CARBON, SILICON, BORON. 



185 



supplied either by a separate process (manufacture of illuminating 

 gas by heating wood or coal in retorts), or generated during the 

 combustion itself. 



In burning a candle, for instance, fat is constantly decomposed by 

 the heat of the flame itself, the generated gases burning continuously 

 until all fat has been decomposed, and the products of decomposition 

 have been burned up, i. e., have been converted into carbon dioxide 

 and water. 



An ordinary flame (Fig. 41) consists of three parts or cones. The 

 inner portion is chiefly unburnt gas ; the second is formed of partially 

 burnt and burning gas ; the outer cone, showing scarcely any light, is 

 that part of the flame where complete combustion takes place. The 

 highest temperature is found between the second and third cone. 



The light of a flame is caused by solid particles of carbon heated 

 to a white heat. The changes that take place in a flame are difficult 

 to study, but sufficient has been done experimentally to permit the 

 conclusion to be drawn that the separation of carbon in a flame is due 

 to dissociation of some of the hydrocarbons, of which ethylene 

 (ethene) is the most important. It is well known that when ethylene 

 (C 2 H 4 ) is heated it yields acetylene (C 2 H 2 ), which in turn gives carbon 

 and hydrogen. Evidence that acetylene is present in a gas flame is 

 furnished by the fact that when a Bunsen flame " strikes 

 back," that is, burns at the base of the tube so that 

 incomplete combustion of the gases takes place, a large 

 quantity of acetylene is formed. 



If a sufficient amount of air be previously mixed with 

 the illuminating gas, as is done in the Bunsen burner, no 

 separation of carbon takes place, and, therefore, no light 

 is produced, but a more intense heat is generated. A 

 similar effect is produced by the aid of the blow-pipe or 

 by means of the blast lamp, which serve to direct a cur- 

 rent of air directly into the cone of the flame. The 

 luminous and non-luminous Bunsen flame, using the 

 same flow of gas, must produce the same amount of heat 

 for a definite amount of gas burned, since the end- 

 products of combustion are the same in both cases. 

 But the non-luminous flame is much shorter than the 

 luminous one, and thus the heat is concentrated within a smaller 

 space, and, therefore, the temperature is much higher than in the 

 luminous flame. 

 * The cause of non-luminosity of a flame when air or oxygen is 



FIG. 41. 



Structure of 

 flame. 



