If we throw upon a plate of polished metal a prismatic spectrum (deprived, as nearly as possible, of its chemical power, by being passed through a deep yellow solution--which possesses this property in a very remarkable manner, as will be explained when we come to the examination of the chemical action of the sun’s rays)--it will be found, if we afterwards expose the plate to the action of vapour, very slowly raised from mercury, that the space occupied by the red rays, and those which lie without the spectrum below it, will condense the vapour thickly, while the portion corresponding with the other rays will be left untouched. This affords us evidence of the power of solar heat to produce, very readily, a change in the molecular structure of solid bodies. If we allow the sun’s rays to permeate coloured glasses, and then fall upon a polished metallic surface, the result, on exposing the plate to vapourisation, will be similar to that just described. Under yellow and green glasses no vapour will be condensed; but on the space on which the rays permeating a red glass, or even a blackened one, fall, a very copious deposit of vapour will mark with distinctness the spaces these glasses covered. More remarkable still, if these or any other coloured bodies are placed in a box, and a polished metal plate is suspended a few lines above them, the whole being kept _in perfect darkness_ for a few hours, precisely the same effect takes place as when the arrangement is exposed to the full rays of the sun. Here we have evidence of the radiating heat of bodies, producing even in darkness the same phenomena as the transmitted heat-rays of the sun. We must, however, return to the examination of some of these and other analogous influences under the head of actino-chemistry. From these curious discoveries of inductive research we learn some high truths. Associated with light--obeying many of the same laws--moving in a similar manner--we receive a power which is essential to the constitution of our planet. This power is often manifested in such intimate combination with the luminous principle of the solar rays, that it has been suspected to be but another form of the same agency. While, however, we are enabled to show the phenomena of one without producing those which distinguish the other, we are constrained to regard heat as something dissimilar to light. It is true that we appear to be tending towards some point of proof on this problem; but we are not in a position to declare them to be forms of one common power, or “particular solutions of one great physical equation.”[59] In many instances it would certainly appear that one of these forces was directly necessary to the production of the other; but we have also numerous examples in which they do not stand in any such correlation.