624 LECTURE X. 



'^^ raised from water will elevate it to the height of one fifth of an inch, and 

 will require a force of 504- grains for each square inch, in order to overcome 

 the apparent attraction of the water; and for mercury the utmost height is 

 about one seventh of an inch. (Plate XXXIX. Fig. 5S9, 540.) 



A detached portion of a liquid may stand on any surface, which it is not 

 capable of wetting, at a height which is different according to its magnitude, 

 and to the attraction of the surface. If the drop is very small, its form may 

 be nearly spherical; but when its extent becomes considerable, its height 

 must always be less than that at which the liquid would separate from a hori- 

 zontal surface; and it will approach the nearer to this limit, as its attraction 

 to the surface on which it stands is weaker. Thus a wide portion of mercury 

 stands on glass at the height of ^^ of an inch, and on paper nearly at -f; 

 and a portion of water will stand on a cabbage leaf, or on a table strewed 

 with the seeds of lycopodium, nearly at the height of one fifth of an inch. 

 (Plate XXXIX. Fig. 541.) 



For the operation of a powder like lycopodium, it appears to be only necessary, 

 that it should possess a weaker power of attraction than water, and should, 

 therefore, be incapable of being readily wetted by it: each particle of the 

 powder, being then but partially in contact with the water, will project 

 beyond its surface, and prevent its coming into contact with any of the 

 surrounding bodies, while the surface assumes such a curvature as is suffi- 

 cient to withstand the pressure of the internal parts. (Plate XXXIX. Fig. 

 542.) 



When a dry and light substance of any kind is placed on the surface of 

 water, its weight is not sufficient to bring it within the distance at which 

 cohesion commences, and it floats, surrounded by a slight depression. Any 

 substance of this kind, or any other substance surrounded by a depression^ 

 as a ball of glass or iron floating on mercury, appears to be attracted by another 

 similar substance in its neighbourhood; for the depression between the Iwo 

 substances is increased,and the pressure of the fluid on that side is consequently 

 lessened, so that they are urged together, by a force which varies inversely 

 as the square of the distance. And in the same manner, when two bodies, 

 surrounded by an elevation, approach each other, they exhibit an attractive 



