26 



MINERALS AND GEOLOGY 



the gaseous compounds pumped up by the wick. The carbon 

 comes ignited in the form of minute particles, and these, with the 

 liberated hydrogen and undecomposed gas, are driven partly out- 

 wards, and partly downwards, or into the blue cup-shaped portion c, 

 which lies at the base of the flame. At the latter spot, the carbon, 

 meeting with a certain supply of oxygen, is converted into carbonic 

 oxide, a compound of equal combining weights of carbon and oxygen. 

 Finally, in the flame-border or outer envelope, d, of a pale pinkish 

 colour, only discernible on close inspection, complete combustion, i. e. y 

 union with oxygen, of both gases, carbon and hydrogen, takes place. 

 The carbon burns into carbonic acid, a compound of two combining 

 weights of oxygen with one of carbon (hence, now commonly 

 known as carbon dioxide) ; and the hydrogen, uniting with oxygen, 

 forms aqueous vapour. If a cold and polished body, for example, be 

 brought in contact with the edge of a flame of any kind, its surface 

 will exhibit a streak or line of moisture. 



Now these different parts of flame, possess, to some extent, different 

 properties. The dark inner cone is entirely neutral or inert. Bodies 

 placed in it, become covered with soot or unburnt carbon. The 

 luminous or yellow cone possesses reducing powers. Its component 

 gases, requiring oxygen for their combustion, are ready to take this 

 from oxidized bodies placed in contact with them. The luminous 

 cone, however, in its normal state, has not a sufficiently high 

 temperature to decompose oxidized bodies, except in a few special 

 cases ; but its temperature, and consequently its decomposing or de- 

 oxidizing power, becomes much increased by the action of the blow- 

 pipe, as shown below. The blue portion of flame possesses also re- 

 ducing powers, but of comparatively feeble intensity, as the carbon 

 is there able to obtain from the atmosphere a partial supply of 

 oxygen. Finally, in the outer or feebly luminous envelope, in 

 which complete combustion takes place, the flame attains its highest 

 temperature ; and, having all the oxygen it requires from the sur- 

 rounding atmosphere, it exerts an oxidizing influence on bodies 

 placed in contact with it, since most bodies absorb oxygen when 

 ignited in the free air. 



In subjecting a body to the action of the blowpipe, we seek, (1) 

 either to raise its temperature to as high a degree as possible, so as 

 to test the relative fusibility of the substance ; or (2) to oxidize it, or 



