A body is termed hot, when its heat exceeds that of the part, with which we touch it; cold, when its heat is less than this. The terms hot and cold are therefore relative ones, and the qualities denoted by them run into each other without any precise distinguishing limits. We may consequently refer cold to heat, and, if we admit the doctrine of vibrations, we are to suppose, that the small parts of all bodies are agitated by subtle vibrations; and that when these vibrations exceed those of the part with which we touch them, they are called warm or hot; when they fall short, cold.
This may be regarded as a gross, general position, which presents itself upon the first consideration of this matter. But then, as, according to this definition of heat, all those objects of taste and smell, which excite strong vibratory motions in the organs, ought to excite heat, we must inquire farther into the vibratory motions of bodies termed hot in common language, and into the difference between these and the vibrations excited in the nerves of taste and smell by sapid and odorous bodies.
I conjecture therefore, that the vibrations belonging to heat are in general quicker and shorter, than the peculiar ones excited by tastes, smells, and colours; also that the last, or the vibrations of the rays of light, are quicker than those of tastes and smells. We may conceive farther, that all the vibrations of the small particles of the medullary substance, and interjacent æther, from whatever cause they arise, grow quicker as they grow shorter, i.e. weaker; or, according to the conjecture just made, that in declining they tend to those which impress the sensation of heat. For vibratory motions of different lengths can be isochronous only according to one law, viz. that of the accelerating force being in the simple proportion of the distance from the middle point of the vibration, as when a heavy body vibrates in a cycloid; whereas, if the accelerating force be in any less ratio than this, short vibrations will be quicker than long ones. Lastly, we are to conceive, that when two vibrations of different kinds, or frequences, are impressed at the same time, they must reduce one another to some single intermediate one, unless the quicker be so much more numerous than the slower, as to be comprehended within them, so that both may be performed together without opposition or confusion.
Let us now inquire how far the several effects of heat and cold upon our bodies are agreeable either to the notion of vibrations in general, or to the particular conjectures of the last paragraph.
First, then, we may expect that heat will rarefy the solids and fluids of the body, and the last more than the first, which is agreeable to experience. For the increase of the agitations will make the small particles recede from one another, and that more
