NATURE 711 



duce its importance still further, and see in matter nothing more than 

 the geometrical locus of singularities in the ether. Lord Kelvin, for 

 instance, holds what we call matter to be only the locus of those points 

 at which the ether is animated by vortex motions. Riemann believes it 

 to be the locus of those points at which ether is constantly destroyed; 

 to Wiechert or Larmor, it is the locus of the points at which the ether 

 has undergone a kind of torsion of a very particular kind. Taking 

 any one of these points of view, I ask by what right do we apply to 

 the ether the mechanical properties observed in ordinary matter, 

 which is but false matter ? The ancient fluids, caloric, electricity, etc., 

 were abandoned when it was seen that heat is not indestructible. But 

 they were also laid aside for another reason. In materializing them, 

 their individuality was, so to speak, emphasized gaps were opened 

 between them ; and these gaps had to be filled in when the sentiment 

 of the unity of Nature became stronger, and when the intimate rela- 

 tions which connect all the parts were perceived. In multiplying the 

 fluids, not only did the ancient physicists create unnecessary entities, 

 but they destroyed real ties. It is not enough for a theory not to 

 affirm false relations; it must not conceal true relations. 



Does our ether actually exist? We know the origin of our belief 

 in the ether. If light takes several years to reach us from a distant 

 star, it is no longer on the star, nor is it on the earth. It must be 

 somewhere, and supported, so to speak, by some material agency. 



The same idea may be expressed in a more mathematical and more 

 abstract form. What we note are the changes undergone by the mate- 

 rial molecules. We see, for instance, that the photographic plate ex- 

 periences the consequences of a phenomenon of which the incan- 

 descent mass of a star was the scene several years before. Now, in 

 ordinary mechanics, the state of the system under consideration de- 

 pends only on its state at the moment immediately preceding; the 

 system therefore satisfies certain differential equations. On the other 

 hand, if we did not believe in the ether, the state of the material uni- 

 verse would depend not only on the state immediately preceding, but 

 also on much older states ; the system would satisfy equations of finite 

 differences. The ether was invented to escape this breaking down 

 of the laws of general mechanics. 



Still, this would only compel us to fill the interplanetary space with 

 ether, but not to make it penetrate into the midst of the material 

 media. Fizeau's experiment goes further. By the interference of 

 rays which have passed through the air or water in motion, it seems 

 to show us two different media penetrating each other, and yet being 

 displaced with respect to each other. The ether is all but in our 

 grasp. Experiments can be conceived in which we come closer still 

 to it. Assume that Newton's principle of the equality of action and 

 re-action is not true if applied to matter alone, and that this can be 



