ETHER of Sir Isaac Newton. When we have separated the actions of bodies upon each other, so far that the effects appear to us to be simple, we resolve the causes of motion into two ; namely, a disposition of bodies to come together,. called at traction, and a disposition to recede from each other, called repulsion. Impulse, or the communication of motion by ap parent contact, will not constitute a pe culiar case, because we know that bo dies cannot be, or are not, in any of our observations, brought close to each other. But as in all our philosophising we endeavour to simplify the general principles, it becomes a question, whe ther the effects of attraction and repul sion may not depend upon the same cause ; and as we have many gross in stances of bodies being urged together by the action of fluids, it naturally occurs to enquire, whether the apparent attrac tions in nature may not be caused by some fluid medium. Sir Isaac Newton was strongly of this opinion, as appears by his letter to Boyle, published in Birch's life of that philosopher, as well as by the famous paragraph at the end of his " Principia," and one of the queries at the end of his " Optics," in the pre face to the second edition of which he re marks, that he does not take gravity for an essential property of bodies. In the query here mentioned, he proceeds up on the supposition of an elastic medium pervading all space ; a supposition which he advances with considerable confidence, and which lie supports by very strong arguments, deduced as well from the phenomena of light and heat, as from the analogy of the electric and mag netic influences. This medium he sup poses to be much rarer within the dense bodies of the sun, the planets, and the comets, than in the empty celestial spa ces between them, and to grow more and more dense at greater distancesfrom them, so that all these bodies are natural ly forced towards each other by the ex cess of pressure.
The effects of ravitation might be produced by a medium thus constituted, if its particles were repelled, by all ma terial substances, with a force decreasing like other repulsive forces, simply as the distances increase; its density would then be every way such as to produce the ap pearance of an attraction varying like that of gravitation : such an ethereal me dium would therefore have the advantage of simplicity in the original law of its ac tion, since the repulsive force, which is known to belong to all matter, would be sufficient, when thus modified, to account for the principal phenomena of attrac tion.
It may be questioned whether a me dium, capable of producing the effects of gravitation in this manner, would also be equally susceptible of those modifications which we have supposed to be necessary for the transmission of light : in either case it must be supposed to pass through the apparent substance of all material bodies with the most perfect freedom, and there would, therefore, be no occa sion to apprehend any difficulty from a retardation of the celestial motions; the ultimate impenetrable particles of matter being perhaps scattered as thinly through its external form as the stars are scatter ed in a nebula, which has still the dis tant appearance of an uniform light, and of a continuous surface : and there seems no reason to doubt the possibility of the propagation of an undulation through the Newtonian medium, with the actual ve locity of light. It must be remembered,
that the difference of its pressure is not to be estimated from the actual hulk of the earth, or any other planet alone, but from the effect of the sphere of repul sion of which that planet is the centre; and we may then deduce the force of gravitation from a medium of no very enormous elasticity.
A similar combination of a simple pres sure with a variable repulsion is also ob servable in the force of cohesion ; and Dr. Young, in his Lectures, remarks, that supposing two particles of matter float ing in such an elastic medium, capable of producing gravitation, to approach each other, their mutual attraction would at once be changed from to co hesion, upon the exclusion of the portion of the medium intervening between them: this supposition is, however, as he adds, directly opposite to that which assigns to the elastic medium the power of passing freely through all the interstices of the ultimate atoms cohering in this manner ; but that, as we see some effects so nearly resembling them, which are unquestion ably produced by the pressure of the at mosphere, we can scarcely avoid sus pecting that there must be some analogy in the causes.
Two plates of metal, which cohere e nough to support Bach other in the open air,will often separate in a vacuum. When a boy draws along a stone by a piece of wet leather, the pressure of the atmo sphere seems tube materially concerned. The well-known experiment of the two exhausted hemispheres of Magdeburgh affords a still more striking instance of apparent cohesion, derived from atmo spherical pressure : and if we place be tween them a thick ring of elastic gum, we may represent the natural equilibri um between the forces of cohesion and of repulsion ; for the ring would resist any small additional pressure, with the same force as would be required for se parating the hemispheres, so far as to al low it to expand in an equal degree ; and at acertain point the ring would expand no more, the air would be admitted, and the cohesion destroyed in the same manlier as when a solid of any kind is torn asunder.
But all suppositions founded on these analogies must be considered as merely conjectural; and our knowledge of every thing which relates to the intimate con. stitution of matter, partly from the intri cacy of the subject, and partly for want of sufficient experiments, is at present in a state of great uncertainty and imper fection.