Theory of Molecular Action according to Newton's Law, 203 



sumption that — 5, &c. are not zero. In the contrary case, 

 ax 1 



as Mr. Earnshaw had previously pointed out (Art. 8), " the 



displacements of particles would not bring into action any 



forces of restitution." Another part of the objection relates 



to the boundaries of the medium, or rather of space. " If the 



particles of aether exert a repulsive action on each other, they 



will naturally endeavour to disperse themselves throughout 



all space, and form a medium coextensive with the boundaries 



of the universe. Here, then, a formidable difficulty presents 



itself to our notice. If the medium be of finite dimensions 



it must be inclosed in an envelope capable of restraining the 



expansive energy of the whole mass of particles. The more 



extensive the medium, the greater must be the strength of the 



envelope. Is it probable that the constitution of the universe 



is such as to require that the whole should be enclosed in a 



huge vessel of inconceivable strength?" (Art. 20.) The author 



then goes on to remove the difficulty by assuming a law of 



force, partly attractive, partly repulsive. 



In replying to these objections we will reverse their order. 



a. The difficulty thrown out relative to the equilibrium of 

 the remote parts of space is one which has often presented 

 itself, but from a consideration of which philosophers have, in 

 general, cautiously abstained. The Newtonian system of the 

 universe is beset with difficulties of a similar nature, which, 

 although by no means satisfactorily removed, are never re- 

 garded as subversive of the hypothesis. We must, I conceive, 

 be content with a theory capable of explaining phaenomena 

 which come within the limits of our own observation, without 

 requiring that it should penetrate to the boundaries of the 

 universe, if, in truth, such boundaries exist. I shall consider 

 myself, therefore, at liberty to pass over this objection, with 

 merely requesting that, should it be pressed, I may be informed 

 how it is got over in the Newtonian system. I shall merely 

 add that the molecular hypothesis does not assume that all 

 the particles act with attractive, or all with repulsive forces. 



b. We proceed to examine the circumstances under which 

 the equilibrium may be neuter. It appears to me that this is 

 really the state of things in nature, and accordingly, when re- 

 plying to Mr. Earnshaw before the Philosophical Society of 

 Cambridge, a little more than two years ago, I argued in 

 support. I then expressed my belief that, in a medium of 

 symmetry, no force whatever is put in play on a particle by its 

 displacement alone. Subsequent investigation has confirmed 

 me in my conjecture. So far as I had proceeded in the in- 

 vestigation I found that V appeared to be constant, so that all 



