S84 FOURTH RKl'ORT — 1834. 



pressures in the interior of a fluid, M. Poisson finds equations 

 of equilibrium relative to the surface of separation of two fluids 

 incumbent one on the other, and thence, by supposing one of the 

 fluids suppressed, the equation of the free surface of a single 

 fluid. The principal conclusion arrived at in the memoir is 

 thus stated in Art. 31 : " Capillary phagnomena are due to the 

 molecular action resulting from the calorific repidsion and an 

 attractive force, and modified not only by the form of the sur- 

 faces as in Laplace's theory, but moreover by a particular state 

 of compression* of the fluid at its supei-ficies." The variation 

 of density near the surface, it is shown, must be extremely rapid. 

 Also, " the molecular attraction in fluids as well as solids extends 

 further than the calorific repulsion." (Art. 30.) 



The above conclusion is confirmed in various waj^s, and the 

 consequences that flow from it with reference to capillary phae- 

 nomena are fully discussed in the woi-k of M. Poisson entitled 

 Nouvelle Theorie de V Action Capillairef. The object of this 

 treatise is to bring the theory of capillary phaenomena to the 

 greatest degree of perfection that the power of analysis and the 

 existing knowledge of facts will permit. In the first chapter the 

 author proves that if the rapid variation of density near the 

 surface were neglected, the fluid within the tube would remain 

 horizontal, and there would be neither elevation nor depression. 

 He shows also the necessity of having regard to the variable 

 compression experienced by the fluid near the surface of the 

 tube, and i-eaching to the extent of the action dvie to the solid. 

 Whence it follows that the principles of Laplace's theory are 

 defective, notwithstanding it is so successful in the explanation 

 of phaenomena. In Arts. 18 and 19, M. Poisson obtains, on the 

 supposition of incompressibility, the equation of Dr. Young re- 

 lative to the angle of contact, which, as we have seen, was also 



* It is not perhaps difficult to see, without the aid of analytical calculation, 

 that if bodies be assumed to be composed of i«oZa<e(? atoms held in places of 

 equilibrium by attractive and repulsive forces, the sphere of whose sensible in- 

 fluence is very small, there must be a rapid change of density at their surfaces. 

 If experiment should ever be able to detect such a change, this assumed con- 

 stitution of bodies would be rendered extremely probable. 



t Paris 1831. Extracts from this work, with Remarks by H.F. Link, will be 

 found in Poggendorif's Annalen der Physik und Chemie, bd. xxv. 1S;32, p. 270, 

 and bd. xxvii. 1833, p. 193. 



M. Poisson states in the preamble of his forthcoming treatise on "The Ma- 

 thematical Theory of Heat," which appeared recently in a Paris weekly scien- 

 tific Journal {L'Insti/ut, No. for May 24, 1834,) that that theory forms the 

 second i^art of a " Treatise on ]\Iathematical Physics," in which he proposes to 

 treat, without restriction to any predetermined order, different physical ques- 

 tions whicli admit of the application of analysis. The " New Theory of Capil- 

 lary Action " is the first part. 



