CHAPTER I.

THE THEORY OF HEAT; OR, THE GENERAL EFFECTS

OF A SUPPOSED CALORIFIC FLUID.

W

WHEN we approach a common fire, we feel a fenfation which we call beating. When we recede from the fire, and approach a quantity of ice, we feel another fenfation, which we call cooling. On a closer examination it will appear that these words beating and cooling, or heat and cold, are relative expreffions; for the very fame body may feel cold to one perfon and hot to another; or it may feel both cold and hot to the fame perfon. Let, for instance, a perfon warm one of his hands near the fire, and cool the other hand in fnow; then let him put both hands in water of a middling temperature, and the fame water will feel cold to one of his hands, and hot to the other.

It is impoffible to give a more precife definition of those fenfations, than what is

common meaning of the words.

conveyed by the

But with respect to the visible effects which are produced by thofe respective approximations, to a fire and to the ice, or to the different degrees of heating and cooling,

we may give a more determinate answer; viz. we may fay, that all the effects of heating may be reduced to an enlargement of the buik, or to the feparation of the parts, of all forts of bodies; and that, on the contrary, all the effects of cooling may be reduced to a contraction of the bulk, or to a mutual approximation, of the parts, of all forts of bodies.

A human body, and every part of an animal body, a stone, a piece of metal, a piece of glass, or, in fhort, every other body, whether folid or fluid, grows larger by heating, and fmaller by cooling; but different bodies are expanded more or lefs by the fame degrees of heating, and are contracted more or less by the fame degrees of cooling. Bodies are not only expanded differently by the fame degrees of heating, or contracted differently by the fame degrees of cooling; but by thofe means they do alfo acquire different forms. Thus a piece of ice heated to a certain degree, becomes fluid water; by increafing the heat the water is increafed in its bulk, and after a certain period the water becomes an elastic fluid; viz. fteam. By continuing to increase the heat, that fteam becomes continually larger and larger; nor do we know the limits of its expanfibility. The like effects, in a contrary order, are produced by cooling; viz. a quantity of fteam grows fmaller and fmaller, until it becomes liquid water, and at laft the water becomes a folid; viz. içe.

The

The converfe of the above-mentioned law has likewife been pretty well proved by means of experiments; namely, that if a certain fubftance be compreffed into a narrower space, a quantity of heat will come out of it, and will be communicated to the furrounding bodies; and, on the contrary, if a certain fubftance be expanded into a larger fpace, it will abforb a quantity of heat from the furrounding bodies; for thofe furrounding bodies will thereby be cooled. Thus, if you wet your hand, and then expofe it to the ambient air, the water, in the act of expanding itself into vapour, abforbs a quantity of heat from the hand, which is thereby fenfibly cooled. If air that has been compreffed by art in a strong veffel, be let out of it through an aperture, that air, in the act of expanding itself, will absorb a quantity of heat. If a piece of metal be compreffed, heat will be produced. If the fteam of water be condenfed, heat will be depofited on the bodies which are in contact with it.

The acceffion of heat, by placing the particles of matter farther from each other, diminishes their mutual attraction; viz. the attraction of aggregition, in confequence of which their attraction for other bodies; viz. the attraction of affinity, grows ftronger; hence, heating to a certain degree effects decompofitions and compofitions, which in general have been called combuftions; but when the heated fubftances have not fuch affinities, or when they

are not heated enough to render their affinities active, fo as to form decompofitions and new combinations, then the fubftances are not faid to have undergone a combustion, or to be burnt; but they are faid to be heated, or rarefied, or ignited; (viz. rendered red hot) or foftened, or liquified, or evaporized, &c. according as any of those effects is produced or attended to.

Heat penetrates bodies of every fort; for whatever body is placed near a common fire, is expanded, or foftened, or ignited; or, in fhort, it fhews fome of the effects of heating; and the fame thing is true with respect to cooling; but this heating does not penetrate all forts of bodies with equal quicknefs; it paffes through certain bodies quicker or eafier, than through others; hence the former are faid to be better conductors of beat, than the latter; we are not however acquainted with any body which may be faid to be a perfect nonconductor of heat. The same thing may be understood of cooling.

With refpect to the communication of heat, it has been observed, that if an heated body be placed amongst colder bodies, or heat be produced by certain bodies in certain proceffes amongst colder bodies, that heat will gradually pafs from the former bodies to the latter, fo as to render the former bodies lefs hot, and the latter, hotter, than they were before; and as there is not a perfect nonconductor of heat, therefore nothing can effectually

fectually prevent that expanfion, or that distribution, of heat; though it may be much obftructed and impeded by the interpofition of bad conducting bodies.

So far the effect is well known, and is daily proved by common experience. But there is another phenomenon attending the communication of heat, which is neither very obvious, nor so easily obferved. This is, that in the diftribution of heat amongst a variety of fubftances, fome bodies abforb more of it than others, though they be all placed exactly in the fame fituation; hence different bodies are faid to have different capacities for abforbing heat.-An example will eafily illuftrate this remarkable property.

If a pound of water heated to a certain degree, for instance, to 60 degrees, be mixed with another pound of water which has been heated 120 degrees, the 60 degrees of heat, which the latter has above the former, will be divided alike between those equal quantities of water; viz. 30 degrees will be communicated to the former pound of water, and the other 30 degrees of heat will remain with the latter; hence the whole will appear to have 90 degrees of heat. Now, if a pound of water heated to 60 degrees, be mixed with a pound of quickfilver, heated to 120 degrees, the mixture will appear to have (not 90 degrees as above) but only 62 degrees of heat; which fhews, that of the 60 degrees of heat, which the mercury had more