Capillary Attraction, 33^ 



and the reciprocal action which their particles exert upon 

 each other. We come now to consider a class of these phenom- 

 ena of considerable extent and variety, and which are the more 

 deserving of attention, as they are susceptible of a rigorous 

 calculation. 



If a disk of glass, marble, or metal, &c., be suspended to the 

 scale of a balance, and counterpoised by an equal weight in the 

 opposite scale, upon being made to touch the surface of a liquid 

 capable of moistening it, it will be found to adhere with a certain 

 force, and to require an additional weight in the opposite scale of 

 the balance to detach it. This adhesion is not produced by the 

 pressure of the air, for it takes place equally well in a vacuum, 

 We infer, therefore, that it is the particles of the solid which attach 

 themselves to the particles of the fluid by virtue of a force of af- 

 finity. But there is to be inferred also a similar action between 

 the particles of the fluid itself. Indeed when the disk is capable 

 of being moistened by the liquid, as is the case when glass is 

 used with water or alcohol, the disk, upon being withdrawn 

 brings with it a small liquid film, or lamina, which adheres to it. 

 It is not then, strictly speaking, the solid which is detached from 

 the liquid, it is this small lamina which is separated from 

 the particles immediately below it. Now the force employed 

 thus to detach it, is incomparably more considerable than the 

 proper weight of this lamina ; consequently the excess of force 

 proves the existence of an internal adhesion in the liquid which 

 would keep the small lamina united to the rest of the liquid mass 

 independently of gravity. 



According to the notions which we have formed of the re- 

 ciprocal action of the particles of bodies upon each other, 

 the force in question seems to be of the same nature and to 

 have a sensible effect only at very small distances. This is 

 moreover proved by experiment. Whatever be the thickness 

 of the disk, so long as the form and substance are the same, the 

 force required to detach it from a given liquid, is also the same. 

 Accordingly, beyond a certain thickness, probably less than any 

 within the reach of human art to attain, any augmentation has 

 no effect capable of being appreciated. Whence it will be seen 

 that this action is not capable of producing sensible effects, ex- 

 Meek. 43 



