110 APPENDIX. 
to the theory which regards heat as motion among 
the particles of matter as I am now. In this 
spirit Prop. v., p. 104, was written, the correct- 
ness of which I have since established experi- 
mentally. 
There are many phenomena which cannot be 
accounted for by the theory which recognises 
heat as a substance ; and there are several, which, 
though sometimes adduced as triumphant objec- 
tions to the other theory, tend, when rightly con- 
sidered, only to confirm it. The heat of fluidity 
may very naturally be regarded as the momen- 
tum or mechanical force necessary to overcome 
the aggregation of particles in the solid state. 
The heat of vaporization may be regarded, partly 
as the mechanical force requisite to overcome the 
aggregated condition of atoms in the fluid state, 
and partly as the force requisite to overcome at- 
mospheric pressure. Again, the heat of combi- 
nation is only the manifestation, in another form, 
of the mechanical force with which atoms com- 
bine : on the other hand, the phenomena of elec- 
trolysis by the voltaic battery give us positive 
proof that the mechanical force of the current 
requisite to procure the decomposition of an elec- 
trolyte is the equivalent of the heat due to the 
