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vulture to his prey, before he himself is visible even as a speck in the heavens; or of those in the antennæ of insects which warn them of the approach of danger : so likewise beings may exist on earth, in the air, or in the waters, which hear sounds our ears are incapable of hearing, and which see rays of light and heat of which we are unconscious. Our perceptions and faculties are limited to a very small portion of that immense chain of existence which extends from the Creator to evanescence. The identity of action under similar circumstances is one of the strongest arguments in favour of the common nature of the chemical, visible, and calorific rays. They are all capable of reflection from polished surfaces, of refraction through diaphanous substances, of polarization by reflection and by doubly refracting crystals; none of these rays add sensibly to the weight of matter; their velocity is prodigious, they may be concentrated and dispersed by convex and concave mirrors; light and heat pass with equal facility through rock-salt, and both are capable of radiation; the chemical rays are subject to the same law of interference with those of light; and although the interference of the calorific rays has not yet been proved, there is no reason to suppose that they differ from the others in this instance. As the action of matter in so
many cases is the same on the whole assemblage of
rays, visible and invisible, which constitute a solar beam, it is more than probable that the obscure, as well as the luminous part, is propagated by the undulations of an imponderable ether, and consequently comes under the same laws of analysis.
Liquids, the various kinds of glass, and probably all substances, whether solid or liquid, that do not crystallize regularly, are more pervious to the calorific rays according as they possess a greater refracting power. For example, the chlorid of sulphur, which has a high refracting power, transmits more of the calorific rays than the oils which have a less refracting power : oils transmit more radiant beat than the acids, the acids more than aqueous solutions, and the latter more than pure water, which, of all the series, has the least refracting power, and is the least pervious to heat. M. Melloni observed also that each ray of the solar spectrum follows the same law of action with that of terrestrial rays having their origin in sources of different temperatures, so that the very refrangible rays may be compared to the heat emanating from a focus of high temperature, and the least refrangible to the heat which comes from a source of low temperature. Thus, if the calorific rays emerging from a prism be made to pass through a
layer of water contained between two plates of glass, it will be found that these rays suffer a loss in passing through the liquid as much greater as their refrangibility is less. The rays of heat that are mixed with the blue or violet light pass in great abundance, while those in the obscure part which follows the red light are almost totally intercepted. The first, therefore, act like the heat of a lamp, and the last like that of boiling water.
These circumstances explain the phenomena observed by several philosophers with regard to the point of greatest heat in the solar spectrum, which varies with the substance of the prism. It has already been observed that Sir William Herschel, who employed a prism of flint glass, found that point to be a little beyond the red extremity of the spectrum, but, according to M. Seebeck, it is found to be upon the yellow, upon on the red, or at the dark limit of the red, according as the prism consists of water, sulphuric acid, crown or flint glass. If it be recollected that, in the spectrum from crown glass, the maximum heat is in the red part, and that the solar rays, in traversing a mass of water, suffer losses inversely as their refrangibility, it will be easy to understand the reason of the phenomenon in question. The solar heat which comes to the anterior face of the prism of water consists of rays of all degrees of refrangibility. Now, the rays possessing the same index of refraction with the red light suffer a greater loss in passing through the prism than the rays possessing the refrangibility of the orange light, and the latter lose less in their passage than the heat of the yellow. Thus, the losses, being inversely proportional to the degree of refrangibility of each ray, cause the point of maximum heat to tend from the red towards the violet, and therefore it rests upon the yellow part. The prism of sulphuric acid, acting similarly, but with less energy than that of water, throws the point of greatest heat on the orange; for the same reason the crown and flint glass prisms transfer that point respectively to the red and to its limit. M. Melloni, observing that the maximum point of heat is transferred farther and farther towards the red end of the spectrum, according as the substance of the prism is more and more permeable to heat, inferred that a prism of rock-salt, which possesses a greater power of transmitting the calorific rays than any known body, ought to throw the point of greatest heat to a considerable distance beyond the visible part of the spectrum-an anticipation which experiment fully confirmed, by placing it as much beyond the dark limit of the red rays as the red part is distant from the bluish-green band of the spectrum.
When radiant heat falls upon a surface, part of it is reflected and part of it is absorbed, consequently the best reflectors possess the least absorbing powers. The absorption of the sun's rays is the cause both of the colour and temperature of solid bodies. A black substance absorbs all the
rays of light, and reflects none; and since it absorbs at the same time all the calorific
it becomes sooner warm, and rises to a higher temperature, than bodies of any other colour. Blue bodies come next to black in their power of absorption. Of all the colours of the solar spectrum, the blue possesses least of the heating power; and since substances of a blue tint absorb all the other colours of the spectrum, they absorb by far the greatest part of the calorific rays, and reflect the blue where they are least abundant. Next in order come the green, yellow, red, and, last of all, white bodies, which reflect nearly all the rays both of light and heat. The temperature of very transparent fluids is not raised by the passage of the sun's rays, because they do not absorb any of them, and as his heat is very intense, transparent solids arrest a very small portion of it.
Rays of heat proceed in diverging straight lines from each point in the surfaces of hot bodies, in the same manner as diverging rays of light dart from every point of the surfaces of those that are