REPORT ON CAPILLARY ATTRACTION. 267 



periment that the concave surface of these fluids differed little 

 from that of a hemisphere*. It would seem from this theory that 

 the intensity of the solid's attraction for the fluid must exceed 

 that of the fluid for itself, in order that the fluid may ivet the 

 solid. 



The preceding are the principal facts which Laplace explains 

 in his first published Treatise on Capillary Action. The expla- 

 nations of some few others are added by way of corollaries at the 

 end of the Treatise. One of these, which serves to exhibit the 

 effect of the convexity of the fluid surfaces, may be mentioned 

 here. 



If a capillary tube be plunged to a small depth in water, and 

 then, with its lower extremity closed by the finger, be taken out, 

 on withdrawing the finger the fluid will be seen to sink in the 

 tube, and to form a drop at the lower end. But when it has 

 ceased to descend, the height at which it rests above the extre- 

 mity of the tube is always greater than the elevation due to ca- 

 pillary action when the tube is just dipped in the fluid. The 

 reason of this excess is, that the effect of the convexity of the 

 drop, which takes place in the upward direction, is added to the 

 effect of the concave surface within the tube. 



Hence it follows that if a slender siphon with unequal arms 

 be filled with water, when the fluid is just on the point of run- 

 ning from the longer arm it has to overcome the capillary 

 actions due to the concavity formed at the extremity of the 

 shorter arm and the convexity at that of the longer arm ; and 

 unless the difference of the,lengths of the arms be greater than 

 the sum of the lengths of the fluid columns which these two 

 actions will sustain at the respective extremities, the fluid will 

 not run. 



The theory of Dr. Young respecting the phsenomena of capil- 

 lary tubes is contained in an "Essay on the Cohesion of Fluidsf," 

 read before the Royal Society, December 20, 1804, and inserted 

 in the second volume of his Lectures on Natural Philosophy. 

 His views resemble those advanced by Segner and Monge. 

 Like these two mathematicians, he considers the phaenomena to 

 be referable to the cohesive attraction of the superficial particles 

 of the fluids, in so far as it gives rise to a uniform tension of 



* It would be difficult to decide by experiment whether the surface be nearly 

 or exactly a hemisphere, because when the angle of contact is very small, the 

 line of contact is not readily discernible. The method of determining the angle 

 of contact by reflection, proposed by Dr. Young in his Lectures on Natural 

 Philosophy, (vol. ii. p. 66G,) is preferable to measuring the sagitta of the sur- 

 face, which was the method adopted bj' Haiiy. 



t Philosophical Transactions, 1805, p. 65. 



