"Scientific men," says Mayer, "do not doubt the existence of such an ether." The presence of such "material ether--dense, elastic, and capable of motion--subject to and determined by mechanical laws,"[156] is demanded for the explanation of radiant heat, light, and actinism. No other theory ever proposed has so beautifully and completely accounted for all the facts. Its reality must be admitted, until the positions established by Huyghens, Young, Fresnel, Foucault, and Fiziau are shown to be untenable. All the prominent experimental physicists of the present day agree in teaching that light and heat are transmitted by vibrations or wave-like motions in a material medium universally diffused through space, and permeating all material bodies. Light and heat are the ceaseless thrill which the distant orbs collectively create in the ether, and which constitute what has been called the _temperature of space_. If the existence of such material medium as the assumed ether be denied, we can not account in any conceivable or rational manner for the transmission of light and heat from the sun. And now, if the space between the celestial bodies contain no other matter than that necessary for the transmission of light, "that alone," says Littrow, "is sufficient, in the course of time, to alter the motion of the planets, and the arrangements of the solar system itself; the fall of all the planets and comets into the sun, and the destruction of the present state of the solar system, must be the final result of this action."[157] But it is further claimed by Helmholtz, Mayer, and Sir William Thomson that the phenomena presented by Encke's comet furnish "direct proof" of the existence of such resisting medium. The observations on this comet made during the past thirty or forty years show that the periods of its revolution are continually diminishing at the rate of 0.11° per revolution of nearly 3-1/3 years. In other words, the comet's mean distance from the sun is diminishing by slow and regular degrees. The solution which Encke himself proposed, and which Herschel informs us "is generally received,"[158] is that resistance is experienced from the medium in which the comet moves; such resistance diminishing its actual velocity and also its centrifugal force, thus giving the sun greater power to draw it nearer. It will, therefore, fall into the sun. A similar fate, says Helmholtz, threatens all the planets. "The analogies of nature, and the ascertained facts of physical science, forbid us to doubt that _every star_, and, indeed, _every body of every kind_ moving in any part of space, has its relative motion impeded by the air, gas, vapor, medium, or whatever we call the substance occupying space immediately around them, just as the motion of a rifle-bullet is impeded by the resistance of the air."[159]