If a quantity of pouteled nitrate of ammonia (a very soluble salt) be placed in a vessel, an equal weight of water added, and the whole stirred for a minute or two with a test-tube containing water, the heat required for the solution of the salt will be abstracted from *all bodiei in contact with the solution, and the water in the test-tube will be frozen. In this sense, the compound is called a freezing mixture. For additional illustrations of heat bee ming latent, See FREEZING MIXTURE.
Of course the converse of this may be expected to hold, and latent heat to become sensible when a liquid becomes solid. As an example, when a saturated solution of sulphate of soda begins to deposit crystals of the salt, the temperature rises very con siderably; and it is the disengagement of latent heat that renders the freezing of a pond a slow process, even after the whole of the water has been reduced nearly to the freez ing-point.
Vaporization.—Almost all that has been said on the subject of fusion is true of vaporization, with the change of a word or two. Thus, however much heat we apply to a liquid, the temperature does not rise above the boiling-point. Heat, then, becomes latent in the act of vaporization, or rather is converted into change of state. It is found by experiment that 510 units of heat (each sufficient to heat a pound of water 1° C.) disappear In the conversion of a pound of water into steam. Hence a pound of steam at 100° C. is sufficient to raise 5.4 pounds of water from zero to the boiling-point.
Communication of Heat.—There are at least three distinct ways in which this occurs, and these we will take in order.
Conduction.—Why is it that if one end of a poker and of a glass or wooden rod be put into a fire, we can keep hold of the other end of the latter much longer than we can of the for mer? The reason is, that heat is more readily transmitted in the iron from particle to par ticle, than it is in glass or wood. This is conduction. It is to lie noticed, however, that in this experiment a great portion of the heat which passes along each rod is given off into the air by the surface. The mathematical theory of conduction has been most exquis itely investigated by Fourier, and after him by Poisson, but on the supposition that the rate at which heat passes froth a warmer to a colder portion of the body is proportional to the dtrerence of temperature. As most of the experiments which have been made with the object of ascertaining the conductivity (not conductibility, the erroneous word in common use) of different bodies have been made in this way, it is not surprising that our knowledge on this point is very meager indeed. We know that silver conducts
better than most other metals, and that the metals in general conduct better than other solids; but here our ends. It is satisfactory to know, however, that the defects o4 old methods are now fully acknowledged, and that the important element of conductivity will shortly be accurately known for all important substances. Forbes has recently shown that the conductivity of iron diminishes as its temperature increases; and the same is probably true of other bodies. This invalidates the con elusions of the mathematical theories above mentioned, but the necessary corrections will be easily applied when the experimental data are completely determined.
In conjunction with their radiating power (see next section), the conductivity of bodies is most important as regards their suitableness as articles of clothing for hot or cold climates, or as materials for building or furnishing dwelling-houses. We need but refer to the difference between linen and woolen clothing, or to the difference (in cold weather) of sensation between a carpet and a bare floor, in order to show how essential the greater or less conducting power of bodies is to our everyday comfort.
Badintion.—By this is understood the passage of heat, not from particle to particle of one body, but through air or vacuum, and even through solid bodieS (in a manner, and with a velocity quite different from those of conduction) from one body to another. There can be no doubt whatever as to radiant heat being identical with light, differing from red light, for instance, as red light differs from blue; i.e., having (see LIGHT) longer waves than those corresponding to red light. This idea might easily have arisen during the contemplation of a body gradually heated. At first, it remains dark, giving off only rays of heat; as its temperature increases, it gives us, along with the heat, a low red light, which, by the increase of the temperature, is gradually accompanied by yellow, blue, etc. rays, and the incandescent body (a lime-ball, for instance) finally gives off a light as white as that of the sun, and which, therefore, contains all the colors of sun light in their usual proportions. In fact (see FoucE), there is great reason to believe that the sun is merely a mass of incandescent Melted matter, and that the radiations it emits, whether called heat or light, merely differ in quality, not in kind. Taking this view of the subject at the outset, it will be instructive to compare the properties of radiant heat with those of light throughout.