Page:Popular Science Monthly Volume 78.djvu/587

Rh bridge between it and the sun. Sun-spots are regarded by many astronomers as a falling-in of a portion of the photosphere, but the reasons for this belief are not conclusive, and it is legitimate to believe in a movement in the opposite direction due to an explosive force within the effulgent crust which, as in the case of volcanoes, occasionally relieves the tension beneath that crust or in its cavities. Our periods of greatest heat often follow sun-spot activity—not that sun-spots are hotter than the rest of the sun's surface, but the matter which is sent forth intercepts and stores heat before it can pass beyond the limits of our orbit to be absorbed by extraneous systems.

A last consideration which makes the chemical theory of the ether untenable is the fact that if space were filled with gases, the temperature near heated bodies like the sun would be very great, and powerful currents would be set up which would be detected by optical if not by other means. A material ether would possess some degree of viscosity and would necessarily interfere with the progress of bodies, and this negative acceleration would create heat of friction, dissipated but undestroyed. This leads us to the consideration of the physical requisites of an ether in which matter, as we know it, must "fit," before we examine any of the other theories which have been offered within recent years.

All the experience which has been acquired through telescope, microscope and spectroscope, with and without the aid of the camera, leads to the belief that the all-pervading medium must be uniform and homogeneous. While this may be taken for granted, it by no means implies that the medium must be continuous; in other words, each of its components does not necessarily have to stand in any physical relation to its neighbors, for this would imply a force of affinity or of cohesion which one may be unwilling to grant, as it yields a purely material conception and carries one back to the chemical theory, which has been laid to one side for the present.

Some of the most eminent physicists have adopted the view that the universal medium must be solid; this belief is based on the manner of propagation of light and other high-frequency energies, which take place without appreciable dispersion in space. But, on the other hand, in all theories but one, this medium is expected to be of a nature which will offer little or no resistance to bodies moving through it. At first sight it is hard to reconcile this requirement with the nature of a perfect elastic solid such as we picture to ourselves. It has been suggested that the medium must be somewhat like pitch which shows no track of a body which has passed through it, and we are asked to conceive our planet—not to mention our humble selves—moving at a rate of eighteen miles per second through it, and, what is still more incredible.