ON COHESION. 627 



que mass ; the unlou, however, appears to be nearly of the same kind as the 

 camniou cohesion of aggregation ; and if the lead were softened into an 

 amalgam by the addition of aiercury, the cohesion of the two masses would 

 liecome 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 eacli pthex as to touch 

 throughout. The interposition of a fluid usually increases the apparent at- 

 traction of such substances, but this circumstance has already been explained 

 from the eifect of the capillary contraction of its surface; and when the 

 substances are wholly immersed in a fluid, the cohesion is little if at all in- 

 creased. 



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 of the relative situation of the particles, so that they cannot be withdrawn 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

 to each other, or be withdrawn further from each other, and the resist- 

 ance is nearly the same,as if the same substancc,in a fluid state, were inclosed 

 in an unalterable vessel, and forcibly compressed or dilated,- Thus the resist- 

 ance of ice to extension or compression is found by experiment to diifer 

 very, little from that of water contained in a vessel ; and the same effect may 

 be produced e\'en when the solidity is not the most perfect which the sub- 

 stance 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 iiingnitude of the lateral adhesion is so much limited as to allow a greater 

 facility of extension or compression, and it may yet retain a pow er of restor- 

 ing the bodies to their original form by its reaction. This force may even be 

 the pitncipal or perhaps the only source of the body's elasticity: thus when 

 a piece of elastic gum is extended, the mean distance of its particles is not 

 materially increased, for it is said to become rather more than less dense 

 during its extension ; consequently the change of form is rather to be attributed 

 to a displacement of the particles, than to their separation to a greate*" 



