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with other bodies, I proceed to notice some of the cases 
in which the appearance and disappearance of sensible 
heat that do depend on the action of mechanical forces; 
these are of so wide a range, that only a few of them 
can or need be given. It must be kept in view that the 
latent heat in bodies, besides depending on their con- 
stituent forms, as solids, liquids, vapours, and gases, — is 
also proportionate to their bulks, in the same form ; 
wherefore, by compressing a body of any kind, sensible 
heat is given out, and by expanding it sensible heat is 
re-absorbed, or taken up in a latent state; so that me- 
chanical forces continually acting among bodies around 
us bring about the alternate conditions of heat so familiar 
to everyone. The cause therefore is thus plainly seen 
why sensible heat in bodies is produced by mechanical 
forces. But this affords no ground for assuming the 
generation de novo of any heat by the exertion of any 
force — -either chemical or mechanical. 
We have sensible heat from the solar rays, and that 
produced by combustion, and other chemical changes of 
bodies, but have strong grounds for concluding that all 
forms of sensible heat produced by mechanical forces are 
derived from the latent heat, rendered sensible by com- 
pression or the abrasion of bodies by such forces. 
But the alternation of sensible and latent heat thus 
caused by mechanical action is seen in an endless 
range of compressions and expansions from the lighting 
of a match in the air syringe to the showers of 
sparks issuing from the particles of steel torn off in dry 
grinding , and ignited by the intense heat from the com- 
pressed air between the steel blade and the grindstone. I 
