Light, then, moves (generally) in straight lines. This is easily verified in the case of heat by the use of the thermo-electric pile (q.v.) and its galvanometer. Placing the pile out of the line from a source of heat to an aperture in a screen, no effect is observed; but deflection of the needle at once occurs when the pile is placed in the line which light would have followed if substituted for the heat.
A concave mirror, which would bring rays of light proceeding from a given point to a focus at another given point, does the same with heat, the hot body being substituted for the luminous one, and the pile placed at the focus. Heat, then, is reflected accord ing to the same lams as light. A burning lens gives a capital proof of the sun's heat and light being subject to the same laws of refraction. When the solar spectrum (q.v,) is formed by means of a prism of rock-salt (the reason for the choice of this material will afterwards appear), the thermo-electric pile proves the existence of heat in all the colored spaces, increasing, however, down to the red end of the spectrum, and attaining its maximum beyond the visible light, just as if heat were (as it must be) light with longer waves.
Some bodies, as glass, water, etc., transmit, when in thin plates, most of the light which falls on them ; others, as wood, metal, colored glass, etc., transmit none or little. A plate of rock-salt, half an inch thick, transmits 90 per cent of the rays of heat which fall on it; while glass, even of a thickness of one-tenth of an inch, transmits very little. In this sense, rock-salt is said to be diathermanous, while glass is said to be adiather mations, or only partially diathermanous. Most of the simple gases, such as oxygen, hydrogen, etc., and mixtures of these, such as air, oppose very little resistance to the passage of radiant heat, but the reverse is the case with compound gases. Some recent experiments by Tyndall seem to show that the vapor of water is exceedingly adiather manons. The question, however, cannot be considered as finally settled, since seine of Tyndall's results are so startling as to require further research and confirmation.
But there are other remarkable phenomena of radiant heat easily observed, which have their analogy in the case of light. 1. Unstained glass seems equally transparent to all kinds of light. Such is the case with rock-salt and heat. 2. Light which has passed
through a blue glass (for instance) loses far less per cent, when it passes through a second plate of blue glass. Similarly, heat loses say 75 per cent in passing through one plate of crown-glass, and only 10 per cent of the remainder (say) in passing through a second. 3. Blue light passes easily through a blue glass, which almost entirely arrests red light. So dark heat passes far less easily through glass than bright heat does. These analogies, mostly due to Melloni, are very remarkable.
Again, light can be doubly refracted, plane polarized, circularly polarked. All these properties have been fOund in heat by principal Forbes (q.v.).
The beautiful investigations of Stokes and Kirchoff on the solar spectrum have shown us that bodies, which most easily absorb light of a particular color, when heated, give off most freely light of that color; and it is easily shown by experiment, that those surfaces which absorb heat most readily also radiate it most readily. Thus, it was found by Leslie, that when a tinned-iron cube full of boiling water had one side polished. another rough ened, a third covered with lampblack, etc., the polished side radiated little heat, the rough ened, more, while: the, blackened side radiated a very great quantity indeed. And again, that if we have (say) three similar thermometers, and if the bulbs be (1) gilded. (2) covered with roughened metal, (3) smoked, and all be exposed to the same radiation of heat, their sensibility will be in the order, 3, 2, 1. A practical illustration of this is seen in the fact, that a blackened kettle is that in which water is most speedily boiled, while a polished one keeps the water longest warm when removed from the tire. Again, if a willow-pattern plate be heated white-hot in the fire, and then examined in a dark room, the pattern will be reversed—a white pattern being seen on a dark ground. This experi ment of Stewart's is very remarkable, and virtually constitutes an anticipation of Kir choff's results leadiug to the explanation of the fixed lines in the spectrum (q.v.). It is this law of radiation and absorption that mainly gives rise to the superior comfort of white clothing to black in winter as well as in summer; radiating less in winter, is absorbs less in summer.