The tern perature of the incandescent bodies with which a Welsbach mantle is impregnated may be assumed as being about 3500° F. The quality of the light depends to a certain extent on the amount of air admitted, which should be just sufficient to ensure combustion of the gas: the burners employed are constructed on this principle. The quality of the light in an incandescent burner depends on the raising of the finely divided rare earths (thoria, ceria, etc.) to the highest degree of incandescence by the agency of a Bunsen burner, which is constructed in such a manner that the amount of air and gas supplied to the burner are in the proportion which will yield a non-luminous flame and give out sufficient heat to effect the object required.