THE UNDERLYING FACTS OF SCIENCE 577 



bridge between it and the sun. Sun-spots are regarded by many astron- 

 omers 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 great- 

 est 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 tempera- 

 ture 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 unde- 

 stroyed. 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. 



Failure oe Material Conceptions of the Ether 



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 rela- 

 tion to its neighbors, for this would imply a force of affinity or of cohe- 

 sion 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 sug- 

 gested 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, 



