OF NATURAL PHILOSOPHY. 227 



tinctly in liquids. They are all, though in a small de- 

 gree, compressible, and recover their former dimen- 

 sions completely when the pressure is removed ; but 

 they cannot be dilated (by mechanical means), and 

 have no tendency, while they remain liquids, to en- 

 large themselves beyond a certain limit, and there- 

 fore they assume a determinate surface while at 

 rest, and their parts actually resist further separ- 

 ation with a considerable force, thus giving rise to 

 the phenomenon of the cohesion of liquids. 



(241.) Both in air and in liquids, however, the 

 most perfect freedom of motion of the parts among 

 each other subsists, which could hardly be the case 

 if they were not separate and independent of each 

 other. And from this, combined with the foregoing 

 considerations, it has been concluded that they do 

 not actually touch, but are kept asunder at deter- 

 minate distances from each other, by the constant 

 action of the two forces of attraction and repulsion, 

 which are supposed to balance and counteract each 

 other at the ordinary distances of the particles, but 

 to prevail, the one, or the other, according as they 

 are forcibly urged together or pulled asunder. 



(242.) In solids, however, the case is very different 

 The mutual free motion of their parts inter se is 

 powerfully impeded, and in some almost destroyed. 

 In some, a slow and gradual change of figure may be 

 produced to a great, extent, by pressure or blows, 

 as for instance in the metals, clay, butter, &c. ; in 

 others, fracture is the consequence of any attempt 

 to change the figure by violence beyond a certain 

 very small limit. In solids, then, it is evident, that 

 the consideration of their intimate structure has a 

 Q 2 



