160 Count Rum ford’s Enquiry concerning the Nature of Heat y 
an augmentation (from without) of that action which occasions 
expansion, 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, how- 
ever 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 independent 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 supposition that their motions are regulated by the same 
laws which regulate the solar system, it is evident that the 
additional motion they must necessarily acquire, in order to their 
taking the fluid form, cannot be lost, but must continue to reside 
in the liquid, and must again make its appearance, when the 
liquid changes its form, and becomes a solid. 
It is well known that a certain quantity of heat is requisite to 
melt a solid; which quantity disappears, or remains latent in the 
