B 



Mechanical Science,' * 195 



glass attached to its aperture, arid were then filled with a liquid 

 substance, having a strongs affinity for another liquid, into which 

 the bac^ was to be immersed, it would not only have the power of 

 absorbinfT the latter liquid into its pores, but also, in certain cases, 

 of forcinp^ it up to the top, and even oat of the glass tube held in 

 a vertical position. On this point, a difference of opinion with 

 regard to the force of capillarity took place : M. Ampere maintain- 

 ing that capillary action would raise the fluid to the top of the 

 tube, but not cause its expulsion ; while M. Poisson maintained 

 that, in certain cases, the latter effect could be produced. The 

 latter has since then published a note, which we transcribe in part, 

 from the Annates de Chimie, xxxv. 98. 



Suppose that two different fluids, A, B, are contained in a 

 vessel, and separated the one from the other by a vertical division ; 

 the heights being in an inverse ratio to the den- 

 sities, so that the points, a and 6, in the two faces 

 of the division, and situated in the same horizontal 

 plane, shall support equal and opposite pressures : 

 suppose also that the division is pierced with one 

 or more holes of small diameter, or, in other words, 

 that it is traversed by several very narrow canals, 

 as a, b, perpendicular to the two faces, and which 

 may be regarded at first as filled with air, or any 

 other fluid. 



If the substance of the division exerts upon each of the two 

 liquids an action superior to the half of that which the liquid has 

 upon itself, each liquid will enter into the canal «, 6, just as it 

 would rise above its ordinary level in a capillary tube of the same 

 size and substance. It would also be urged, by the excess of 

 pressure which it would exert at the extremity of the canal, against 

 the elasticity of the included air. When the two fluids have pene- 

 trated the interior of «, b, the air will be pushed on both sides in 

 different directions by forces each of which is equal to the primitive 

 pressure augmented by the corresponding capillary force, i. e. aug- 

 mented by forces proportional, according to the known theory of 

 M. Laplace, to double the action of the tube on the liquid, less 

 the proper action of the liquid itself It will only be in the case 

 when the capillary force shall be the same on both sides, that the 

 air, after being compressed to a certain degree, will remain at rest : 

 for whenever this force preponderates at one end of the canal, the 

 air will be driven out at the opposite end, and the liquid with the 

 strongest capillary attraction will entirely fill the canal. 



Suppose this liquid to be A, then let us consider the forces which 

 will act on the portion rt, b^ of this liquid. At the extremity a, 

 it will be submitted to the attraction of the exterior fluid A : at 

 the extremity 6, it will be attracted in the opposite direction by the 

 liquid B. Now the two liquids being different, their attractions 

 will be unequal, and we will suppose that that of B, on the matter 



O 2 



