138 INCANDESCENCE. 



very hot place, as is shown at once by holding there a cold 

 chisel, or hatchet, or large carving-knife, or any other piece 

 of polished and cold iron. 



The other substance contained in the hydrocarbon burned 

 in the lamp or candle is carbon. This, so far as it is really 

 burned that is, so far as it finds oxygen to unite with it, 

 forms a suffocating gas, called carbonic acid gas, which it 

 is very injurious to breathe. This gas rises with the vapor 

 of water into the air, and is diffused in the upper part of 

 the room till it gets cold, when it descends and gradually 

 escapes through open doors, or windows, or up the chim- 

 ney. If there is no way of escape for it open, or if there 

 are many lamps, or candles, or gas jets burning in the room, 

 the air becomes gradually so charged with it as to be un- 

 comfortable and unhealthy to breathe. 



But all the carbon does not at once find oxygen enough 

 at hand to combine with it. A portion of it remains for a 

 moment in the flame, where it serves the purpose of fur- 

 nishing a supply of solid particles to emit light. They can 

 not all burn at once, because there is not oxygen enough 

 for all ; so that, while some are burning, and evolving great 

 heat in so doing, the others, while waiting their turn to be 

 supplied with oxygen, as if not willing to be idle and use- 

 less even for a moment, employ themselves in producing 

 and emitting light, which the heat that is supplied to them 

 empowers them to do. At length, however, when they 

 reach the upper and outer margin of the flame, they too 

 obtain their supply of oxygen from the air, and, combining 

 with it, give out more heat, and also form more carbonic 

 acid gas, which arises with the rest into the air. 



Thus, on their way through the flame, after being liber- 

 ated from their previous combination with hydrogen in the 

 hydrocarbon, and before their turn comes to be supplied 

 with oxygen to enable them to form a new combination, 



