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visible-the red, yellow, and blue; each of the five varies in refrangibility and intensity throughout the whole extent, the visible part being overlapped at one extremity by the
chemical, and at
the other by the calorific rays. The action of the chemical rays blackens the salts of silver, and their influence is daily seen in the fading of vegetable colours: what object they are destined to accomplish in the economy of nature remains unknown, but certain it is, that the very existence of the animal and vegetable creation depends upon the calorific rays. That the heat-producing rays exist independently of light is a matter of constant experience in the abundant emission of them from boiling water, yet there is every reason to believe that both the calorific and chemical rays are modifications of the same agent which produces the sensation of light. The rays of heat are subject to the same laws of reflection and refraction with those of light; they pass through the gases with the same facility, but a remarkable difference obtains in the transmission of light and heat through most solid and liquid substances, the same body being often perfectly transparent to the luminous, and altogether impermeable to the calorific rays. The experiments of M. de Laroche show that glass, however thin, totally intercepts the obscure rays of caloric when they 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 rockcrystal, 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 The transmissive power of certain substances having a dark colour 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 diminishes, 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 experiments, 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 altogether ignorant of the perceptions which direct the carrier-pigeon to his home, and the