Defence of the Newtonian Law of Molecular Action. 195 
the different portions of it in order. 1. I am told that an ar- 
gument I have used “ is considered as a strong indication of 
my having allowed other motives than a desire of truth to in- 
fluence me in bringing it forward.” It is quite true I had 
other motives,—motives of delicacy. I did not wish to make a 
conclusion so evidently at variance with the premises stand 
forth prominently in a friendly controversy. In the first 
sketch of my paper it bore a more conspicuous place than I 
afterwards permitted it to do. Mr. Earnshaw remarks that 
it stands in his Memoir as a purely casual observation. I am 
glad to learn he intends it as no more, and an incorrect one 
of course. It appeared to me to be the summing up of the 
argument which I was replying to; for this is the way in which 
it is introduced; “and consequently whether the particles are 
arranged in cubical forms, or in any other manner, there will 
always exist a direction of instability.” (Art. 5). If I am to 
understand that Mr. Earnshaw withdraws this, then have I 
attained the main object of my reply to the objection from in- 
stability, for he then withdraws the arguments on which it is 
founded. I trust to the discernment of my readers to decide 
whether in what I said I stepped “ out of the line of legitimate 
argument.” 
Mr. Earnshaw goes on to say, “unfortunately for the Pro- 
fessor, in this instance he reaps no advantage by stepping out 
of the line of legitimate argument, as his objection is founded 
on the misconception that I have supposed the particles to be 
in equilibrium.” Ireply that I certainly did consider that the 
portion of Mr. Earnshaw’s Memoir which relates to instability 
admitted that the particles are (or at least may be) supposed 
in their position of rest. Had I conceived Mr. Earnshaw 
would not allow this, I certainly should not have thought his 
objections worth answering, and I apologize for having trou- 
bled my readers with a reply. But I must not on that ac- 
count refuse Mr. Earnshaw’s request (top of p. 438), * to point 
out the link of his argument against Newton’s law which vio- 
lates that supposition” (viz. that the particles are not in their 
position of rest). It is to be found in Art. 12 of his Memoir 
(Trans. Camb. Ph. Soc. v. 7), the enunciation of which is as 
follows: —* To find the force of restitution when a particle is 
slightly disturbed from its position of equilibrium.” In this ar- 
ticle it is assumed that A a 0, or the force on the particle 
parallel to any axis is zero; the particle being in its position 
of rest, and the other particles in their positions nof of rest, 
This assumption is manifestly incorrect. It amounts to the 
following: By moving all the particles of a system but one in 
O2 , 
