318 Lord Kelvin on the Motions of Ether 



indeed almost absolutely certain that there are many different 

 kinds of atom, each eternally invariable in its own specific 

 quality ; and that different substances, such as gold, silver, 

 lead, iron, copper, oxygen, nitrogen, hydrogen, consist each 

 of them of atoms of one invariable quality; and that every- 

 one of them is incapable of being transmuted into any other. 

 § 4. The sole properties of an atom are : — (1) its mass 

 (being the measure of the inertia of its translatory motion), (2) 

 its law of mutual force between itself and every other gravi- 

 tational or electrical atom in the universe, varying according 

 to the distance between them. As to the mutual force between 

 ponderable atoms, we have strong reason to believe that this 

 law is practically the Newtonian Law of universal gravitation, 

 for all distances exceeding the millionth of a centimetre. For 

 distances considerably less than the millionth of a centimetre, 

 the Newtonian Law of attraction according to the inverse 

 square of distance merges into repulsions resulting in mutual 

 pressure of two bodies resisting joint occupation of space. 

 For smaller distances, we have attraction again, in the 

 inevitable theory of Boscovich, constituting cohesions and 

 chemical affinities. 



§ 5. The assumption, that the mutual force between two 

 atoms depends merely on the distance between their centres, 

 implies that each atom is utterly isotropic. An seolotropic 

 atom, that is to say, an atom having different attractive and 

 repulsive forces in different directions, is conceivable : and 

 may possibly come in future to have a place in atomic theory. 

 Hitherto it has been universally assumed that every atom, 

 whether gravitational or electrical, is thoroughly isotropic, 

 and I do not propose at present to enter upon any theoretical 

 consideration of seolotropic atoms. 



§ 6. 1 do not propose to enter on any atomic theory of 

 ether. It seems to me indeed most probable that in reality 

 ether is structureless; which means that every portion of 

 ether however small has the same elastic properties as any 

 portion however great. There is no difficulty in this concep- 

 tion of an utterly homogeneous elastic solid, occupying the 

 whole of space from infinity to infinity in every direction. 

 We sometimes hear the " luminiferous ether " spoken of as a 

 fluid. More than thirty years ago I abandoned, for reasons 

 which seem to me thoroughly cogent, the idea that ether is a 

 fluid presenting appearances of elasticity due to motion, as 

 in collisions between Helmholtz vortex rings. Abandoning 

 this idea, we are driven to the conclusion that ether is an 

 elastic solid, capable of equi-voluminal waves in which the 

 motive force is elastic resistance against change of shape. 



