HEAT, in physiology, one of the secondary qualities of bodies, produced by fire, and opposed to cold.
Under the article fire, we considered the sun as the principal source of heat upon the earth's surface, and the confines of the earth and atmosphere: without this, all the bodies upon our globe would doubtless grow rigid, lifeless, and fixed. It is this that stirs within them, as the main spring of their actions. Hence vegetation and animalization are evidently promoted; and hence the ocean and the atmosphere continue in a fluid state.
Heat in us is properly a sensation, excited by the action of fire; or it is the effect of fire on our organs of feeling. Hence it follows, that what we call heat is a particular idea or modification of our own mind, and not any thing existing in that form in the body that occasions it. Heat, says Mr Locke, is no more in the fire that burns the finger, than pain is in the needle that pricks it. In effect, heat in the body that
gives it, is only motion; and in the mind, only a particular idea.
Heat in the hot body, according to 'S Gravefande, is an agitation of the parts of the body, made by means of the fire contained in it: by such an agitation a motion is produced in our bodies, which excites the idea of heat in our mind; so that heat in respect of us is nothing but that idea, and in the hot body nothing but motion. If such motion expel the fire in right lines, it gives us the idea of light; if in a various and irregular motion, only heat.
Heat, with respect to our sensations, or the effect produced on us by a hot body, is estimated by its relation to the organ of feeling; no object appearing to be hot, unless its heat exceed that of our body. Whence the same thing to different persons, or at different times to the same person, shall appear both hot and cold. The degree of heat is measured by the expansion of the air, or spirit in the thermometer.
It has been justly observed, by some of our modern philosophers, that actual or absolute heat, is to sensible or relative heat, the same as motion is to velocity: for absolute heat is nothing but the whole motion of all the parts of the ignited body; and sensible or relative heat, respects only the comparative velocity of the parts. Thus, equal bulks of mercury and water set in a sand-heat, where the heat of the fire may be uniformly communicated to both, will acquire in equal times equal degrees of absolute heat: but the relative heat of the water, or that which is sensible to the finger, will be near 14 times as great as that of the mercury, because the water, having 14 times a less quantity of matter, will admit of velocity so much in proportion greater.
Again, if mercury and water have the same relative or sensible heat, that is, if both are heated in such a manner as to cause an equal ascent in the thermometer, then a quantity of mercury will heat 14 times as much water as the same quantity of water will do; or it will make the same quantity of cold water 14 times hotter than the same quantity of hot water can. All which is easy to be shewn by experiment, and abundantly proves, that heat and fire are wholly owing to the velocity of the parts of the heated or ardent body: on which theory the various phenomena of heat, cold, fire, burning, &c. are rationally accounted for. For, first, we are to consider, that cold and heat are only comparative terms, or that the same thing may either be too hot, or too cold, according to the relative idea or standard-degree. Thus, ice or snow is said to be cold with respect to the finger, but ice or snow is warm if compared to a freezing mixture; so that if (as we commonly do) we make the hand or any part of the body the standard of heat or cold, or the term of comparison; then it is evident, 1. If the parts of any body, applied to the hand, have the same velocity as the parts of the hand, such a body we naturally pronounce is neither hot nor cold. 2. If the particles of the body have a greater velocity than those of the hand, we pronounce it warm, if the excess be small; but hot, if it be great. 3. If the velocity of the parts of the body applied be less than that in the hand, the sensation then is what we call cold, which also may be in various degrees. 4. Hence it is plain, there can be no such thing as absolute cold, but where the particles of matter are absolutely quiescent or at rest. 5. Hence also, there can be no such thing as absolute heat, because no degree of velocity can be assigned but a greater is still assignable, till we come to infinity, where we are quite lost, as having no idea of infinite velocity or heat.
From this theory of heat and cold we may conclude, that there is no body in nature whose parts are not in motion, in some degree, since we have yet been able to discover no ultimate degree or limit of cold; and if any such thing were to be found in nature, it is likely that it would be as impossible to bear or endure the rest, as any extreme degree of heat; both heat and cold naturally tending to destroy the animated part, or rest, in the extreme degrees: cold, by destroying
the vital motion, and fixing the part rigid and inflexible; but heat, by putting the parts into too great an agitation, causing a greater velocity of the fluids, and dissipation and a force of tension in the solids beyond what the natural state of the body can bear; and therefore it will inevitably destroy it.