Page:Proceedings of the Royal Society of London Vol 1.djvu/192

Rh and repulsions of ﬂoating bodies: these attractions are found to vary ultimately in the inverse ratio of the squares of the distances; and they appear to be the same as are found to cause an apparent co- hesion between any moistened surfaces nearly in contact : the mag- nitude of this cohesion, as measured, in a particular case by Morvean, being found to agree with the calculation of the effect of capillary action.

The attraction of a drop of a ﬂuid towards the line of contact of two plates of glass, which was found by Hawkesbee to vary nearly in the inverse ratio of the square of the distance of the plates, was supposed by Newton to indicate an immediate cohesive force, varying in the simple inverse ratio. But Dr. Young has shown that the fundamental law of the equable tension of the surface is sufﬁcient to explain this phenomenon, and to remove the apparent irregularity in the laws of cohesive forces.

The equable tension of the surface is shown to be a consequence which may be mathematically deduced from the existence of a stable equilibrium between the forces of repulsion and of cohesion, which is a necessary condition of liquidity, as the repulsive force always varies more rapidly than the cohesive force. The mutual attractions of solids and ﬂuids are then considered; and Dr. Young agrees with Clairaut, although upon diﬁerent grounds, in aﬂirming that a ﬂuid will be elevated when in contact with any solid of more than half its attractive density. The tension of the common surfaces of a solid and a ﬂuid, or of two continuous ﬂuids, is supposed to be proportional to the difference of the attractive densities; and this supposition is conﬁrmed by some observations, with which the paper is concluded, on the phenomena of oily substances ﬂoating on water.

This paper may be considered as a continuation of Mr. Knight’s former communications respecting the motion of the sap in trees. Du Hamel, and other subsequent naturalists, have shown that trees contain two kinds of sap; and the chief purpose of Mr. Knight’s paper is to prove that one of them (called by Du Hamel suc propre, and by Mr. Knight the true sap,) is generated in the leaf; and that this ﬂuid, in an inspissated state, or some concrete substance deposited by it, exists during the winter in the albumum, from which substance, dissolved in the ascending aqueous sap, is derived the matter which enters into the composition of the new leaves in the spring. To the above-mentioned deposition, Mr. Knight attributes the well-known superiority of winter-felled wood, which superiority has generally been supposed owing merely to the absence of the sap at that season.

Du Hamel has remarked, that trees perspire more when the leaves