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flow from a body whose temperature is lower than that of boiling water; that, as the temperature increases, the calorific rays are transmitted more and more abundantly; and when the body becomes highly luminous, that they penetrate the glass with perfect ease. The very feeble heat of moonlight must be incapable of penetrating glass, consequently it does not sensibly affect the thermometer, even when concentrated; and, on the contrary, the extreme brilliancy of the sun is probably the reason why his heat, when brought to a focus by a lens, is more intense than any that can be produced artificially; and it is owing to the same cause that glass screens, which entirely exclude the heat of a common fire, are permeable by the solar caloric.
The results of de Laroche have been confirmed by the recent experiments of M. Melloni, whence it appears that the calorific rays pass less abundantly, not only through glass, but through rock-crystal, Iceland spar, and other diaphanous bodies, both solid and liquid, according as the temperature of their origin is diminished, and that they are altogether intercepted when the temperature is about that of boiling water. It is singular that transparency with regard to light is totally different from the power of transmitting heat. In bodies possessing the same degree of transparency for light, the quantities of heat which they transmit differ immensely, though proceeding from the same source. The transmissive power of certain substances having a dark color exceeds by four or five times that of others perfectly diaphanous, and the calorific rays pass instantaneously through black glass perfectly opaque to light.
The property of transmitting the calorific rays dimin
ishes, to a certain degree, with the thickness of the body they have to traverse, but not so much as might be expected: a piece of very transparent alum transmitted three or four times less radiant heat from the flame of a lamp than a piece of nearly opaque quartz about a hundred. times as thick. However, the influence of thickness upon the phenomena of transmission increases with the decrease of temperature in the origin of the rays, and becomes very great when that temperature is low-a circumstance intimately connected with the law established by de Laroche, for M. Melloni observed that the differences between the quantities of caloric transmitted by the same plate of glass, exposed successively to several sources of heat, diminished with the thinness of the plate, and vanished altogether at a certain limit, and that a film of mica transmitted the same quantity of caloric whether it was exposed to incandescent platina or to a mass of iron heated to 360°.
Since the power of penetrating glass increases in proportion as the radiating caloric approaches the state of light, it seemed to indicate that the same principle takes the form of light or heat, according to the modification it receives, and that the hot rays are only invisible light, and light luminous caloric; and it was natural to infer that, in the gradual approach of invisible caloric to the condition and properties of luminous caloric, the invisible rays must at first be analogous to the least calorific part of the spectrum, which is at the violet extremity, an analogy which appeared to be greater, by all flame being at first violet or blue, and only becoming white when it has attained the greatest intensity. Thus, as diaphanous bodies transmit light with the same facility whether proceeding
from the sun or from a glow-worm, and that no substance had hitherto been found which instantaneously transmits radiant caloric coming from a source of low temperature, it was concluded that no such substance exists, and the great difference between the transmission of light and radiant heat was thus referred to the nature of the agent of heat, and not to the action of matter upon the calorific rays. M. Melloni has, however, discovered in rock salt a substance which transmits radiant heat with the same facility whether it originates in the brightest flame or luke-warm water, and which consequently possesses the same permeability with regard to heat that all diaphanous bodies have for light. It follows, therefore, that the impermeability of glass and other substances, for heat arises from their action upon the calorific rays, and not from the principle of heat. But, although this discovery changes the received ideas drawn from de Laroche's experiment, it establishes a new and unlooked-for analogy between these two great agents of nature. 'The probability of light and heat being modifications of the same principle is not diminished by the calorific rays being unseen, for the condition of visibility or invisibility may only depend upon the construction of our eyes, and not upon the nature of the agent which produces these sensations in us. The sense of seeing, like that of hearing, may be confined within certain limits; the chemical rays beyond the violet end of the spectrum may be too rapid or not sufficiently excursive in their vibrations to be visible to the human eye; and the calorific rays beyond the other end of the spectrum may not be sufficiently rapid or too extensive in their undulations to affect our optic nerves, though both may be visible to certain animals or insects. We are al
together ignorant of the perceptions which direct the carrier pigeon to his home, and the 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 favor 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 facilty 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 assem blage 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 chloride 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 heat 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 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