458 On the two great powers, (Serr. 
be equal to the attraction in the liquid state, since it must be a perma- 
nent force, the attraction, being always opposed by an equal force, 
would never be able, under any circumstances, to draw the atoms into 
the solid state. 
If the repulsion be considered equal to the attraction, when parti- 
cles are at rest in the solid state, no solid could contract, unless ex- 
posed to an extra-compressing force, of which (as before remarked) 
we have no evidence, and which must only operate at certain times, 
for otherwise no solid could ever expand. 
Since then it has been shewn, that, if an inherent repulsion, exert- 
ed by atoms, be considered, as one of the great opponent forces to 
their attraction, it necessarily involves the introduction of an extra- 
compressing force, which must only operate at certain times ; and since 
no such compressing force can be demonstrated, it is manifest that 
such a repulsion cannot be considered as one of the opponent forces to 
atomic attraction. 
Thirdly.—That the opponent force, to the attraction of atoms, de- 
pends on the power heat alone. 
It has been already proved, that no inherent force of repulsion can 
be supposed to be exerted by atoms, and that such a force would not 
account for the phenomena of repulsion, which could not take place on 
the admission of it. It therefore follows, according to the division, that 
in heat consists the great opponent force to the attraction of atoms. 
It is manifest, that previously to an attempt to explain the action of 
heat, as the source of repulsion, a decided opinion should, if possible, be 
formed of its nature. 
The difficulty of this is apparent, in the fact, that chemical philoso- 
‘ phers are divided between the two opinions, that the phenomena called 
heat depend on vibratory motions in the particles of bodies, or that 
heat is a subtle highly elastic fluid pervading all bodies. 
1. That the phenomena of heat depend on vibratory motions, in the 
particles of bodies, 
The phenomena of heat are of two kinds:—Those, which are appa- 
rent to the senses, and commonly called heat; and those of repulsion. 
The great philosopher Bacon, being unacquainted with most of the 
facts proving the repulsive force of heat, could only judge of its nature 
by those of the former kind. He, observing that great heat was pro- 
duced by the friction and percussion of many bodies, that iron may 
even be rendered red hot by percussion, was led to the conclusion, that 
heat consists in a motion in the particles of bodies. But he did not ap- 
ply his hypothesis to the explanation of repulsion. Of late years a great 
