THEORY. 185 
phosphorescence disappears, and is afterwards re- 
placed by another kind of hight; the body is then 
said to be red-hot, white-hot, ete. 
In every case phosphoric hght 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 hg¢ht 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 ight 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 intoa 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. 
