Sources of 
heat. 
Solar heat, 
Combus- 
tion, 
Heat pro- 
duced by 
mechanical 
means. 
SECT. IIT. 
Sources of Heat. 
Arter having described the properties and effects of 
heat, we must now give an account of the sources 
whence it is derived. Those that are usually enume- 
rated are the following: the rays of the sun,—combus- 
tion,—a change in the capacity of bodies, and their 
chemical union,—the mechanical means of producing 
heat, as friction, percussion, and condensation,—and, 
lastly, the electric and galvanic discharge. The great 
source of heat to our world, and probably to the rest 
of our solar system, is the radiant caloric which is pro- 
jected from the sun. When a sufficient number of rays 
are collected into one spot, either by a concave mirror, 
or by a convex lens, the most powerful heat is excited 
of which we have any conception ; and we are capable 
of producing effects by it, which we cannot otherwise 
accomplish. The effect, however, is necessarily con- 
fined to a small spot ; and therefore it can only be em- 
ployed in refined operations of analysis, or for the pur~ 
pose of delicate philosophical processes. It would ap- 
ar, from an experiment of Rumford’s, that the great 
beat excited in these cases, depends entirely upon the 
concentration of the rays, and not upon any change in 
their nature; because, when he directed a certain por- 
tion of the sun’s rays against a substance edupead lik 
absorbing them, the total amount of heat communi- 
cated to it was the same, whether the rays were re- 
ceived on the surface in a diffused state, or brought 
into a small focus. 
The most important supply of heat which we have 
it in our power to produce at pleasure, is that which 
depends upon combustion. Nearly all the heat which 
we employ for domestic and manufacturing purposes, 
proceeds from the burning of fuel; an operation of a 
strictly chemical nature, in which a union takes place 
between the fuel and the oxygen of the atmosphere, the 
“result of which is a combination of the combustible 
matter with the oxygen, and the extrication of a quan- 
tity of heat and light. It has been a much controvert- 
ed question, whence does the heat proceed in this ope- 
ration? The discussion is strictly chemical, and must 
‘therefore be dismissed with a few observations. A part 
of the heat liberated probably depends upon the less 
capacity of the product of combustion, than of the 
substance supporting it, i. e. of carbonic acid, than of 
oxygen and carbon separately ; but there are instan- 
ces in which the quantity of heat extricated appears to 
“bear no proportion to the bulk of the materials con- 
sumed, or of carbonic acid formed ; and hence we are 
induced to conjecture, that heat exists as an ingredient 
in combustible substances, and is disengaged from them 
in consequence of the changes of composition that take 
place. It has been supposed, that, besides what ne- 
cessarily belongs to it as a gas, oxygen contains caloric 
as one of its constituents, which is liberated by a kind 
of precipitation ; and the phenomena which attend the 
detonation of gunpowder lead us to conclude, that ca- 
loric also enters into the composition of the nitric salts, 
and others that possess similar properties. Of the heat 
produced by a change of capacity, in consequence of 
an alteration Pe — er of a body, or of new combi- 
nations, we have already spoken at sufficien i 
the last Section. ig “raping ah 
There are few subjects connected with heat, which 
are more inexplicable, than the manner in which it is 
HEAT. 
generated by those means which appear to act mecha- § 
nically, as friction, percussion, and condensation. The _ ! 
latter of these operations, condensation, has already ~~ 
come + ore tha notice, in treating of the change of ca- 
acity W ases experience, according as their par- 
lle ies eanapreied together, or allowed to depena 
themselves ; and the same effect, at least to a certain 
extent, seems to follow from the condensation of solids. 
A very familiar instance of this occurs in the heating 
of a bar of iron merely by repeated hammering, where, 
by a few smart blows on the anvil, the smith is able to 
produce a sufficient extrication of heat to kindle a match. 
It is, however, known that iron does not | this 
property of producing heat by being hammered, unless 
it has been yp biae heated for some time in the forge, 
and suffered to cool slowly in the air ; and that, after it 
has been hammered, and Jost part of its heat, its texture 
seems to become harder, and it is rendered less flexible 
than before the operation. The rolling out of metals 
into thin sheets or leaves, and the drawing of fine wires, 
likewise excite heat, which is to be referred to the com- 
pression the substance undergoes during these various 
operations. In all these cases there is some reason to 
suspect, that the bodies have had their capacity dimi- 
nished, and that the heat, which becomes sensible, is to 
be assigned to this oe The effect of 
may perhaps be explained on the same principle, as we 
may conceive, that, whenever percussion takes place, it 
must cause a proportionate condensation. It is to be 
remarked, that liquids seem to be incapable of extrica- 
ting heat, either by any attempts at condensation, or by 
any kind of percussion ; and as they are supposed to be 
nearly incompressible, we may conceive that their ca- 
pacity is not liable to be affected by these means. Gas- 
es, however, which are readily compressible, are cap: 
ble of having heat extricated from them, to a consider- 
able amount, by condensation ; and here we suppose 
that their capacities are changed, and in this way we 
account for the effect. 
This mode of reasoning will not, however, apply to Friction. 
the extrication of heat which is caused by friction. The 
power of friction in producing heat, in various mecha- 
nical operations, is the subject of daily observation, and, 
if not prevented by different expedients, produces the 
most fatal consequences. The motion of wheels, and 
pulleys upon their axes, excites a degree of heat, which 
would quickly set fire to the machinery, was it not pre< 
vented by the application of different lubricating sub- 
stances, which dimainal the friction, by interposing be- 
tween the hard surfaces a body which readily yields to 
the impression. The quantity of heat generated in 
these cases is so considerable, and seems so dispropor- 
tioned to any effect that can be attributed to a eu 
either in the capacity or chemical nature of the body 
employed, as to have been always regarded a serious 
objection to the hypothesis, which regards heat as a 
substance capable of being transferred from one body 
to another, like other material agents. The objection 
was reduced to a more palpable form by Rumford, who Rumfo 
estimated the exact amount of the heat, which was pro- ae 
duced by a certain degree of friction, where all other 
extraneous sources for its admission were carefully 
guarded against. A piece of brass was fixed in a ma- 
chine used for boring cannon, and a steel cylinder was 
pressed against the brass, with a weight equal to 1000 
Ibs. and then made to revolve on its axis with a given 
velocity. After some preparatory experiments, the ap- 
paratus was all inclosed in a vessel of water ; and after 
the friction had been kept up for some time, the water 
ssion Percy 
