where the number of vibrating particles were many, than where few. The same hypothesis is well suited to the rarefaction, fluidity, dissolution, and other changes of texture which heat produces in bodies, according to their various natures. And if we farther consider that all bodies, contiguous to each other, come, after a short time, to the same degree of heat, viz. that of the circumambient air; those which are hotter losing something, and those which are colder gaining; and yet that the air is not necessary for the conveyance of heat, as appears from Sir Isaac Newton’s experiment of the two thermometers above-mentioned; it will appear highly probable, both that heat in bodies is attended by vibratory motions of the small parts, and also that these are communicated to contiguous bodies by vibrations of a subtle fluid, by an argument something different from that urged above, in speaking of the two thermometers; at least the consideration of the equality of temperature, to which all contiguous bodies are known, by common observations, to arrive, will cast some light upon that argument. And upon the whole it will follow, that heat in us is caused or attended by subtle vibrations of the medullary substance, which is the immediate instrument of all the sensations; and that a subtle fluid is concerned in the production of this effect. And what is thus proved of heat, may be inferred to hold in respect of all the other sensations, from the argument of analogy.
2. Light is so nearly related to heat, that we must suppose the argument of analogy to be particularly strong in respect of it: but besides this, we have an independent argument for the existence of vibrations here, also for their communication by a subtle fluid, if we admit Sir Isaac Newton’s hypothesis concerning the cause of the alternate fits of easy reflection and transmission, as I have above remarked.
3. As sounds are caused by pulses or vibrations excited in the air by the tremors of the parts of sounding bodies, they must raise vibrations in the membrana tympani; and the small bones of the ear seem peculiarly adapted, by their situation and muscles, to communicate these vibrations to the cavities of the vestibulum, semicircular canals, and cochlea, in which the auditory nerve is expanded; i.e. to the nerve itself. Now though these are gross vibrations, in respect of those which we must suppose to take place in the æther itself, yet they prepare the way for the supposition of the more subtle vibrations of the æther, and may be the instrument of these, in the same manner as very gross reciprocal motions of bodies in the air are observed to produce, by percussion, those quicker vibrations in which sound consists.
4. We are, in some measure, prepared also for admitting the doctrine of vibrations in the animal functions, from that disposition to yield a sound upon percussion, which appears in some degree in almost all bodies; since this shews, that the disposition
