S72 FOURTH REPORT — 1834. 



differ in no essential respect from that of Dr. Young, as far as 

 regards the principles on which the equation relative to the ca- 

 pillary fluid surface is obtained; and neither possesses an advan- 

 tage over the other in the explanation of pheenomena. But the 

 way in which the latter mathematician accounts on physical 

 principles for the constancy of the angle of contact, (which is 

 the other hypothesis of his theory,) though incomplete, is more 

 satisfactory than anything we meet with on the same subject 

 in Laplace's treatise. The following is his reasoning on this 

 point. 



When the surface of a fluid is free, or exposed to a gas, the 

 superficial tension arising from the cohesive power of the par- 

 ticles acts with full force to produce pressure directed inwards, 

 from which, in fact, arises the tendency observable in small 

 fluid masses to assume a globular form. This contractile power 

 is altered by contact with a solid surface. For instance, if a 

 cube of water had one of its halves congealed, its other proper- 

 ties remaining the same, the other half would retain its form, 

 because the tendency to contract at the edges contiguous to the 

 solid, would be just counteracted by an equal and contrary action 

 of the solid ; and at all other points of contact, the contractile 

 force would vanish for the same reason as at any point in the 

 interior of the fluid. If the solid were of smaller attractive 

 power than the fluid, the tendency to contract, and consequently 

 the tension of the surface in contact, would be proportional only 

 to the difference of the attractive forces, or to the difference of 

 the densities of the solid and fluid, if the law of the forces be the 

 same for both*. The portion of the solid surface which is con- 

 tiguous to, but not touched by, the fluid, will act on the fluid par- 

 ticles situated at the angle of contact, just as the fluid superficies 

 itself does, but in a different degree according to the difference 

 of density. Hence, the conditions of equilibrium are to be sought 

 of three forces acting on a particle at the angle of contact, one 

 in the direction of the surface of the fluid only, another in that 

 of the common surface of the fluid and solid, and the third in 

 the opposite direction along the exposed surface of the solid. If 

 q and g' be the densities respectively of the fluid and solid, and 

 the first force be k g, the second will he k(§ — §'), and the third 

 k §'. If then & be the angle of contact which the free surface of 

 the fluid makes with its surface of contact, the first force re- 



• Dr. Young adduces some facts relating to the spreading of the drops of oil 

 on water in support of the proportion here assigned. (Lectures on Nat. Phil., 

 vol. ii. p. 659.) 



