62 FLAME. 



stance, is undergoing decomposition; and its elements, 

 the combustible substances of which they are com- 

 posed, are combining with the oxygen of the surround- 

 ing air to form new compounds; heat is necessary to 

 both of these two changes, and this heat is evolved 

 by the very changes themselves. 



96. When a candle is lighted a portion of the fuel 

 is melted, drawn up by capillary action through the 

 wick, and decomposed and converted into combustible 

 vapor; this vapor burns as its elements combine 

 with the surrounding oxygen, and the heat which re- 

 sults from this chemical action renders the process 

 continuous, by causing the decomposition of fresh 

 portions of the tallow, &c., and the consequent pro- 

 duction of more combustible vapor. 



97. In ordinary flame, therefore, several things are 

 necessary: air and heat are of course quite essential. 

 The heat evolved by the flame itself, causes a circu- 

 lation of air (61), provides a sort of natural ventila- 

 tion, and insures a constant supply of fresh air to 

 the burning vapors; on the other hand, this circula- 

 tion of air, by bringing fresh oxygen to the burning 

 combustible vapors, causes the evolution of heat 

 enough to insure the combustion of the fresh vapors, 

 which are about to be given out. 



98. Every one knows well how necessary fresh air 

 is to perfect combustion ; the importance of heat is 

 not so evident, though it is really quite as essential. 

 The flame of a candle may be extinguished by a 

 coil of wire, or by bringing a piece of cold metal in 



