out, there is no such thing as action at a distance, therefore the Law of Gravitation demands a medium for its operation, production, and continuity. Newton distinctly points this out in his Letters to Bentley, where he says: "That one body should act upon another through empty space without the mediation of anything else, by and through which their action and pressure may be conveyed from one to another, is to me so great an absurdity that I believe no man who has in philosophical matters a faculty for thinking can ever fall into it." It has already been pointed out (Art. 42), that the only medium which is universal is the Aether medium, and we have therefore to look to the motions and properties of that medium for the solution of the problem as to the physical cause of Gravitation. That such a medium has motions which are as regular as the tides of the sea, or the trade winds of the atmosphere, will be proved later on, when it will be found that Gravitation, with all that that law implies, is due, as Newton and Challis suggested, to the pressure, properties, and motions of the aetherial medium, which is as universal as Gravitation itself. This being so, it is essential that we should set ourselves to find out from the analogies of Nature, what are those properties and motions of the Aether which give rise to the universal Law of Gravitation. This I propose doing by a consideration of three different modes of motion--viz. Heat, a mode of motion; Light, a mode of motion; and Electricity, a mode of motion. I venture to premise, from a careful consideration of these three truths, that we shall be able logically and philosophically to arrive at the simple, yet grand truth which reveals the physical source of all motion of the universe. CHAPTER VI HEAT IS MOTION ART. 60. _Heat is Motion._--On the phenomena of Heat, Newton in his eighteenth query in _Optics_ asks the questions: "Is not the heat of a warm room conveyed through the vacuum by the vibrations of a much subtler medium than air, and is not the medium the same as that medium by which light is reflected and refracted, or by whose vibrations light communicates heat to bodies? And do not the vibrations of this medium in hot bodies, contribute to the intenseness and duration of their heat? And do not hot bodies communicate their heat to contiguous cold ones by the vibrations of this medium propagated from them into the cold ones? And is not this medium exceedingly more rare and subtle than air, and exceedingly more elastic and active?" Thus it can be seen that Newton was of the opinion that heat consists in a minute vibratory motion of the particles of bodies, and that such motion was communicated through what he calls a vacuum by the vibrations of an elastic medium, the Aether, which was also concerned in the phenomena of light.