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XXI. Reply to some Objections against the Tlieory of Molecu- 

 lar Action according to Newton's Law. By the Rev. P. 

 Kelland, M.A., F.R.SS. L. $ E., F.C.P.S., %c, Professor 

 of Mathematics in the University of Edinburgh, late Fellow 

 and Tutor of Queen's College, Cambridge*. 

 Y\THEN I wrote my reply to an anonymous correspondent 

 ** in the Phil. Mag. (S. 3. vol. xx. January 1842, p. 8), I 

 did not contemplate extending my remarks beyond the limits 

 of the objections before me. But finding, as well from the pri- 

 vate communications of my friends, as from what has ap- 

 peared in your Journal, that silence is construed into an ad- 

 mission of the indefensibility of the Newtonian law as applied 

 to molecular actions, I am induced most reluctantly to enter 

 on the defence of the hypothesis. The following remarks are 

 the substance of a paper which I read before the Philosophical 

 Society of Cambridge in 1840, but which, from my extreme 

 dislike to controversy, I never printed. Nor should I have 

 now done so, but for the expressed opinion of two of the first 

 mathematicians in Europe, whom I am proud to number 

 amongst my friends, both of whom have united in urging me 

 either to remove the difficulties which attend the theory, or to 

 point out in what way they may be regarded as not subversive 

 of its truth. It shall be my endeavour in what follows to argue 

 with perfect candour, not against the objections so much as 

 for the theory. I hope nothing I shall say will induce any 

 one to imagine that I undervalue the importance, or the in- 

 genuity of the objections themselves, or that I lightly esteem 

 the memoirs in which they are embodied. Let it be under- 

 stood that I do not attempt to overthrow the arguments of 

 my opponents to any extent further than as they, if admitted, 

 would subvert a theory in which I am deeply interested, and 

 which, indeed, I partly originated f. 



Before I enter on my subject I wish to state expressly what 

 is the hypothesis itself which I am about to defencL It is this: 

 That bodies consist of molecules, simple or aggregated in groups, 

 surrounded by particles ofafiuid which pervades all space. Both 

 the former and the latter molecules are endued with attractive 

 or repulsive forces towards each other, and each system likewise 

 attracts or repels the particles of the other. The law of force 

 in all cases is that of the inverse square of the distance. 

 * Communicated by the Author. 



+ M. Mossotti's paper was printed at Turin in 1836 j mine was read 

 in February of the same year. [M. Mossotti's paper was scarcely known in 

 this country, until its contents, especially as bearing upon the theory of 

 electricity, were announced by Mr. Faraday at the Royal Institution, on the 

 20th of January 1837 (see Phil. Mag. S.3. vol. x. p. 84, 317) : a transla- 

 tion of the entire paper appeared in Taylor's Scientific Memoirs, (vol. i. 

 p. 448) on the 1st of February.— Edit.] 



