(349.) Heat is communicated either by _radiation_ between bodies at a distance, or by _conduction_ between bodies in contact, or between the contiguous parts of one and the same body. The laws of the radiation of heat have been studied with great attention, and have been found to present strong analogies with that of light in some points, and singular differences in others. Thus, the heat which accompanies the sun’s rays comports itself, in all respects, like light; being subject to similar laws of reflection, refraction, and even of polarization, as has been shown by Berard. Yet they are not identical with each other; Sir William Herschel having shown, by decisive experiments, verified by those of Sir H. Englefield, that there exist in a solar beam both rays of heat which are not luminous, and rays of light which have no heating power. (350.) The heat, radiated by terrestrial fires, and by bodies _obscurely_ hot, by whatever means they have acquired their heat (even by exposure to the sun’s rays), differs very materially from solar heat in their power of penetrating transparent substances. This singular and important difference was first noticed by Mariotte, and afterwards made the subject of many curious and interesting experiments by Scheele, who found that terrestrial heat, or that radiated from fires or heated bodies, is intercepted and detained by glass or other transparent bodies, while solar heat is not; and that, being so detained, it heats them: which the latter, as it passes freely through them, is incapable of doing. The more recent researches of Delaroche, however, have shown that this detention is complete only when the temperature of the source of heat is low; but that, as that temperature is higher, a portion of the heat radiated acquires a power of penetrating glass; and that the quantity which does so bears continually a larger and larger proportion to the whole, as the heat of the radiant body is more intense. This discovery is very important, as it establishes a community of nature between solar and terrestrial heat; while at the same time it leads us to regard the actual temperature of the sun as far exceeding that of any earthly flame.