136 INCANDESCENCE. 



CHAPTER XVI. 



INCANDESCENCE. 



THERE are various methods by which an intense white 

 light is produced by artificial means, but most of them, if 

 not all, depend on the same principle as that already ex- 

 plained in the case of the magnesium light that is, in rais- 

 ing particles of a solid substance to incandescence. The 

 two essentials are, first, some method of producing an in- 

 tense heat ; and, secondly, the presence of some solid sub- 

 stance to receive the heat and to emit the light developed 

 by it ; for light, for some mysterious reason, is emitted 

 much more powerfully from a solid substance, however 

 minutely subdivided, than from a gas. 



The same general principle, indeed, is seen to operate in 

 the case of light derived from the lower, as well as in that 

 from the higher temperatures produced by combustion. 

 This is shown quite clearly in the flame of a common lamp 

 or candle. 



The materials used for burning in lamps and candles, as 

 the reader will recollect, usually belong to a class of sub- 

 stances called hydrocarbons. They are so called because 

 they are chiefly composed of hydrogen and carbon. Their 

 burning is, of course, the combination of these substances 

 with the oxygen of the surrounding atmosphere. 



Now hydrogen, in combining with oxygen, produces, un- 

 der favorable circumstances, a very intense heat, and forms 

 by the combination the vapor of water. This vapor rises 

 from the flame and is diffused through the atmosphere. 

 "We do not see it as it arises, but we can show it very plain- 



