Although we do not fear that heat and light can be confounded in the mind, so different are their phenomena,—we have heat rays, as from dark hot iron, which give no light, while in the full flood of the lunar rays the heat is scarcely appreciable by the most delicate instruments;—yet it is important to show how far these two principles have—been separated from each other. Transparent bodies have varied powers of calorific transparency, or transcalescence: some obstructing the heat radiated from bodies of the highest temperatures almost entirely even in the thinnest layers; whilst others will allow the warmth of the hand to pass through a thickness of several inches. Liquid chloride of sulphur, which is of a deep red colour, will allow 63 out of 100 rays of heat to pass, and a solution of carmine in ammonia, or glass stained with oxides of gold, or copper, rather a greater number; yet these transparent media obstruct a large quantity of light. Colourless media obstructing scarcely any light, will, on the contrary, prevent the passage of calorific rays. Out of every hundred rays, oil of turpentine will only transmit 31, sulphuric ether 21, sulphuric acid 17, and distilled water only 11. Pure flint glass, however, is permeated by 67 per cent. of the thermic rays, and crown glass by 49 per cent. The body possessing the most perfect transparency to the rays of heat is diaphanous salt-rock, which transmits 92, while alum, equally translucent, admits the passage of only 12 per cent.[43]
Black mica, obsidian, and black glass, are nearly opaque to light, but they allow 90 per cent. of radiant heat to pass through them; whereas a pale green glass, coloured by oxide of copper,[44] covered with a layer of water, or a very thin plate of alum, will, although perfectly transparent to light, almost entirely obstruct the permeation of heat rays.
We thus arrive at the fact that heat and light may be separated from each other; and if we examine the solar beam by that analysis which the prism affords, we shall find that there is no correspondence between intense light and ardent heat. By careful observation, it has been proved, when we have a temperature of 62° F. in the yellow ray, which ray has the greatest illuminating power; that below the red ray, out of the point of visible light, the temperature is found to be 79°, while at the other end of the spectrum, in the blue ray, it is 56°, and at the end of the violet ray no thermic action can be detected.[45]
From the circumstance, that as we, by artificial means, raise the temperature of any body, and produce intense heat, so after a certain point of thermic elevation has been obtained, we occasion a manifestation of light.[46] It has been concluded, somewhat hastily, that heat and light differ from each other only in the rapidity of the undulations of an hypothetical ether.
It must be admitted that the mathematical demonstrations of many of the phenomena of calorific and luminous power are sufficiently striking to convince us that a wave-movement is common to both heat and light. The undulatory theory, however, requires the admission of so many premises of which we have no proof; its postulates are, indeed, in many cases so gratuitous, that notwithstanding the array of talent which stands forward in its support, we must not allow ourselves to be deceived by the deductions of its advocates, or dazzled by the brilliancy of their displays of learning.
Radiant heat appears to move in waves; but that calorific effects in material bodies are established by any system of undulation, is a deduction without a proof; and the thermic phenomena of matter are as easily explained by the hypothesis of a diffusive subtile fluid.
We have not, however, to prove the correctness of either of the opposing views; indeed, it is acknowledged that many phenomena require for their explanation conditions which are not indicated by either theory.
The earth receives its heat from the sun; a portion of it is conducted from particle to particle into the interior of the rocky crust. Another portion produces warmth in the atmosphere around us, by convection, or the circulation of particles; those warmed by contact with the surface becoming lighter, and ascending to give place to the colder and heavier ones. A third portion is radiated off into space, according to laws which have not been sufficiently investigated, but which are dependent upon the colour, chemical composition, and mechanical structure of the surface.