Some substances emit only one kind, some any kinds, of heat.
Of the first class, perfectly pure rock-salt is an instance. It is monothermic, just as its vapour is monochromatic. It does not, therefore, as maintained by Melloni and Knoblauch, transmit heat from all sources with uniform facility. The amount of absorption effected by rock-salt increases with the thickness of the absorbing plate. This circumstance is also obviously in analogy with the incandescent vapour of the salt, or of potassium, which is known to furnish an almost continuous spectrum.