64 SMOKE. 



placed in the centre of the flame, and the powder 

 ■will not be fired. 



101. The wick of a common candle requires con- 

 stant snuffing, because, being in the very centre of 

 the flame, it is in fact deprived of two necessary 

 conditions to combustion, namely, heat and fresh 

 air ; composition candles with twisted wicks, on the 

 other hand, do not require snuffing, because, as the 

 candle burns, the wick constantly twists outwards 

 towards the edge of the flame; it is thus brought 

 into the hottest part of it, and at the same time is 

 supplied with fresh air, which causes it to burn. 



102. Smoke is merely combustible matter which 

 is unable to burn, because it cannot come in contact 

 with free oxygen, or does not reach it till it is too 

 cold to burn. The production of smoke from a fire- 

 place or furnace, may always be prevented, and in 

 so doing a loss of heat is obviated. A great deal is 

 often said about the burning of smoke ; this is a very 

 difficult thing ; it is far more easy to prevent its 

 formation than to burn it when once formed. It 

 requires, however, constant care and attention to 

 prevent the production of smoke ; there is no reason 

 to doubt that at least nine-tenths of the dense black 

 smoke which contaminates the air of large towns 

 might be altogether prevented. 



103. Carbon is unable to combine with oxygen at 

 common temperatures; it requires to be heated before 

 it can enter into combination with that substance; 

 but when a compound substance containing carbon is 



