OBSERVATIONS ON THE NATURE OF HEAT. 241 
will be necessary to consider how far caloric corresponds with our 
ideas of matter; then, which of the hypotheses gives the most plausi- 
ble explanation of the phenomena dependent upon caloric. 
If we adopt the opinion, as many do, that whatever is capable of 
acting upon our senses is material, the question is at once settled ; 
but, to give greater scope to the argument, it will be better to fix 
upon some characteristics common to all matter, and then to find if 
there is any thing ir caloric resembling or approaching .to these. 
Extent and impenetrability are chosen as the indisputable character- 
istics of all material objects. The first implies, that every atom of 
matter must occupy space; the second, that no two atoms can occupy 
the same space in the same precise instant of time. “ Were this 
latter proposition otherwise,” says Sir John Leslie, “each body or 
atom might be successively absorbed into the substance of another 
till the whole frame of the universe, collapsing into a point, were 
lost in the vortex of annihilation.” 
Does this general and common characteristic of matter, extent, 
apply to caloric, or does caloric occupy space? It decidedly occu- 
pies space: for most bodies, by an increase of density, give out 
caloric; or it is a general law, with a very few exceptions, that | 
bodies passing from a larger to a smaller bulk evolve caloric ; or 
the reverse, bodies passing from a smaller to a larger bulk necessa- 
rily absorb, or take in, caloric. Thus, according to the experiments 
of Mr. Watt, water, by conversion into steam, is enlarged about 1800 
times. It may be urged that this is all very plain when caloric is 
viewed in connection with matter; but does it occupy space uncon- 
nected with matter, as we can conceive an atom or a number of 
atoms of any elementary substance to do? This question certainly 
cannot be answered with the same clearness as that respecting caloric 
in connection with material objects. That it can, however, be an- 
swered in the affirmative, will be abundantly evident to any unbiassed 
mind who considers the following fact: the transmission of caloric 
in vacuo, as shewn by Pictet, by placing a thermometer in the ex- 
hausted receiver of an air-pump; and by Count Rumford, by placing 
the same in a Torricellian vacuum, the most perfect that can be 
found. Now, whatever passes through a complete void naturally 
occupies a portion of that, unless it be analogous to mental pheno- 
mena, which few would be willing to admit of caloric. Therefore, 
with the idea that caloric is material there is nothing preposterous 
in saying that extent is one of its essential properties. 
The other essential property of matter is impenctrability, or that 
VOL. IX., NO. XXVI. 31 7 
