X. AN ESSAY 



ON THE 



COHESION OF FLUIDS. 



BY 



THOMAS YOUNG, M. D. FOR. SEC. R. Sw 



Read Dece7nber 9.0, 1804. 



I. GENERAL PRINCIPLES. 



It has already been asserted, by Mr. Monge 

 and others, that the phenomena of capillary 

 tubes are referable to the cohesive attraction 

 of the superficial particles only of the fluids 

 employed ; and that the surfaces must conse- 

 quently be formed into curves of the nature 

 of linteariae, which are supposed to be the 

 results of a uniform tension of a burlace, re- 

 sisting the pressure of a fluid, either uniform, 

 or varying according to a given law. Seg- 

 ner, who appears to have been the first that 

 maintained a similar opinion, has shown in 

 what manner the principle may be deduced 

 from the doctrine of attraction, but his de- 

 monstration is complicated, and not per- 

 fectly satisfactory; and in applying the law 

 to the forms of drops, he has neglected 

 to consider the very material effects of the 

 double curvature, which is evidently the 

 cause of the want of a perfect coincidence 

 of some of his experiments with his theory. 

 Since the time of begner, little has been done 

 in investigating accurately and in detail the 

 various consequences of the principle. 



VOL. II. 



It will perhaps be most agreeable to the ex- 

 perimental philosopher, although less con- 

 sistent with the strict course of logical argu- 

 ment, to proceed in tlie first place to the 

 comparison of this theory with the pheno- 

 mena, and to inquire afterwards for its foun- 

 dation in the ultimate properties of matter.. 

 But it is necessary to premise one observation, 

 which appears to be new, and which is equally 

 consistent with theory and with experiment; 

 that is, that for each combination of a solid 

 and a fluid, there is an appropriate angle of 

 contact, between the surfaces of the fluid, 

 exposed to the air, and to the solid. This 

 angle, for glass and water, and in all cases 

 where a solid is perfectly wetted by a fluid,, 

 i& evanescent : for glass and mercury, it is 

 about 140°, in common temperatures, and 

 when the mercury is moderately clean. 



II. FORM OF THE SURFACE OF A FLUID. 



It is well known, and it results immediately 

 from the composition of forces, that where 

 a line is equably distended, the force that 

 it exerts, in a direction perpendicular to 

 its own, is directly as its curvature; and 

 4 o 



