Rev. P. Kelland on Mossotti's Theorx) of Molecular Action. 9 



objected to, and by putting them in a proper light, I shall be 

 considered, I trust, to have offered the best apology for them. 



I proceed then to examine the difficulty raised relative to 

 the second set of equations (p. 385. of vol. xix.). Let us sup- 

 pose, for the sake of argument, that we are driven to the alter- 

 native of admitting " either that the molecular action ex- 

 pressed by the first terms of these equations is included in 

 that expressed in the second, in which case the first terms are 

 superfluous and the equations incorrect j or that besides the 

 forces given in the hypothesis, we must take into account 

 certain other forces whose nature is wholly unknown, viz. 

 those by which the pressure on the molecules is produced." 



If by the forces spoken of in the last clause, it is implied 

 that the forces assumed to exist by M. Mossotti could not 

 include pressure in addition to the effects previously taken 

 account of, I have, for the present, no other reply to offer 

 than this : — That M. Mossotti supposes the pressure to take 

 place by actual contact. I say, this is my explanation ^o?- the 

 ■present. The fact that finite sums are replaced by integrals 

 introduces other considerations, on which we will not dwell 

 now. The pressure which MM. Poisson and Cauchy trace 

 to molecular action, is a pressure much of the same kind as 

 this. These authors refer it to its cause^ M. Mossotti esti- 

 mates its effect. Now it appeared to M. Mossotti, and it does 

 so to myself, that this effect cannot be treated as &. force; for 

 it does not depend on the relative positions of the active par- 

 ticles, or on the absolute amount of their proper action. It 

 depends almost entirely on the action of other particles on the 

 pressing one. 



But if it be meant that M. Mossotti's expressions for the ac- 

 tions of his forces, inasmuch as they comprehend the cause, 

 must give also the effect of pressure, we conceive that a simple 

 illustration will clearly show the contrary. Imagine a single 

 particle of matter to exist with a single particle of eether by 

 its side; this will be a system of equilibrium; but if we esti- 

 mate the force on the particle of matter, we shall find it to be 

 one of attraction towards the particle of aether. How then is 

 the equilibrium preserved ? It appeared to M. Mossotti that 

 \!i\Q pressure of the particles sustained it; and this pressure 

 (if it exists) is not in any legitimate sense of the word 2i force. 



On the second set of equations, I shall say but little. The 

 subject is one of great difficulty, and requires ample develop- 

 ment. I am afraid, too, I do not understand rightly what 

 your Correspondent's objection amounts to, in sayino- « the 

 conclusion is obvious, and coincides with that which we de- 

 duced from the consideration of (2.)." Are we to understand 



