REPORT ON CAPILLARY ATTRACTION. 2.01 



Transactions of the Cambridge Philosophical Society*, his 

 theory has undergone some modification, the phaenomena both 

 of endosmose and exosmose, and the variation of the maximum 

 difference of the heights of the fluids according to the difference 

 of their densities, being accounted for independently of any 

 particular mode of communication of the fluids in the capillary 

 spaces. 



I beg leave to close this Report with proposing a query sug- 

 gested by the existing state of the theory of capillary attraction. 

 How does it happen that the principles with which Laplace's 

 theory sets out conduct to two fundamental equations the same 

 in form as those of M. Poisson's theory ? Ought not the lat- 

 ter, seeing that the law of the molecular forces is quite ar- 

 bitrary, to embrace every possible method of arriving at these 

 equations ; and should we not expect that the method of Laplace 

 is not inconsistent with the other, but a particular case of it ? 

 The most probable supposition respecting the molecular forces 

 of fluids is, that the attractive force is comparatively small, de- 

 creases much more slowly with the distance than the repulsive, 

 and is sensible to a much greater distance from the centre to 

 which it is directed. The hypothesis of incompressibility cor- 

 responds to the limiting case, when the repulsive force being 

 infinitely great at first, decreases by very large gradations as 

 the distance from the centre increases, and within a very small 

 space becomes less than the attractive force. As the above law 

 of the forces as well as the limiting case of it are embraced by 

 M. Poisson's theorjf, we may perhaps hence see why the sup- 

 position of incompressibility conducts to the same form of 

 the principal equations. In some objections that have been 

 made to the principles of Laplace's theory, it does not appear 

 to have been sufficiently considered that by supposing the fluid 

 to be incompressible, he does in fact take account of a molecular 

 repulsion. It remains to be determined whether the variation 

 of density, which on the hypothesis of a disjoined molecular 

 constitution of bodies must obtain at their surfaces, be such as 

 to admit of the supposition of incompressibility as a near ap- 

 proximation to the truth. But this there are at present no 

 experimental means of determining. The experiments of Has- 

 senfratzf, from which he inferred that glass by being pounded 

 became specifically lighter, are not confirmed by those of Gay- 

 Lussact- As no variation of density has been hitherto detected, 

 we have a sort of negative evidence that the depth of the super- 



* vol. V. part ii. f Ann. Ch. Gilh. I. p. 515. 



X See Nouvelle Theorie de V Action CapUlaire, p. 6. 



u2 



