, CAPILLARY ACTION. 



Dr Jurin* shewed that the height at which the liquid is suspended 

 depend* on the section of the tube at the surface of the liquid, and is inde- 

 ndent of the form of the lower part of the tube. He considered that the 

 ^upp^nn of the liquid is due to "the attraction of the periphery or section 

 of the mfr"* of the tube to which the upper surface of the water is con- 

 tiguous and coheres." From this he shews that the rise of the liquid in tubes 

 of the same substance is inversely proportional to their radii. 



Newton devotes the 31st query in the last edition of his Opticla to 

 molecular forces, and instances several examples of the cohesion of liquids, such 

 M the suspension of mercury in a barometer tube at more than double the 

 height at which it usually stands. This arises from its adhesion to the tube, 

 and the upper part of the mercury sustains a considerable tension, or negative 

 pressure, without the separation of its parts. He considers the capillary phe- 

 nomena to be of the same kind, but his explanation is not sufficiently explicit 

 with respect to the nature and the limits of the action of the attractive force. 



It is to be observed that, while these early speculators ascribe the phe- 

 nomena to attraction, they do not distinctly assert that this attraction is 

 sensible only at insensible distances, and that for all distances which we can 

 directly measure the force is altogether insensible. The idea of such forces, 

 however, had been distinctly formed by Newton, who gave the first example 

 of the calculation of the effect of such forces in his theorem on the alteration 

 of the path of a light-corpuscle when it enters or leaves a dense body. 



Clairautf appears to have been the first to shew the necessity of taking 

 account of the attraction between the parts of the fluid itself in order to 

 explain the phenomena. He does not, however, recognise the fact that the 

 distance at which the attraction is sensible is not only small but altogether 

 insensible. 



Segner J introduced the very important idea of the surface-tension of liquids, 

 which he ascribed to attractive forces, the sphere of whose action is so small 

 "ut nullo adhuc sensu percipi potuerit." In attempting to calculate the effect 

 of this surface-tension in determining the form of a drop of the liquid, Segner 

 took account of the curvature of a meridian section of the drop, but neglected 

 the effect of the curvature in a plane at right angles to this section. 



* Phil. Trans., 1718, No. 355, p. 739, and 1719, No. 363, p. 1083. 

 t Clairaut, Thcone de la Figure de la Terre, Paris, 1808, pp. 105, 128. 

 J Segner, Comment. Soc. Reg. Gotting., i. (1751), p. 301. 



