 the cheeks distended with air during a number of inspirations and expi- 

 rations, performed through the nostrils. Next, attempt the same with 

 the mouth piece of the blowpipe between the lips : now, as this provides 

 an exit for the air in the mouth, unless a fresh supply be kept up from 

 the lungs, the cheeks will soon collapse ; in order to prevent this, at the 

 moment of expiration through the nose, a sufficient quantity of air must 

 be allowed to enter the mouth to preserve their distention. By this 

 means, the air in the mouth is constantly subject to the same compres- 

 sion, and flows in an uniform mannner from the little orifice. Having 

 acquired the habit of keeping up a continued current of air from the 

 blowpipe, the beak is now brought within the border of the flame of the 

 lamp or candle. We immediately perceive before the orifice a long 

 and conical blue flame, environed by an outer cone more resembling 

 the ordinary flame of a candle or lamp. It is at the apex of the 

 blue cone that the most intense heat is produced. Much practice, 

 however, is required in order to obtain the maximum heat of this in- 

 strument. If the current of air is too strong, the heat is dissipated 

 as soon as produced ; if too feeble, there is a deficiency of air for the 

 effect. 



Oxidation takes place when we bring the matter of assay before the 

 apex of the exterior flame, where the combustible matter coining from 

 the lamp or candle has ceased to attract oxygen. The heat required in 

 the matter of assay is only that of incipient redness ; and one of the 

 larger orifices of the beak is found best for producing this temperature. 

 Reduction, on the other hand, requires a more elevated temperature, 



