ON COHESION. 481 



to allow a greater facility of extension or compression, and it may yet 

 retain a power of restoring the bodies to their original form by its reaction. 

 This force may even be the principal 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 greater distance from each other, and the resistance 

 must be derived from the lateral adhesion only : some other substances 

 also, approaching more nearly to the nature of liquids, may be extended to 

 many times their original length, with a resistance continually increasing ; 

 and in such cases there can scarcely be any material change of the specific 

 gravity of these substances. Professor Robison has mentioned the juice 

 of bryony as affording a remarkable instance of such a viscidity. 



It is probable that the immediate cause of the lateral adhesion of solids 

 is a symmetrical arrangement of their constituent parts : it is certain that 

 almost all bodies are disposed, in becoming solid, to assume the form of 

 crystals, which evidently indicates the existence of such an arrangement ; 

 and all the hardest bodies in nature are of a crystalline form. It appears, 

 therefore, consistent both with reason and with experience to suppose that 

 a crystallization more or less perfect is the universal cause of solidity. We 

 may imagine that when the particles of matter are disposed without any 

 order, they can afford no strong resistance to a motion in any direction, 

 but when they are regularly placed in certain situations with respect to 

 each other, any change of form must displace them in such a manner, as 

 to increase the distance of a whole rank at once ; and hence they may be 

 enabled to cooperate in resisting such a change. Any inequality of tension 

 in a particular part of a solid is also probably so far the cause of hardness, 

 as it tends to increase the strength of union of any part of a series of par- 

 ticles which must be displaced by a change of form. 



The immediate resistance of a solid to extension or compression is most 

 properly called its elasticity ; although this term has sometimes been used 

 to denote a facility of extension or compression, arising from the weakness 

 of this resistance. A practical mode of estimating the force of elasticity 

 has already been explained, and according to the simplest statement of the 

 nature of cohesion and repulsion, the weight of the modulus of elasticity 

 is the measure of the actual magnitude of each of these forces ; and it fol- 

 lows that an additional pressure, equal to that of the modulus, would 

 double the force of cohesion, and require the particles to be reduced to half 

 their distance in order that the repulsion might balance it ; and in the 

 same manner an extending force equal to the weight of half the modulus 

 would reduce the force of cohesion to one half, and extend the substance to 

 twice its dimensions. But, if, as there is some reason to suppose, the 

 mutual repulsion of the particles of solids varies a little more rapidly than 

 their distance, the modulus of elasticity will be a little greater than the 

 true measure of the whole.eohesive and repulsive force : this difference will 

 .not,* however, affect the truth of our calculations respecting the properties 



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