On the Clustering of Gravitational Matter in Any Part of the Universe
IN the Mathematical and Physical Science Section of the British Association, Lord Kelvin delivered a discourse on “The Absolute Amount of Gravitational Matter in any large Volume of Interstellar Space.” Gravitational matter, according to our ideas of universal gravitation, would be all matter. Now was there any matter which was not subject to the law of gravitation? He thought he might say with absolute decision that there was. They were all convinced, with their President, that ether was matter, but they were forced to say that the properties of molar matter were not to be looked for in ether as generally known to them by action resulting from force between atoms and matter, ether and ether, and atoms of matter and ether. Here he was illogical when he said between matter and ether, as if ether were not matter. It was to avoid an illogical phraseology that he used the title “gravitational matter.” Many years ago he had given strong reason to feel certain that ether was outside the law of gravitation. They need not absolutely exclude, as an idea, the possibility of there being a portion of space occupied by ether beyond which there was absolute vacuum—no ether and no matter. They admitted that that was something that one could think of; but he did not believeany living scientific man considered it in the slightest degree probable that there was space surrounding our universe beyond which there was no ether and no matter. Well, if ether went through all space, then it was certain that ether could not be subject to the law of mutual gravitation between its parts, because if it were subject to mutual attraction between its parts its equilibrium would be unstable, unless it were infinitely incompressible. But here again he was reminded of the critical character of the ground on which they stood in speaking of or giving very definite propositions beyond what they saw or felt by experiment. He was afraid he must here express a view different from that which Prof. Rücker announced in his address, when he said that continuity of matter implied absolute resistance to condensation. They had no right to bar condensation as a property of ether. While admitting ether not to have any atomic structure, it was postulated as a material which performed functions of which they knew something, and which might have properties allowing it to perform other functions of which they were not yet cognisant. If they considered ether to be matter, they postulated that it had rigidity enough for the vibration of light, but they had no right to say that it was absolutely incompressible. They must admit that sufficiently great pressure all round could condense the ether in a given space, allowing the ether in surrounding space to come in towards the ideal shrinking surface. When he said that ether might be outside the law of gravitation he assumed that it was not infinitely incompressible. He admitted that if it were infinitely incompressible, then it might be subject to the law of mutual gravitation between its parts; but to his mind it seemed infinitely improbable that ether was infinitely, incompressible, and it appeared more consistent with the analogies of the known properties of molar matter, which should be their guides, to suppose that ether had not the quality of exerting an infinite1y great force against compressing action of gravitation. Hence if they assume that it extended through all space, ether must be outside the law of gravitation, that is to say, truly imponderable. He remembered the contempt and self-complacent compassion with which sixty years ago he himself, he was afraid and most of the teachers of that time looked upon the ideas of the elderly people who went before them, who spoke of “the imponderables.” He feared that in this, as in a great many other things in science, they had to hark back to the dark ages of fifty, sixty or a hundred years ago, and that they must admit there was something which they could not refuse to call matter, but which was not subject to the Newtonian law of gravitation. That the sun, stars, planets, and meteoric stones were all of them ponderable matter was true, but the title of his paper implied that there was something else. Ether was not any part of the subject of his paper; what he dealt with was gravitational matter, ponderable matter. Ether they relegated, not to a limbo of imponderables, but to distinct species of matter which had inertia, rigidity, elasticity, compressibility, but not heaviness. In a paper he had already published he had given strong reasons for limiting to a definite amount the quantity of matter in space known tp astronomers. He could scarcely avoid using the word “universe,” but he meant our universe, which might be a very small affair after all, occupying a very small portion of all the space in which there is ponderable matter.