I20 



SCIENCE. 



[Vol. XVIII. No. 447 



And it is worthy of remark, as a matter of congratulation, 

 showing how far scientific men have emerged fron> the in- 

 tellectual pugilism of the last century, that this audacious 

 departure was met with pleased surprise, instead of angry 

 polemics against a new heresy. 



The modern theory of the ether took its origin with the 

 undulatory theory of light. It had for a tangible basis the 

 observed fact that light requires time for transmission. 

 Whether measured over long distances through interplan- 

 etary spaces, or through short distances on the surface of the 

 earth, the time required for transmission is proportionate to 

 the distance. It was to such facts that men finally came to 

 look for a justification of the assumption of an all-pervading 

 medium. The modern theories of the nature of the ether 

 are based wholly on the results which must be produced by 

 this invisible machinery, instead of upon an assumed dictum 

 that Nature abhors a vacuum. Perhaps no teaching of sci- 

 ence is now more firmly established than the doctrine of the 

 existence of an ether, and that it is capable of transmitting 

 energy by virtue of peculiarities which must be as definite as 

 those which characterize a train of cog-wheels. 



But when one comes to assemble all of the results which 

 remain to be accounted for and explained, it becomes ex- 

 ceedingly difficult to construct a mental image, in three- 

 dimensional space, of the machinery capable of producing 

 them all. 



Green's idea of the ether makes it an incompressible, fric- 

 tionless, structureless jelly, sometimes called a "solid," 

 which opens out and allows the particles of ordinary matter 

 to sweep through without appreciable resistance. Thomas 

 Young likened the operation to the sweeping of the wind 

 through the leaves and branches of a forest. Certain well- 

 known electrical experiments of Faraday and Cavendish 

 :seem to require the assumption that electricity is, or is some 

 function of, an incompressible medium. On the other hand, 

 the slowing up of light in space occupied by matter indicates 

 that the ether within must be either more dense (as Fresnel 

 believed) or less elastic than that existing in free space. It 

 is certainly very difficult to understand what there can be 

 in the molecules of matter which can increase the density of 

 an incompressible medium, as the experiments of Fresnel 

 seemed to require ; nor is it as yet easy on any hypothesis to 

 account for those condensed films of bound ether which are 

 carried along with the particles of moving matter. They 

 seem to be differentiated with equal sharpness from the free 

 ether which sweeps through matter, and from the spinning 

 aggregation of ether vortices which Thomson assumes may 

 perhaps make up the molecule or the atom. Certainly it 

 would seem that a vortex ring in a medium so devoid of 

 friction that these vortices are permanent, could hardly drag 

 :along with it portions of the surrounding medium, from 

 which the analysis of Helmholtz shows it must be wholly 

 and forever differentiated. And the matter is not simplified 

 by the beautiful experiment of Michelson and Morley. It 

 appears that the frictionless ether adheres in a layer around 

 the earth as a whole, or at least that it was entangled in and 

 •carried along with the matter composing the building in 

 which their experiments were made. 



If, however, one forms a Torricellian vacuum in a barom- 

 eter tube of either transparent' or opaque material, it is easily 

 and completely shown, by an inclination of the tube, that 

 the ether flows freely through matter. Whether the ether 

 be incompressible or highly compressible, it seems to be as 

 impossible to compress it in a chamber surrounded by mat- 

 ter as it would be to compress water or air in a fisher's net. 



The fact that a rotational phenomenon, such as must exist 

 in the field of a steel magnet, is maintained indefinitely 

 without the expenditure of energy, must certainly justify 

 the assumption that the ether is frictionless, that the ether- 

 vortex atom is a possibility. 



The effects which seem to be nearest to a mechanical ex- 

 planation are those which result in heat or light and electri- 

 cal and magnetic induction. It is possible to construct ma- 

 chinery which will represent the conditions for propagation 

 of a magnetic induction in a plane radial to a conducting 

 wire. A train of cog-wheels separated by elastic idle- wheels 

 which articulate with them, or a series of smooth rimmed 

 fly wheels connected by elastic bands, will do the work. It 

 becomes more difficult when we spread this paraphernalia 

 into three-dimensional space. 



No one, of course, thinks of the geared ether models of 

 Maxwell and Fitzgerald as anj'thing more than an aid to a 

 conception of the nature of the action to be explained. 

 When we come down to the working drawings we find great 

 room for conjecture, and some demand for invention. Do 

 the pai'ticular cog-wheels which slip on each other without 

 friction at the surface and within the body of a perfect con- 

 ductor ever get outside of the body into free space where 

 they must gear rigidly with each other? If so, why do they 

 behave so differently in the two places? What happens to 

 this gearing when masses of matter which it permeates are 

 set into rotation? Is there any difference between the earth's 

 magnetism and the motion of masses of ether at the earth's 

 surface? It is exceedingly difficult to understand how a 

 frictionless medium in which a magnetic spin is permanent, 

 can offer resistance to shear, unless the rigidity involved is 

 due to motion. 



Another function which the ether should perform is the 

 transmission of gravitation. The theory which has attracted 

 most attention, the only one suggested which has been seri- 

 ously considered, is the one first announced by La Sage of 

 Geneva, and elaborated by Preston and others. It seems to 

 require that the ether shall partake of the nature of the gas, 

 the mean free path being of interplanetary dimensions. 



Such a medium it is not difficult to admit as a possibility. 

 The theory accounts for the gravitation of bodies towards 

 each other as due to the difference in the bombardment of 

 bodies on the exposed and sheltered sides. Each body shields 

 the other, so that gravitating bodies are pushed together. It 

 is, however, necessary that the particles at the centre of the 

 earth shall have the same resultant differential pressure di- 

 rectly exerted upon them, causing them to gravitate towards 

 the sun, as if the surrounding mass of the earth were re- 

 moved. It is, in fact, necessary to assume that nearly all 

 the ether particles which plunge into the eartli's figure pass 

 straight through the earth without encounter with matter. 



It is, however, of some interest to know quantitatively 

 about what velocities must be involved in such an impact 

 theory of gravitation. DeVolsoa Wood has made a compu- 

 tation of the density of an elastic medium capable of trans- 

 mitting a pulsation with the velocity of light, and of trans- 

 mitting from the sun to the earth 2.8 calories per minute per 

 square centimetre of surface. While it seems to me that 

 some fault may be found with his analysis, still the results 

 reached by him seem to be of the proper order of magnitude. 



The density of the ether turns out to be about ^^-^ pounds 

 per cubic foot, so that a mass equal in volume to that of the 

 earth would contain about a pound and seven-tenths This 

 value for density lies well within the limits which Sir Wil- 

 liam Thomson assigned to the same quantity. 



