By the definition of a unit of heat, it is at once seen that the specific heat of water is unity; and, in general, the specific heats of other bodies are less, and are therefore to be expressed as proper fractions. For example, if equal weights of water and mercury be mixed, the first at 0°, the second at 100°, the resulting temperature will not be 50° (ns it would have been had both bodies been water), but 3°.23 nearly—in other words, the amount of heat which raises the ,temperature of 1 pound of water 3°.2, is that which would raise that of 1 pound of mercury 96°.77, or the specific heat of mercury is of that of water. The following may be given as instances of the great differences which experiment has shown to exist among bodies in respect of specific heat: Water, 1.000; turpentine, .426; sulphur, .203; iron, .114; mercury, .033.
It is mainly to the great specific heat of water that we are indebted for the compara tively small amount of it required to cool a hot body dropped into it; for its compara tively small loss of temperature when it is poured into a cold vessel, and the enormous effects of the water of the ocean in modifying climate.
It has been found generally, with a few exceptions, that the specific heats of bodies are nearly inversely as their atomic weights (q.v.). Hence all atoms require the same amount of heat to produce the same change in their temperature. Thus, for simple bodies, we have atomic weight of mercury, 100 its specific heat, .083; product, 3.3: atomic weight of iron, 28; its specific heat, .114; product., 3.2. A similar remark may be made, it appears, with reference to compound bodies of the same type; but, in gen eral, the product of the specific heat and the atomic weight differs from one type to another.
Latent Heat, Fusion, Solution, and are now prepared to consider the somewhat complex effects produced•by heat on the molecular constitution of bodies; and, conversely, the relations of solidity, fluidity, etc., to heat. All bodies (except car .
bon, which has been softened only) have been melted, by the application of a proper amount of heat. The laws of this fusion are: 1. Every body has a definite melting-point, assignable on the thermometric scale, if the pressure to which it is be the same.
2. When a body is melting, it retains that fixed temperature however much heat may be applied, until the last particle is melted. The last result is most remarkable. The heat applied does not raise the temperature, but produces the change of state. Hence it seemed
to disappear, as far as the thermometer is concerned, and was therefore called latent heat.
A pound of water at 79° C. added to a pound of water at 0' C., produces, of course, 2 pounds of water at 39'.5. But, a pound of water .at 79° C. added to a pound of ice at 0° C., produces 2 pounds of water at 0°. Heat, then, has.disappeared in the production of a change from solidity to fluidity. And this we might expect from the conservation of energy for actual energy in the shape of beat must be consumed in pro ducing the potential energy of the molecular actions in the fluid. For every pound of ice melted,without change of temperature, 79 units of heal are thus converted into change of molecular arrangement.
We give a few instances of latent heat of fusion: 'Water (as above), 79.0; zinc, 28.1; sulphur, 9.4; lead, 5.4; mercury, 2.8.
In law 1, it is mentioned that constancy of pressure is necessary. In fact, the freez ine. (or melting) point of water is lowered by increase of pressure, while those of sulphur and wax are raised; but these effects, though extremely remarkable, are very small. Most bodies contract on solidifying; sonic, however, as water, cast-iron, type-metal, etc., expand. Thus, a severe frost setting in after copious rain splits rocks, etc., by the expansion of freezing water; and thus also we obtain in iron the most delicate and faithful copy of a mold, and in the fusible alloy a clear-cut copy of a type. The modern dynamical theory of heat enables us to see that a perpetual motion would be procurable, if bodies which contract on solidifying had not their melting point raised by pressure, and vice verso.
Analogous to the fusion of a solid is its solution in a liquid, or the mutual conversion into liquids of two solids which are intimately mixed in powder. Here, also, we should expect actual energy in the shape of heat, to be used up in producing the potential energy of the fluid state; and, indeed, such is always the case. Such changes of arrangement destroy heat, or produce cold; but this iu many cases is not the effect observed, as heat is generally developed by the loss of potential energy, if there be chemical action between the two substances. Hence in general, the observed effect will be the difference of the heat generated by chemical • .ction, and that absorbed in change of state.