HEAT 569 molecules of oxide of silver are dissociated at a comparatively low temperature, while the molecules of water require a white heat to pro- duce dissociation between the hydrogen and oxygen atoms composing it. The elements of oxide of lead are also dissociated by a compar- atively low temperature when oxygen is exclu- ded, while to cause dissociation between the atoms of oxygen and iron in oxide of iron re- quires so high a heat that separation is difficult, except in the presence of a third body, an ex- ample of which is the reduction of iron ore by charcoal or anthracite, in which, however, the heat of dissociation is not reached. Advan- tage is taken in the arts of the expansion which heat produces in bodies for various industrial operations, such as the placing of tires on wagon wheels and the moving of immense weights or resistances, as the drawing toward each other of the walls of buildings. (See EXPANSION.) The construction of instruments for the mea- surement of heat is also founded upon the prop- erty of expansion. (See PYROMETER, and THER- MOMETER.) It sometimes occurs that at the temperature at which a liquid solidifies there is expansion instead of contraction, as in the solidification of iron and bismuth, and also of water, a familiar example, in which the expan- sion is made obvious in the floating of the less dense ice ; and mechanical advantage is often taken of this property of expansion during con- gelation to rend asunder masses of rocks or iron vessels. The phenomena and philosophy of combustion are treated under the title COM- BUSTION ; the expanding force of heat, particu- larly with reference to liquids and solids, un- der BOILING POINT and EXPANSION ; the trans- mission of radiant heat, especially in connec- tion with the diathermanous properties of dif- ferent bodies, under DIATHERMANCY ; the gen- eration of heat by mechanical means and by electricity, and its correlation with these forms of energy, under CORRELATION OF FORCES, ELEC- TRICITY, FRICTION, and GALVANISM ; and the causes of solar heat and its continuance or dis- sipation, under SUN. The remainder of this article will therefore be principally devoted to a consideration of the- more general laws of ra- diant heat, of the conduction of heat, and of specific and latent heat. I. KADIANT HEAT. The undulatory theory of radiation will be treated in the article LIGHT, and only such of its laws will be given here as are necessary for the treatment of the subject, and some of the reasons which indicate the identity of the two forces. A beam of light from the sun, or from any highly incandescent body, consists of a great number of rays propagated by transverse vibrations in- the ethereal particles. These vi- brations are of variable amplitude, correspond- ing to the particular kinds of rays, and these rays have the property of being refracted when passing from one medium to another in an ob- lique direction, as when passing from air into glass, and again from glass into air or any other medium. Those rays which consist of vibra- tions of greater amplitude have been found the least refrangible, and also to be those which in a greater degree than the others produce the effects of heat. When a beam of light is dispersed by a triangular prism made of rock salt, a highly diathermanous substance, there is formed a luminous spectrum of various colors in which heat is more or less distributed, abound- ing more in the red or least refrangible light than in other portions ; but far more in that part of the spectrum which is composed of in- visible rays of still less refrangibility than the red. It is estimated that the amount of heat contained in the invisible or non-luminous part beyond the red rays is more than seven times as great as that in the luminous part. Here, then, is a proof that rays of light and rays of heat are transmitted together in ordinary ra- diation of compound light. Now, if they are found to travel with the same velocity, their identity becomes probable, and this is shown by the fact that during an eclipse of the sun, at the conclusion of total obscuration, heat makes its appearance simultaneously with the rays of light ; and finally, when it is found that the rays of light and heat observe the same laws of reflection, refraction, interference, and polarization, the conclusion is irresistible that the only difference between the two is that the less refrangible rays possess the greater heating power. Eadiation of both light and heat is propagated in straight lines in a homogeneous medium, and unlike sound may be transmitted through a vacuum, a fact which indicates that it employs a different medium. If a sphere of glass, a, fig. 1, have a thermometer, 5, sealed into it, with its bulb placed in the centre of the sphere, and if the air be exhausted through the tube c, which is afterward closed by the flame of a blowpipe, and then the sphere be surrounded by a heated body, as a piece of tin foil, the thermometer will indicate a rise of temperature. The radiation of heat follows FIG. 1. FIG. 2. three important laws : 1. Its intensity is pro- portional to the intensity of the source. 2. It is inversely as the square of the distance. 3. Its intensity is less in proportion to the obli- quity of the surface of the body emitting the rays. The first law is demonstrated by placing
