620 recTURE l. 



stances, and from' the suspension of a drop from any solid, to which its upper 

 surface adheres with sufficient force. Without cohesion, indeed, a liquid 

 would be only a very fine powder, except that the particles of powders have 

 not the power of moving with perfect freedom on each other, which consti- 

 tutes fluidity. The apparent weakness of the cohesion of liquids is entirely 

 owing to tliis mobility, since their form may be clianged in any degree with- 

 out considerably increasing the distances of their particles, and it is only 

 under particular circumstances that the effects of their cohesion can become 

 sensible. 



When a liquid is considered as unlimited in its extent, the repulsion of its 

 particles, situated in all possible directions with regard to each other, may 

 be supposed in all cases precisely to balance the cohesion, which is derived 

 from the actions of particles similarly situated; and this must also be the 

 state of the internal parts of every detached portion of a liquid, where they 

 are so remote from the surface as to be beyond the minute distance which is 

 the limit of the action of these forces. But the external parts of the drop 

 will not remain in the same kind of equilibrium: they may be considered as 

 a thin coating of a liquid surrounding a substance which resists only in a 

 direction perpendicular to its surface, and does not interfere with the mutual 

 actions of the particles of the liquid. Now since the repulsive force increases 

 as the distance diminishes, it must be exerted more powerfully by the nearest 

 particles, while the cohesion is directed equally towards all the particles within a 

 certain distance, and wherever the surface is curved, the joint cohesive force 

 Avill be directed to a remoter part of the curve than the repulsive force 

 opposed to it, so that each particle will be urged, by the combination of these 

 forces, towards the concave side of the curve, and the more as the curvature 

 is greater; hence the coating of the liquid, thus constituted,must exert a force 

 on the parts in contact with it, precisely similar to that of a flexible surface, 

 which is every where stretched by an equal force ; and from this simple prin- 

 ciple we may derive all the effects produced by a cohesion of this kind, which, 

 from its being most commonly observed in the ascent of water in capillary 

 tubes, has been denominated capillary attraction. (Plate XXXIX. Fig. 

 -531.) 



It is, therefore, a general law, that the surface of every detached portion 

 of a fluid must every where have such a curvature, as to be able to withstand 



