206 Prof. Kelland's Reply to some Objections against the 



So far, then, as we have proceeded, we have obtained, as 

 our result, that V is constant. We have thus strengthened 

 the argument, if any exists, based on the neutrality of the 

 equilibrium. But what is the argument? Mr. Earnshaw 

 says (Art. 8), " the displacements of particles placed in such 

 positions as those here considered would not bring into action 

 any forces of restoration ; on which account the particles 

 would not vibrate." Mr. O'Brien says, too, " I have shown 



that if such be the case the whole universe is in a 



state of neuter equilibrium." [Phil. Mag. June, p. 487.] The 

 only shadow of an argument contained in these quotations 

 exists in the words "on which account the particles would 

 not vibrate." What would they do then? and why? It 

 really is hard that I should be obliged to make the objection's 

 and answer them too. I hope Mr. Earnshaw will point out, 

 in a future communication, whereon he supposes the in- 

 ference to hang. So far as is stated nothing more appears 

 than this : a particle is moved, no instantaneous force is put 

 in play by the motion ; therefore the particle cannot vibrate. 

 Now to this argument we reply, — 1st, that the statement em- 

 bodies a proposition which is very difficult of proof: for 

 although the particle receives no instantaneous force, it cer- 

 tainly communicates one to the adjacent molecules. On those 

 in advance it acts more powerfully, on those behind less so, 

 than when in its position of rest. Motion will therefore ensue. 

 Whether the particles will vibrate or not we do not affirm ; 

 the onus of proving that they will not, rests with those who 

 make the assertion. But 2nd, suppose it could be proved 

 that the particles will not vibrate, what follows? I repeat that 

 we do not attempt to explain how vibrations are generated. 

 It is not to be conceived that the motion of a single particle 

 should produce a system of transverse vibrations; and he who 

 rejects every hypothesis which will not admit such to be the 

 case, excludes virtually (if I mistake not) the possibility of the 

 existence of such vibrations. All that can be made to follow 

 from the above inference, therefore, appears to be, that the 

 motion of a single particle cannot put in play a system of vi- 

 brations. This is a very different thing indeed from what is 

 supposed to be made out by it, viz. " that the constitution of 

 such a medium is incapable of transmitting light, a phaenome- 

 non due to vibration." When it shall have been shown to be 

 incapable of transmitting vibrations, it will be time to reject 

 it; but nothing of the kind has as yet been attempted. 



c. From what has preceded, it will be evident that we con- 

 ceive the constitution of media to be such that the equilibrium 

 is of the kind technically called neuter ; yet as we are desirous 



