However, as it is of great importance in the present subject, I thought it best to give it a distinct place and consideration.
Before I enter upon the proof of it, it will be proper to premise something by way of explanation, concerning the æther, and the qualities of the medullary substance just mentioned.
Sir Isaac Newton supposes, that a very subtle and elastic fluid, which he calls æther, for the sake of treating upon it commodiously under an appropriated name, is diffused through the pores of gross bodies, as well as through the open spaces that are void of gross matter. He supposes likewise, that it is rarer in the pores of bodies than in open spaces, and even rarer in small pores and dense bodies, than in large pores and rare bodies; and also that its density increases in receding from gross matter; so, for instance, as to be greater at the hundredth of an inch from the surface of any body than at its surface; and so on. To the action of this æther he ascribes the attractions of gravitation and cohesion, the attractions and repulsions of electrical bodies, the mutual influences of bodies and light upon each other, the effects and communication of heat, and the performance of animal sensation and motion. My business in these observations is only with the last; but the reader will do well to consult what Sir Isaac Newton has himself advanced concerning the existence of this æther, and the properties and powers which he has ascribed to it in the last paragraph of his Principia, the Questions annexed to his Optics, and a Letter from him to Mr. Boyle, lately published in Mr. Boyle’s Life. As to myself, I am not satisfied that I understand him perfectly on this subject. I will hint a few things partly from him, partly from my own reflections, concerning the existence and properties of this æther.
Since a thermometer kept in vacuo varies with the heat and cold of the room in which it is placed, as much as another surrounded by air; and since the small parts of hot bodies probably vibrate to and fro, and by thus vibrating keep up the heat for a certain time; one may conjecture that a subtle medium remains after the air is exhausted, and that heat is communicated to the thermometer suspended in vacuo, by the vibrating motions of this medium. See Opt. Qu. 18.
The greater density of the æther at a distance from bodies than at their surface, may be conjectured from the various phænomena solved by this supposition; which phænomena may also be alleged as probable evidences of the existence of the æther. See Opt. Qu. and the Letter to Mr. Boyle.
The great subtlety and elasticity of the æther may be inferred from the motions of the planets and quick propagation of light, if we first suppose its existence, and concurrence in the propagation of light, and efficacy in causing gravity. And from its great elasticity we may infer, that it is extremely susceptible of vibrations and pulses, in the same manner as common air. See Opt. Qu.
Since the gross bodies that lie upon the surface of the earth
