478 LECTURE L. 



their distance is -r-f,-, and in a tube, when its diameter is 7 ! T of an inch. 

 (Plate XXXIX. Fig. 537, 538.) 



A liquid may also adhere to a horizontal surface which is gradually 

 raised from it, until the hydrostatical pressure becomes sufficient to over- 

 power the cohesion of its superficial parts ; the internal part of the fluid 

 being usually raised, not immediately by the force of cohesion, but by the 

 pressure of the atmosphere. The solid bears the whole weight of the 

 liquid, which is elevated above the surface ; and when the surface is 

 perfectly wetted, this weight is equal, at the moment of separation, to the 

 hydrostatical pressure, or rather suction, corresponding to the height ; but 

 in other cases the weight may be somewhat greater than the hydrostatical 

 pressure on the surface of the solid, on account of the elevation which 

 surrounds the body, and which is not compensated by the excavation 

 immediately under it. A surface thus raised from water will elevate it to 

 the height of one fifth of an inch, and will require a force of 50| 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. 539, 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 consider- 

 able, its height must always be less than that at which the liquid would 

 separate from a horizontal 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 T ^ 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 sufficient 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 two 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 force of a similar nature, the pressure of the atmo- 



