172 



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 Morveau, 

 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 fun- 

 damental 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 different grounds, in affirming 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. 



Concerning the State in which the true Sap of Trees is deposited during 

 Winter. In a Letter from Thomas Andrew Knight, Esq. to the 

 Right Hon. Sir Joseph Banks, Bart. K.B. P.R.S. Read January 

 24, 1805. [Phil. Trans. 1805,^. 88.] 



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 sue 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 depo- 

 sited by it, exists during the winter in the alburnum, 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 



