412 



CAPILLARY ATTRACTION. 



PLATE CX. 

 Fig. 11. 

 Theory of 

 bogner. 



Theory of 

 Monge. 



Dr Young's 

 Theory. 



that of the fluid, the resultant will be HF in Fig. 1 1 : 

 Attraction. an( j therefore, asinthecaseofmercury.thesurface must 

 be depressed, in order to be perpendicular to the force. 

 Tlic subject of capillary attraction was next taken 

 up by Segner in 1751, who referred all the pheno- 

 mena to the attraction of the superficial particles of 

 the fluids. He deduces this principle from the doc- 

 trine of attraction. He supposes the attraction of 

 the tube to be insensible at sensible distances ; and 

 he has shewn that the curvature of each part of the 

 surface of a fluid is proportional to its distance from 

 the general level ; and without much error, he has 

 obtained from experiments the magnitude of this cur- 

 vature at a given height both for water and mercury. 

 M. Monge has followed Segner in ascribing the ca- 

 pillary phenomena to the cohesive attraction of the 

 superficial particles of the fluids ; and he maintains 

 that the surfaces must be formed into curves of the 

 nature of lintearias, resulting from the uniform ten- 

 sion of a surface resisting the pressure of a fluid, 

 which is either uniform, or varies according,to a gi- 

 ven law. 



In a very ingenious paper on the cohesion of 

 fluids, which was read by Dr Young in the Royal 

 Society in 1805, that able mathematician has given a 

 new theory of capillary attraction. He has reduced 

 all the phenomena of cohesion to the joint operation 

 of a cohesive and a repulsive force, which balance each 

 other in the internal parts of a fluid, where the par- 

 ticles are brought so near that the repulsion is exactly 

 equal to the cohesive force by which they are attract- 

 ed ; and he assumes only, that the repulsion is more 

 increased than the cohesion, by the mutual approach 

 of the particles. By this means Dr Young has con- 

 nected together a variety of facts which had hitherto 

 been unexplained; and we regret that our limits will 

 not permit us to give a more detailed account of his 

 ingenious speculations. 



More than a year after the publication of Dr 

 Young's paper, M. La Place published a Supple- 

 ment to the Mecanique Celeste, upon capillary attrac- 

 tion, where he has proposed a theory which has led 

 him to several conclusions that Dr Young had already 

 obtained by a more simple route. It is a very singu- 

 lar circumstance, that La Place should take no no- 

 tice whatever of Dr Young's labours, as the volume 

 of the Transactions which contained them, and se- 

 veral periodical works in which they were noticed, 

 must naturally have found their way to Paris. La 

 Place supposes, from Exp. 11. and 18. that capillary 

 action, like the refractive force, and the chemical affi- 

 nities, is only sensible at imperceptible distances ; 

 that a narrow ring of glass immediately above the 

 surface ot the fluid, exerts its force on the water; and 

 that this force, combined with the weight of the wa- 

 ter and the cohesion of its particles, produces the 

 concave surface or meniscus of fluid with which the 

 column is always terminated. He supposes this me- 

 niscus to be sustained by the action of the glass, while 

 it exerts its own attraction on the fluid particles im- 

 mediately below it, by means of which their gravity 

 is diminished, and the water consequently rises in the 

 tube ; and he has determined the form of the meniscus 

 to be that of a hemisphere, and its attraction to be 

 equal to that of a spherule of water of the same dia- 

 meter. Hence the attraction of the meniscus v. ill be 

 inversely as its diameter, or the diameter of the tube, 

 that js, as tfie weight of the elevated column, and 



Theory of 

 La Place. 



therefore the heights of ascent must be inversely as the Capillary 

 diameter of the tube. " Since it has hitherto been Attraction, 

 usnal with natural philosophers," says La Place, ft to 

 consider the concavity and convexity of the surface* 

 of fluids in capillary spaces, as a second iry effect of 

 capillary attraction only,' and not as the principal 

 cause of phenomena of this kind, they have not at- 

 tached much importance to the determination of the 

 curvature of these surfaces. But the theory whiclv 

 has been here Advanced, having shewn that all these 

 phenomena depend principally on the curvature, it 

 becomes of consequence to examine it." In opposi- 

 tion ' to the high authority of La Place, we agree\ 

 with Professor Playfair in thinking, " that the prin- 

 cipal and primary cause is that attraction, which 

 sustains the meniscus, and enables it to act on the 

 water below without being drawn out of its place. 

 It is not the concavity of its surface that makes the 

 water in the tube press less in the bottom than if its 

 surface were plain ; but it is the attraction of the 

 glass that produces in a manner equally direct, both, 

 the concavity and the diminution of pressure." The 

 fact mentioned in Exp. 12, has been ascribed by La 

 Place to the action of the drop upon the column, in 

 consequence of its convexity ; while Mr Playfair sup- 

 poses the additional elevation to be occasioned by the 

 action of the bottom and outside of the tube upon 

 the drop, by which the column of water is lifted up 

 to a higher level. We are disposed, however, to think, 

 that the column of water, after being raised above its' 

 ordinary height in the tube, as in Exp. 12, is pre- 

 vented from obeying the force of gravity by the force 

 with which the drop below adheres to the bottom of 

 the tube, and the force by which it resists any change 

 of form ; for the descent of the column to its usual 

 height could only take place, either by detaching 

 the drop altogether from the tube, or by giving it a' 

 more spherical, or a more elongated form. If the 

 other explanations were true, then the column might 

 be raised above its usual height in the tube, by pla- 

 cing a drop of water on the outside, and allowing 

 it to descend to the bottom of the tube, where 

 it would exert its force, according to La Place, or 

 be acted upon by the tube, according to Mr Play- 

 fair, which is not the case. For further information 

 on this subject, see Hooke's Micographia. Jurin, 

 Phil. Trans. No. 355, and No. 363. Hamilton's 

 Lectures, ii. p. 47. Hauksbee, Phil. Trans. 1706, 

 p. 223 ; 1709, p. 258 j 171 1, p- 395 ; 1712, p. 413 ; 

 1712, p. 539 ; 1713, p. 151. Taylor, Phil. Trans. 

 1712, p. 538 ; 1721, p. 209. Bullfmger, Com. Pe- 

 trop, ii. 233 ; iii. 281. Muschenbroek de tubis vitreis y 

 Diss. Phys. 271. Weitbrecht, Com. Pelrop. viii.26'1 ; 

 ix. 275. Gellert, Id. xii. 293, 302. Segner, Com. 

 Gotting, 1751, i. p. 301 ; La Lande sur la cause de 

 I' elevation des liqueurs, 12. Paris, 1770. Morveair 

 Rozier, Journ. i. 172, 4-60. Dutour, Rozier Journ. 

 xi. 127 ; xiii. Sup. 357 ; xiv. 216 ; xv. 46, 234 ; xvi. 

 85 ; xix. 137, 287. Milon Journ. Phys.liv. 128 ; and 

 Repertory of Arts, xvi. 427. Von Arnim in Gilbert's 

 Journal, iv. 376. Halistrom, Id. xiv. 425. Clairaut 

 Theorie de la Figure de la Terre tirees des principes 

 dc I' Hydrostatique, . 59. Dr T. Young on The 

 Cohesion of Fluids, Phil. Trans. 1805, and in his 

 Nat. Phil. ii. p. 649. La Phce's Mecanique Celesti. 

 Sup. au Dizieme Liv. Playfair't Outlines of Nat. 

 Phil. vol. i. p. 176, 184. See alao ADHESION and 

 HYDRODYNAMICS. (/3) 



