io8 Inquiry concerning the Nature of Heaf, 



contracted ; but if the motions of these particles were 

 increased, we might conclude, a priori, that the volume 

 of the body would be expanded. 



We have not sufficient data to enable us to form dis- 

 tinct ideas of the nature of the change which takes place 

 when a solid body is melted ; but as fusion is occa- 

 sioned by heat, that is to say, by an augmentation (from 

 without) of that action which occasions expansiou, if 

 expansion be occasioned by an increase of the motions 

 of the constituent particles of the body, it is, no doubt, 

 a certain additional increase of those motions which 

 causes the form of the body to be changed, and from 

 a solid to become a fluid substance. 



As long as the constituent particles of a solid body 

 which are at the surface of that body do not, in their 

 motions, pass by each other, the body must necessarily 

 retain its form or shape, however rapid those motions 

 or vibrations may be ; but as soon as the motion of 

 these particles is so augmented that they can no longer 

 be restrained or retained within these limits, the regular 

 distribution of the particles which they acquired in 

 crystallization is gradually destroyed, and the particles 

 so detached from the solid mass form new and inde- 

 pendent systems, and become a liquid substance. 



Whatever may be the figures of the orbits which the 

 particles of a liquid describe, the mean distances of 

 those particles from each other remain nearly the same 

 as when they constituted a solid, as appears by the 

 small change of specific gravity which takes place when 

 a solid is melted and becomes a liquid ; and, on a sup- 

 position that their motions are regulated by the same 

 laws wh'ich regulate the solar system, it is evident that 

 the additional motion they must necessarily acquire, in 



