and on the Elementary Colors of the Solar Spectrum. 
ces; then, making the barrel red hot in the fire, he observed at- 
tentively the moment when the incandescence commenced to 
manifest itself in the barrel, and the substance which it contained. 
He could discover no difference of time between the two phe- 
nomena, for the gun-barrel and the substance under trial became 
simultaneously luminous as the temperature was rising, and also 
simultaneously lost this property when, after being removed from 
the fire, and carried into a'dark place the temperature fell by 
radiation. 
themselves in the liquid condition without boiling at 977° F. 
Professor Draper excepts from this law the fluorid of calcium 
and carbonate of lime which become luminous much sooner. 
These cases do not in reality belong to the phenomena we are 
how considering, but to those of phosphorescence, which require 
a lower degree of heat, according to the nature of the substance. 
I may add, that regarding the question in a general point of view, 
We should also except those cases in which the development of 
light arises in chemical combinations. 
