COHESIOX OF FLUIDS. 85 



repulsion of a wet and a dry body does not appear to Mow ^^IJ^^^J^^PP^^, 

 the same proportion : for it by no means approaches to infi- and repulsions 

 nitv upon the supposition of perfect contact ; its maximum is ^^^°^*"*^ 

 measured by half the sum of the elevation and depression on 

 the remote sides of the substance?, and as the distance increa- 

 ses, this maximum is only diminished by a quantity, which is 

 initially as the square of the distance. The figures of the 

 solids concerned modify also sometimes the law of attraction, 

 so that, for bodies surrounded by a depression, there is some- 

 times a maximum, beyond which the force again diminishes : 

 and it is hence that a lij^ht body floating on mercury, in a ves- 

 sel little larger than itself, is held in a stable equilibrium with- 

 out touching the sides. Th-e reason of this will become ap- 

 parent, when we examine the direction of the surface neces- 

 siirily assumed by the mercury in order to preserve the appro- 

 priate angle of contact, the tension acting with less force when 

 the surface attaches itself to the angular termination of the 

 float in a direction less horizontal. 



The apparent attraction produced between solids by the in- 

 terposition of a fluid does not depend on their being partially 

 immersed in it ; on the contrary, its effects are still more pow- 

 erfully exhibited in oilier situations ; and, when the cohesion 

 between two solids is increased and extended by the interven- 

 tion of a drop of water or of oil, the superficial cohesion of 

 these fluids is fully sufficient to explain the additional effect. 

 When wholly iarvmersed in water, the cohesion between two 

 pieces of glass is little or not at all greater than when, dry; 

 but if a small portion only of a fluid be interposed, the carved 

 surface, th:it it exposes to the air, will evidently be capable 

 *f resisting as great a force as it would support from the prcs- 

 s\ue of the column of fluid that it is capable of sustaining in 

 a. vertical situation ; and in order to apply this force, we must 

 employ in the separation of the plates, as great a force as is 

 equivalent to the pj'essure of a column appropriate to their 

 dist:ince. Morveau found that two discs of glass, 3 inches 

 French in diameter, at the distance of one-tenth of a line, ap- 

 peared to cohere with a force of 4719 grains, wiiich is equiva- 

 k'ni to the pressure of a column 23 lines in height; hence the 

 product of the height and the distance of the pktes is 2.3 lines 

 instead of 2.6'5, which was the result of iNIongc's experiments 



on 



