BED HEAT AND WHITE HEAT. 131 



CHAPTER XV. 



THE MAGNESIUM LAMP. 



THE brightness of the light produced in any sense by 

 combustion seems to depend upon two things first, the 

 intensity of the heat developed by the combustion ; and, 

 secondly, upon the presence of solid particles to be raised 

 to what is called a white heat by this intensity. Gaseous 

 substances, though the heat may be very great, emit usual- 

 ly a comparatively faint light, as is observed in the case of 

 the flame of hydrogen or of alcohol, which substances in 

 combustion, though the heat produced is very great, give 

 rise chiefly to incandescent gases. But in the case of mag- 

 nesium there is not only a Very intense heat, but this heat 

 takes effect upon the solid particles of magnesia as fast as 

 they are produced, and causes them to emit a light of the 

 greatest possible brilliancy. 



When any solid is heated in a furnace, we observe that 

 it first begins to emit a reddish-colored light, or, as we say, 

 it becomes red hot. When the heat is raised to a much 

 higher degree, the light that radiates from it becomes 

 brighter and whiter, and we say it is white hot. This 

 would seem to be the secret of the very intense light 

 given out by the combustion of magnesium. The com- 

 bustion produces an extremely high degree of heat, and 

 this takes effect on the solid particles of magnesia as fast 

 as they are produced, raises them to the most intense in- 

 candescence, and causes them to emit the very brilliant 

 and dazzling radiation which we see. 



This white heat, moreover, is not only different in degree 



