Heat. 
Introducto- 
ry observa- 
tions, 
Term heat 
employed 
both for 
cause and 
effect. 
How em- 
ployed in 
this article: 
Importance 
of the sub- 
ject. 
666 7 i 
ADE ASE 
Hear is a term which was originally employed to ex- 
ress the effects produced by a peculiar condition of bo- 
ies, when they communicated the sensation of warmth. 
It was also perceived that hot bodies, or those that had 
the power of communicating warmth, possessed other 
properties, such as that of expanding the substances to 
which the warmth was imparted, of converting certain 
solids into liquids, and certain liquids into the state of 
vapour, and many other operations of great importance 
in the system of nature. Philosophers soon began to 
speculate upon the cause of these phenomena, and, with 
a degree of inaccuracy, to which we are liable in the in- 
fancy of our scientific pursuits, they employed the same 
word to signify both the cause,and the effect produced, It 
was commonly said, that heat occasioned the warmth and 
expansion of bodies, and likewise that heat was excited 
in bodies by the addition of some peculiar kind of mat- 
ter, or by a certain modification of their particles. The 
more precise nomenclature of the moderns has tended 
to correct this error, and has led to the invention of a 
new term, caloric, to designate the cause, while the 
word heat is, strictly speaking, only applicable to 
the effect. As, however, in all the older authors the 
former phraseology necessarily exists, as it is still adopt- 
ed in popular language, and as there is no danger of 
falling into any error, since the distinction has been so 
fully pointed out, the word heat is frequently employed 
in its double sense, even by the latest and most correct 
writers, and it will be used in this way in the following 
article. 
We have already given some account of the nature 
and effects of caloric under the head of Curemistry ; 
but it is an agent of such extensive importance in the 
operations of nature,—it produces such powerful effects 
both upon organized and unorganized matter,—it is so 
intimately connected with the existence of life, both 
animal and vegetable,—and is so essential to all the 
processes by which we act upon the bodies around us, 
when we convert them to our support or utility;—that 
it well deserves to be farther discussed, and made the 
subject of a separate article. The importance of the ob- 
ject has produced a consequent share of attention to it 
from the modern experimentalists ; and there is per- 
haps no one topic on which more curious, and, we may 
add, more unexpected results have been obtained, than 
have ensued from the researches into caloric. The 
names of Black, Crawford, Rumford, Pictet, Gay-Lus- 
sac, Prevost, Dalton, and Leslie, among many others 
which will be afterwards referred to, must suggest the 
recollection of the many ingenious and elaborate trains 
. of experiments, that have occupied the attention of phi- 
Plan of the 
article. 
losophers, during the last 50 years. It will be to an ac- 
count of what has been done in this period that we shall 
principally confine ourselves in the following pages ; 
for the experiments and hypotheses that were publish- 
ed before this time, are rather to be regarded as curious 
historical records of opinions, than as affording much 
that is important in the actual advancement of know- 
ps: 
_ We shall arrange our observations on heat under four 
heads: 1st, The properties of heat; 2d, The effects of 
heat ; 3d, The sources of heat ; and, 4th, The nature of 
heat. In the course of the article, we shall take an op- 
Paes: of tracing the gradual developement of the 
ading opinions that have successively prevailed on 
these topics, as well as the most important experiments 
by which they have been da i § ; 
SECT. I. 
On the Properties of Heat. 
Ir might, at first_view, appear more regular to begin 
by investigating the nature of heat, before we described 
its properties and effects; but it is so difficult to ascer« 
tain its nature, and the knowledge which we possess, 
or rather the conjectures which we form concerning it, 
are so entirely derived from the observations that we 
are able to make of its properties and effects, that the 
order of treating the subject. which we have adopted, 
will be found, we apprehend, the most convenient. For 
the present, we may consider heat, or caloric, to be a 
principle or pores: existing in bodies, which gives rise 
to many of their most important actions, and modifies 
their effects upon other substances. 
The properties of caloric are of three kinds: Those Arrange. 
that are strictly mechanical, or such as may be con- ment c 
ceived analogous to the laws of gravitation or impulse ; Propet 
chemical, or those that tend directly to effect a chemi- heat. 
cal change in bodies; and a third class, which may be 
regarded as specific, and which do not bear an exact 
resemblance to either of the two former. 
Among the mechanical properties of caloric are its 1, Mechani 
radiation, reflection, and refraction, which bear a very cal prope 
near resemblance to the same affections of light. Some ties. 
obscure intimation of the radiation of heat may be Radiation. 
met with in the authors of the latter part of the 17th a 
and beginning of the 18th centuries, but the subject 
was not een attended to until the time of Scheele. gcheete’s 
This distinguished philosopher, in his investigation of experimen' 
the nature of fire, performed some new and decisive ex- on radian 
periments, which completely established the existence b** 
of this property, and chewed how it differed from the 
power which hot bodies possess of communicatin 
warmth by contact. He found, that when glass is 
interposed between the face and a quantity of burn- 
ing fuel, although the light passes through without 
interruption, the heat, at least for a certain space 
of time, is entirely stopped. Heat, he observed, ra- 
diates through air, without communicating any warmth 
to it, and its passage does not appear to be interrupted 
by any currents in the atmosphere. He found, that a 
transparent mirror, which concentrates the rays of light, 
does not produce any increase of temperature in the fo- 
cus, until it has absorbed a sufficient portion of heat, 
but it then becomes a radiating body, and emits heat in 
certain directions. The conclusion which may be de- 
duced from Scheele’s experiments is, that caloric is sent 
off in rays from all hot bodies, and moves through the 
air with great velocity, but that in this transmission, it 
does not necessarily communicate warmth to it, and is 
not diverted from the straight course by any currents 
or motion of the air itself: (On Fire, p. 70, et seg.) His On the 
experiments also tended considerably to elucidate ano- ration of 
ther point respecting heat, which had been the subject heat and 
of much controversy, whether it was not identical with : 
light, or only differing from it in gas pt of some 
slight modification of its properties. Heat and light 
are so frequently observed in connection with each 
other, emanating from the same sources, and produced, 
as it would appear, by the same agents, that the opi- 
of Heat 
* ee 
