There are many remarkable results dependent entirely on the colours of bodies, which are not explicable upon the idea of difference in mechanical arrangement. We know that different colours are regulated by the powers which structures have of absorbing and reflecting light; consequently a blue surface must have a different order of molecular arrangement from a red one. But there are some physical peculiarities which also influence heat radiation, quite independently of this _surface_ condition. If we take pieces of red, black, green, and yellow glass, and expose them when the dew is condensing, we shall find that moisture will show itself first on the yellow, then on the green glass, and last of all upon the black or red glasses. The same thing takes place if we expose coloured fluids in white glass bottles or troughs, in which case the surfaces are all alike. If against a sheet of glass, upon which moisture has been slightly frozen, we place glasses similarly coloured to those already described, it will be found that the earliest heat-rays will so warm the red and the black glasses, that the ice will be melted opposite to them, long before any change will be seen upon the frozen film covered by the other colours. The order in which heat permeates coloured media, it has already been shown, very nearly agrees with their powers of radiation. These most curious results have engaged the attention of Melloni, to whose investigations we owe so much; and from the peculiar order of radiations, which present phenomena of an analogous character to those of the coloured rays of light, obtained by him from dissimilarly coloured bodies, he has been led to imagine the existence of a “heat-colouration.” That is, the heat-rays are supposed to possess properties like luminous colour although invisible; and, consequently, that a blue surface has a strong affinity for the blue heat-rays, a red surface for the red ones, and so on through the scale. The ingenuity of this hypothesis has procured it much attention; but now, when the Newtonian hypothesis of the refrangibility of light is nearly overturned, we must not, upon mere analogy, rush to the conclusion that the rays of heat have different orders of refrangibility, which Melloni’s hypothesis requires.[58] Can anything be more calculated to impress the mind with the consciousness of the high perfection of natural phenomena, than the fact, that the colour of a body should powerfully influence the transmission of a principle which is diffused through all nature, and also determine the rate with which it is to pass off from its surface. Some recent experiments have brought us acquainted with other facts connected with these heat-radiations, and the power of heat, as influenced by the calorific rays, to produce molecular changes in bodies, which bear most importantly on our subject.