The hypothesis that gravitation is transmitted through the same medium as light, or indeed through any medium, is encumbered with serious if not insuperable difficulties. All transmission of whatever kind--of a letter by the post, a gunshot, a sound, a wave of light, an electro-magnetic disturbance--_occupies time_. It has a velocity--sometimes a very great one, as in the case of light; still it is a measurable velocity. But, according to Herschel, the pull which the sun exerts on the earth is delivered _instantaneously_. Were it not so there would be "a continually progressive increase of the major axis of the earth's orbit, and therefore of the length of the year."[280] Surely it must be obvious to every one that the _instantaneous_ transmission of the sun's attractive force to the planet Neptune, three thousand millions of miles distant, through a physical medium like the ether, would be as great a miracle as action at a distance through a perfect void. But the advocates of this hypothesis have not thereby escaped the difficulties of action at a distance. The majority of physicists regard the luminiferous ether as consisting of "discrete particles"--"elementary molecules of inconceivable minuteness and tenuity." These ultimate particles or atoms of highly attenuated matter must have some magnitude, some extension, however inconceivably minute. If extended, they must have some form, and must occupy separate positions in space. If they are capable of motions--undulatory, rotatory, or spiral motions--they can not be in mutual contact. Conceive, then, two such atoms, and draw around each an imaginary circle. Let these circles touch at the middle point between the two, and ask yourself the question, What exists there? On the hypothesis under consideration you are bound to answer pure, empty space--that is, pure nothing. "But if there is no matter between the atoms, then all their actions, one upon the other, must be exerted across a void--that is, through a medium of nothingness;" in other words, through no medium at all. Now the size of the interval makes no difference in the argument. "Whether that interval be the 92 billionth of an inch, or the 92 millions of miles or thereabouts between the earth and the sun, it is still action at a distance, and no escape."[281]