LATENT HEAT. LATENT LIGHT. It is stated in the article on Light, that both light and heat are certainly due to the undulations of an elastic ether which pervades space ; and that the rays, both of light and heat are propagated with enormous velocity, and can be refracted, reflected, polarized, &c., by similar means. We have suggested, therefore, that heat may be an undulation of the same universal ether as that in which light is propagated, the difference being that the waves of heat are longer than those of light. There is however this difference between light and heat, that whereas bodies when exposed to radiant heat become heated, that is to say become themselves sources of radiant heat, bodies exposed to light do not in general become self-luminous, or sources of light ; (we say in general because solar phosphori offer a few exceptions to the rule). This may be in consequence of the light received by bodies being converted into heat ; the short waves of light which vibrate in the interstices of a body being lengthened into waves of heat by the mechanical effect of the atoms of the body vibrating amongst them.
When a body is heated it in general expands, and contracts on cooling, that is, radiates heat while returning to its former dimensions. It is in the highest degree probable that the Sun once extended far beyond the limits of the planetary system, its temperature being then very little above that of space ; that it gradually contracted in dimen tions, and by so doing became a source of heat and light to the planets which were from time to time detached from it ; that this contraction of volume may be still going on, while the amount of light and heat propagated by the condensation of the solar materials may be for ages to come continually on the increase, after which a reac tion may occur similar to what has happened in the case of the earth, which was once in a fluid incandescent state, propagating light and heat into space by its contraction of volume, which volume at last attained a minimum, the surface of the globe cooled into a solid crust, and the earth became as we find it, non-luminous and opaque.
Matter, then, in motion can act upon ether and communicate vibrations to it ; on the other hand, ether in motion can act upon matter and communicate vibrations to it, altering its form, volume, &c. Among these actions and reactions between gross matter and
subtle ether we must endeavour to seek for an explanation of the phe nomena due to the imponderable agents.
When a body is heated above the temperature of surrounding ob jects it gradually cools until an equal temperature is obtained amongst them, in which state the atoms of ether pulsate in unison with those of the bodies.
Heated bodies do not all cool at the same rate. The time in which a body at a certain temperature cools, when suspended in vacuo, to a certain lower temperature, is called its " Specific Heat." The following table shews the specific, heats of different bodies.
Water . . . . 1000 Zinc . . . . 93 Sulphur 188 Silver . . . . 56 Glass 117 Mercury . . . 33 Iron 110 Platinum. . . 31 Copper 95 Lead . . . . 29 Experiments seem to indicate that the specific heat of a body varies inversely as its atomic weight. For instance, the atomic weight of nlphur is 16, of lead 103 ; the specific heats of these substances ing as 188 : 29; and the ratio of 29 : 188 being nearly equal to kt of 16 : 108, Now we come to the particular subject of this article, viz., " La tent He,at." Strong brine, i. e. a saturated solution of common salt in water, freeze,s at 0° of Fahrenheit's scale ; water at 32°. Suppose then that two vessels, one containing water, the other brine, and each having a thermometer immersed in the liquid marking the temperature, say at 60°, are taken out-of-doors on a winter's day, when the temperature is 20°. The thermometers at once begin to fall ; that in the brine goes on continually falling until it reaches 20°, while that in the water exhibits a curious phenomenon ; it first sinks gradually until the water begins to freeze, then remains stationary until the wh.ole of the water is congealed or crystallized into ice, (or it may iise a little during the process,) and afterwards goes on falling as before until the temperature of the ice becomes 20°., It remains therefore to explain this curious phenomenon, which accompanies the change of form of a body from the liquid to the solid state.