ON COHESION. ^21 



the hydrostatical pressure which acts against It; and hence we may calculate 

 in many cases the properties of the curve which it must form ; but in other cases 

 the exact calculation becomes extremely intricate, and perhaps impracticable. 

 A drop descending in a vacuum would be perfectly spherical; and if its mag- 

 nitude were inconsiderable, it would be of the same form when descending 

 through the air; a small bubble rising in a hquid must also be spherical; but 

 where the drop or the bubble is larger, its curvature will be greatest where 

 the internal pressure is greatest, or where the external pressure is least, and 

 in different cases this pressure may be differently distributed. Where a drop 

 is suspended from a solid, its length may be such that the pressure at its 

 upper part may become negative, and its surface will then be concave instead 

 of convex: and when a bubble rises to the surface of a liquid, it often carries 

 with it a film of the liquid, of which the weight is probably smaller than the con- 

 tractile force with which the surface resists the escape of the air, so that, from 

 the magnitude of the contractile force, we may determine the greatest possible 

 weight of a bubble of given dimensions. A slight imperfection of fluidity 

 probably favours the formation of detached bubbles, by retarding the ascent 

 of the air, but it has a still greater effect in prolonging their duration when 

 formed. (Plate XXXIX. Fig. 532.) 



In order to determine the forms of the surfaces of liquids in the cases which most 

 commonly occur, it is necessary to examine how they are aflfected by the action of 

 other liquids, and of solids of different descriptions. Supposing the horizontal sur- 

 face of a liquid to be in contact with a vertical plane surface of a solid of half the 

 attractive power, it will remain at rest in consequence of the equilibrium of at- 

 tractions ; for the particles situated exactly at the junction of the surfaces may be 

 considered as actuated by three forces ; one deduced from the effect of the liquid, 

 the other two from that of the two equal portions of the solid above and below the 

 surface of the fluid; and itmaybe shown that the combination of thesethree forces 

 will produce a joint result in the direction of gravity; consequently the direction 

 of the surface must remain the same as when it is subjected to the force of gravity 

 alone, since the surface of every fluid at rest must be perpendicular to the joint 

 direction of all the forces acting on it. But if the attractive power of the solid 

 be more than half as great as that of the liquid, the result of the forces will be 

 inclined towards the solid, and the surface of the liquid, in order to be per- 

 pendicular to it, must be more elevated at the side of the vessel than else- 



VOL. I. 4 H 



