CALOfllC. 
process consists in an almost infinite num- 
ber of repeated compositions and decom- 
positions. We see, too, that when heat is 
applied to one extremity of a body, the 
temperature of the strata of that body must 
diminish equably, according to their dis- 
tance from the source of heat. Eveiy per- 
son must have observed that this is always 
the case. If, for instance, we, pass our 
hand along an iron rod, one end of which 
is held in the fire, we shall perceive its tem- 
perature gradually diminishing from the end 
in the fire, which is hottest, to the other 
extremity, which is coldest. Hence the 
measure of the heat ti ansmitted, must al- 
ways be proportional to the excess of tem- 
perature communicated to that side of the 
conductor which is nearest the source of 
beat. The passage of caloric througli a 
body by its conducting power must have 
a limit ; and that limit depends upon the 
number of doses of caloric, with which the 
stratum of the body nearest the source of 
heat IS capable of combining. If the length 
of a body be so great, that the strata of 
which it is composed exceed the number 
of (loses of caloric with which a stratum is 
capable of combining, it is clear that ca- 
kii'ic cannot possibly be conducted through 
the body ; that is to say, the strata farthest 
distant fi om the source of heat cannot re- 
ceive any increase of temperature. This 
limit depends, in all cases, upon the quan- 
tity of caloric with which a body is capable 
of combining before it changes its state. 
All bodies, as far as we know at present 
are capable of combining indefinitely with 
caloric ; but the greater number, after the 
addition of a certain number of doses, 
cirange their state. Thus ice, after com- 
bining with a certain quantity of caloric, is 
changed into water, which is converted in 
its turn to steam, by the addition of more 
caloric. Metals also, when heated to a 
certain degree, melt, are volatilized, and 
oxydated : wood and most other combusti- 
bles catch fire, and are dissipated. As to 
the rate at which bodies conduct caloric, 
that depends upon the specific nature of 
each particular body ; the best conductors 
conducting most rapidly, and to the greatest 
distance. When bodies are arranged into 
sets, we may lay it down as a general rule, 
that the densest set conduct at the greatest 
rate. Tims the metals conduct at a greater 
rate tl.an any other bodies. But in consi- 
dering the individuals of a set, it is not al- 
ways the densest that conducts best: as 
bodies;; conduct caloric in consequence of 
their alMty for it, and as all bodies have 
an affinity for caloric, it follows as a conse- 
quence, that all bodies must be conductors, 
unless their conducting power be counter- 
acted by some other property. 
Alt solids are conductors; because all 
solids are capable of combining with va- 
rious doses of caloric before they change 
their state. This is the case in a very re- 
markable degree with all earthy and stony 
bodies ; it is the case also with metals, 
with vegetables, and with animal matters. 
This, however, must be understood with 
certain limitations. All bodies are indeed 
conductors ; but they are not conductors 
in all situations. Most solids are conduc- 
tors at the common temperature of the at- 
mosphere; but when heated to tlie tem- 
perature at which they change their state, 
they are no longer conductors. Thus at 
the temperature of 60°, sulphur is a con- 
ductor; but when heated to 214°, or the 
point at which it melts or is volatilized, it 
is no longer a conductor. In the same 
manner ice conducts caloric when at the 
temperature of 20°, or any other degree 
below the freezing point ; but ice at .32° is 
not a conductor, because the addition of 
caloric causes it to change its state. 
With respect to liquids and gaseous bo- 
dies, it would appear at first sight that 
they also are all conductors ; for they can 
be heated as well as solids, and heated too 
considerably without sensibly changing their 
state. But fluids differ from solids in one 
essential particular : their particles are at 
full liberty to move among themselves, and 
they obey the smallest impulse; while the 
particles of solids, from the very nature of 
these bodies, are fixed and stationaiy. One 
of the changes which caloric produces on 
bodies is expansion, or increase of bulk ; 
and this increase is attended with a pro- 
portional diminution of specific gravity. 
Therefore, whenever caloric combines with 
a stratum of particles, the whole stratum 
becomes specifically lighter than the other 
particles. This produces no change of si- 
tuation in solids ; but in fluids, if the heated 
stratum happens to be below the other 
strata, it is pressed upwards by them, and 
being at liberty to move, it changes its 
place, and is buoyed up to the surface of 
the fluid. In fluids, then, it makes a very 
great difference to what part of the body 
the source of heat is applied. If it be ap- 
plied to the highest stratum of all, or to 
the surface of the liquid, the caloric can 
only make its way downwards, as through 
solids, by the conducting power of the 
fluid ; but if it be applied to the lowest 
