stance. If then, as would be the case, it should fall into the sun, this heat would be increased by the fall four-hundred-fold. Now, it makes no difference in the aggregate evolution of heat whether this cessation of motion is sudden or gradual; and if we can find in Nature any agencies tending to retard the revolutions of the planetary bodies, they must inevitably sooner or later fall into the sun. In such a case it can hardly be doubted that we have found a cause sufficient to produce again the disintegration and diffusion of matter.
The wave-theory of light and radiant heat presupposes the existence of an ethereal medium pervading all space. It must be a medium of material atoms held in equipoise by a balance of forces, for it is evident there could be no wave-motion unless there was something to move, and something, too, having the attributes of matter in a state of extreme mobility or fluidity. There is no other conceivable way by which light could reach us from the sun and stars except through this all-pervading form of matter. And if there is a material medium, of whatsoever exceeding tenuity it may be, still it must pre sent something of resistance to everything passing through it. It resists the passage of light eight minutes in 90,000,000 miles, thus proving its materiality by its resistance to force, which is one of the definitions of matter. If one could conceive of any force passing through an absolute vacuum, it could only be conceived of as passing instantaneously—there is absolutely nothing to detain it. Again, heat and its allied forces are only effects, and the subject is and can be only matter. There is no physical truth better established than that the forces can exist only where matter is in some form. It is not essential that this form of matter be subject to the ordinary laws of gravitation. The probability is, that it differs entirely from anything that we have experience of. It would seem that the atoms composing the ether of space, instead of attracting each other like those of ordinary matter, must repel each other. At least this supposition would account for what there is remarkable in connection with the ethereal medium. But, whatever theories we may adopt in regard to it, this is certainly true, that the revolutions of the heavenly bodies must be continually opening passages through it, and that a certain part of the force of those revolutions must be expended in pushing it aside. The centrifugal force is thus lessened, and the bodies are drawn nearer to the sun. In consequence, the periods of their revolutions are shortened. This has not as yet become noticeable in the case of the planets, from the fact that the slow contraction of their bulk by the loss of internal heat through volcanoes, thermal springs, and other sources, has the contrary effect of increasing the velocity of revolution, and thus counterbalancing the retardation of friction. The fact that the two effects are thus nearly counterbalanced proves the retardation, for otherwise we know that the acceleration would be observable. In the case, however, of the light cometary bodies, it has been shown that they suf-
