#25 ]l:cctu!RE t,. 



traction to usefiU puifroses: it is well known, that in order to its speetlj. 

 operatian, it Tequires to be previously moistened, by the .assistauce of a little 

 pressure, otherwirse it exhibits the same appearance of repulsion that is ob- 

 servable in many other cases where the contact is imperfect. The absorp- 

 tion of moisture hy sugar depends on the same principle, and .here the tuibes 

 are so minute, that the height of ascent appears to be almost unlimited. 



The magiritude of the cohesion between flnids and solids, as well as of 

 the particles of fluids with each other, is more directly shown by an ex- 

 periment on the continuance of a column of mercury, in the tube of a 

 l)arometer, at a height considerably greater than that at which it usually 

 stands, on account of the pressure of the atmosphere. If the mercury has 

 been well boiled in the tube, it may be made to remain in contact with the 

 closed end, at the height of 70 inches or more; and by agitation only it may be 

 made to cohere so strongly, as to occupy the whole length of the tube of a 

 common barometer, which is several inches more than the height at which 

 the pressure of the atmosphere sustains it. A small siphon may also convey 

 mercury from one vessel into another in the vacuum of an air pump: and in 

 both these cases it is obvious that no other force than cohesion can retain 

 the upper surface of the mercury in contact with the glass, or its internal 

 parts in contact with each other. 



The force of cohesion may also be exerted by solid substances on other 

 solids, either of the same kind, or of diiferent kinds. Thus two masses of 

 lead, when once united by pressure, assisted by a little friction, require a 

 very considerable force to separate them, and it may be shown either by 

 measuring this force, or by suspending the lead in the vacuum of the air- 

 pump, that the pressure of the atmosphere is not materially concerned in 

 producing this appearance of cohesion, since its magnitude much exceeds 

 that of the atmospherical pressure. A cohesion of this kind is sometimes of 

 practical utility in the arts ; little ornaments of laminated silver remaining 

 attached to iron or steel, with which they have been made to cohei'e by the 

 powerful pressure of a blow, so as to form one mass with it. 



The contact of two pieces of lead, although intimate enough to produce a 

 considerable cohesion, is by no means so complete as to unite the parts into 



