Remarks on M. Mossotti's Theory of Molecular Action. 387 



mass, are well known, and need not here be discussed. To 

 return, the equilibrium of the element dx dy dz of the fluid 

 is maintained by the respective equality of the impressed 

 moving forces ~Kp dx dy dz, Y p d x dy dz, Z p dx dy d z, 

 to the differences of pressure over the opposite faces of the 



element, or to-r^ dx .dy dz; -3-dy.dzdx'. and -/- dz.dx 

 ' dx * dy * d% 



dy. Now this pressure is nothing but the result of molecular 



action between contiguous particles. Hence it is clear that 



the expressions X. Y. Z do not include all the forces which 



act upon the particles of the fluid : they exclude precisely 



the molecular forces. 



But if we examine the terms on the right-hand side of(l.), 

 we shall find that they include all the forces, of whatever 

 kind, which act upon the particles of the aether. The con- 

 clusion from this is obvious, and coincides with that which we 

 derived from the consideration of (2.). 



In fact the errors in both cases arise from the same source, 

 namely, that after considering explicitly all the forces of the 

 system, the author introduces the idea of fluid pressure, itself 

 the result of a particular mode of molecular action. 



If this were a question to be decided by a reference to au- 

 thority, we might remark, that in the various memoirs which 

 have of late years been published on the molecular view of the 

 undulatory theory of light, the luminiferous aether is con- 

 sidered simply as a congeries of molecules; and further, that 

 where, as in a memoir by Poisson, in those of the Institute for 

 1830, and in some of Cauchy's, the pressure of fluids or the 

 tension of solids is introduced in connexion with molecular 

 action, they appear as results derived from it, and not as co-or- 

 dinate principles. 



The preceding observations contain little that is not familiar 

 to most of those who take any interest in mathematical phy- 

 sics, but perhaps the celebrity of M. Mossotti's memoir may 

 give them some degree of interest, and at any rate they may 

 serve to point out the necessity of distinct views of the connex- 

 ion between the theory of molecular action and the ordinary 

 principles of equilibrium. 



R. L. E. 



September 24, 1841. 



2 C2 



