THEORY. 185 



phosphorescence disappears, and is afterwards re- 

 placed by another kind of light ; the body is then 

 said to be red-hot, white-hot, etc. 



In every case phosphoric light is developed 

 first, and is followed by the second kind of lumi- 

 nosity, unless chemical decomposition is required 

 to take place before phosphorescence ensues.* 



These two kinds of light have never been ex- 

 amined physically to ascertain whether they differ 

 in any essential property, but we know that they 

 must differ. Researches undertaken with this 

 view would be exceedingly interesting. We know 

 that the light of incandescent solid bodies, that is, 

 bodies heated red-hot, and the light of the electric 

 spark, exhibit great diversity in the number and 

 position of Wollaston's dark lines, already referred 

 to in this work. The velocity of electric light is 

 also known to be greater than solar light in the 

 ratio of three to two, according to Wheatstone's 

 admirable experiments. Again, we know that 

 a solid or liquid incandescent body possesses light 



* For instance, when native gypsum is heated on charcoal 

 before the blowpipe, some curious phenomena occur. After the 

 compound has lost its water, it melts into a beautiful transparent 

 bead, which becomes opaque on cooling. If this be strongly 

 heated in the flame of reduction, the sulphate of lime loses its 

 oxygen ; at the same time, its point of fusion is considerably 

 raised, and it is transformed into sulphuret of calcium. At this 

 moment, with greater heat, it begins to melt again, and at the 

 same time a very brilliant phosphorescence is observed. 



