480 LECTURE L. 



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 hoth these cases it is ohvious 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 he exerted by solid substances on other 

 solids, either of the same kind, or of different 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 re- 

 maining attached to iron or steel, with which they have been made to 

 cohere 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 one mass ; the union, however, appears to be nearly of the same kind as 

 the common cohesion of aggregation ; and if the lead were softened into 

 an amalgam by the addition of mercury, the cohesion of the two masses 

 would become precisely the same as the internal cohesion of each mass. 

 Harder substances, such as marble or glass, cohere but weakly, perhaps 

 because their surfaces are never so perfectly adjusted to each other as to 

 touch throughout. The interposition of a fluid usually increases the 

 apparent attraction of such substances, but this circumstance has already 

 been explained from the effect of the capillary contraction of its surface ; 

 and when the substances are wholly immersed in a fluid, the cohesion is 

 little, if at all, increased. 



The immediate cause of solidity, as distinguished from liquidity, is the 

 lateral adhesion of the particles to each other, to which the degree of hard- 

 ness or solidity is always proportional. This adhesion prevents any change 

 of the relative situation of the particles, so that they cannot be withdrawn 

 from their places, without experiencing a considerable resistance from the 

 force of cohesion, while those of liquids may remain equally in contact 

 with the neighbouring particles, notwithstanding their change of form. 

 When a perfect solid is extended or compressed, the particles, being retained 

 in their situations by the force of lateral adhesion, can only approach 

 directly to each other, or be withdrawn further from each other, and the 

 resistance is nearly the same as if the same substance, in a fluid state, were 

 inclosed in an unalterable vessel, and forcibly compressed or dilated. Thus 

 the resistance of ice to extension or compression is found by experiment 

 to differ very little from that of water contained in a vessel ; and the same 

 effect may be produced even when the solidity is not the most perfect 

 which the substance admits ; for the immediate resistance of iron or steel 

 to flexure is the same whether it may be harder or softer. It often happens, 

 however, that the magnitude of the lateral adhesion is so much limited as 



