628 LECTURE L. 



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: i^'is certain that 

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

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

 and all the hardest bodies in nature are of a crys^fline form. It appeaffe, 

 therefore, consistent both with reason and with Experience to supposS-ttfkf k 

 crystallization more or less perfect is the universal cause of soli{fftj|^,j „ 

 may imagine that when the particles of matter ai^disposed without am 

 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 rankat once; and hence they may be enabled to coope- 

 rate 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 particles which must be dis- 

 placed by a cliange 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 follow? 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 leduce the force of cohe- 

 sion to one halfjand extend the substance to twice its dimensions. But, if, as 



