Properties 
of Heat, 
‘Rumford’s 
experiments 
on radiation, 
Leslie’s ex- 
668 
be confident that no light could be emitted: and this 
he found to radiate heat, and to raise the thermometer 
very perceptibly. The experiments of Pictet may 
therefore be considered as completely establishing the 
point, that heat is sent off in right lines from bodies, 
where light cannot be supposed to be present. He also 
rendered the radiating power of caloric still'more ob- 
vious, by shewing, that when rays.of ‘heat impinge 
against a body, if it have a polished surface, they are 
reflected from it; but, if the surface be ‘such as not to 
admit of reflection, they enter into it, and raise its 
temperature. In pursuance of ‘this idea, he found, 
that when the bulb of the thermometer which he em~ 
ployed was blackened, it rose more rapidly, or absorb- 
ed more heat, than when its surface was clean and 
bright ; thus proving, that the heat from boiling water 
was, in this respect, similar to the heat in the sun’s 
rays, or that emitted during combustion: (Essay on 
Fire, § 51, et seg.) We have already referred to the 
experiments of Dr Herschel, in which he analysed the 
sun’s rays, and separated the part which produces heat, 
from that which excites the sensation of colour. These 
experiments proved, in a very decisive manner, the ra- 
diating power of the calorific part of the solar beam ; 
and he afterwards made experiments of a similar kind, 
upon different species of heat, extricated from bo- 
dies on the surface of the earth, such as burning fuel 
and red hot iron. Phil. Trans. 1800, p. 316. 
An interesting train of experiments, on the manner 
in which heat radiates, or escapes from the surfaces of 
bodies, was-performed by Rumford, in which he shew- 
ed, with his usnaladdress and dexterity, the different 
effects which are produced by a difference in the nature 
of the radiating surface: on Trans. 1804, p. 89 et 
seq.) By a singular coincidence, nearly about the same 
time that this essay was published in the Phil. Trans. 
a still more complete view of the subject was taken by 
Professor Leslie. They both supported their peculiar opi- 
nions by a number of well contrived and well execut- 
ed experiments, which led'to many’ curious and unex- 
pected results, and which have, in some measure, al- 
tered our previous notions respecting the nature of 
heat, or at least respecting some of its most remark- 
able properties. It had been before known, that the na- 
ture of the surface of a body materially affects its power 
of admitting caloric to enter into it; and this power 
was now extended to the emission or radiation of heat. 
There is so much similarity between the ‘experiments 
of Rumford ‘and Leslie, that it will not be necessary to 
refer to both of them, considering the narrow limits te 
which we are confined in this article ; and.as those of the 
at ter are generally the most decisive, and, for the most 
art, performed with the most accurate instruments, 
we shall principally employ-them in our examination of 
the subject, 
Professor Leslie's researches illustrate, in a striking 
periments on Manner, the effect of the peculiar nature of the surfaces of 
radiation. 
bodies, both upon the emission and reception of radiant 
heat, bodies of equal temperatures discharging and ab- 
sorbing it in very different degrees. When a body 
sends off rays of heat, we may conceive that it parts 
with a portion of caloric that “was previously united to 
it; and that when it receives the rays of heat, a quan- 
tity of caloric beeomes combined with it, which was 
before in a free state. These two operations, although 
the reverse of each other, seem to exist in the same pro- 
portion, and in all respects to bear an exact ratio to each 
Differential other. Professor Leslie employed in his experiments a 
thermome- 
ters 
species of air thermometer of anew construction, which, 
besides possessing the advantage of being an instru« 
HEAT. 
ment of great delicacy, and being 1g sensible to very mi- Proper 
nute variations of heat, has also the useful ee of of 
indicating, at all times, any variation that occurs in the 
temperature of the. portion of air in which it is immer. 
sed, and of adapting, as it were, its own scale to this 
new temperature, so that the apparent effect is the 
same, whatever be the actual temperature at which the 
experiment is performed. It was from this 
that he gave to his instrument the name of 
thermometer, as not indicating the actual degree of heat, 
but only the degree in which it differs from that of the 
atmesphere, At is shewn in Plate CXLI. Fig.1.) Rum- . 
ford employed an instrument very similar in its nature, 
which he called a i 
In ascertaining the q Experi 
surfaces of bodies, Professor © generar examined. ™ents with 
the rays after had been reflected by concave mirrors., * Canc 
Those which he used were con of polished block 
tin ; and by means of a mould, upon which they } q 
formed, they were made to constitute portions of the i | 
parabolic’curve. The substance from which the heat 
was emitted was boiling water, contained in a cubical 
canister of block tin. was provided with a 10 
meter to ascertain its temperature ; and the apparatus be= 
ing placed in the focus of an elliptical tin reflector, the 
effect was noticed upon the differential thermometer, si« 
tuated in the opposite focus. ‘The canister had four. 
sides of equal dimensions ; and these being prepared in 
different ways, either polished, or left rough, varnished, 
or covered with paper, or some other substance, afforded _ 
an opportunity of accurately examining and com 
the effect of different kinds of surfaces on the radiation of 
heat: (Inquiry, p. 17.) ‘The apparatus is shewn in Plate 
CCLXXXIX, Fig.11. Professor Leslie begins by ascer« 
taining what was the effect of the canister of boiling water, 
when simply placed in the focus of the reflector, in what 
length of time the maximum of heat was produced, and 
how long the processof cooling occupied. He likewiseob- 
serpadthe effect produced on the thermometer, by em- 
ploying water of different temperatures, and also by the 
degree in which the vermpermepne of the water exceeds that 
of the temperature of the air of the chamber in which 
the apparatusis situated. The vessel that contained the 
water was a cube of'6 inches ; and when it was at the 
‘boiling ‘heat, and was,placed at the distance of about 3 
feet from the mirror, the rise of the differential thermo- 
meter was equivalent to what would have been 14.5? 
of Fahrenheit’s seale. He also found that the, greater 
was the excess of the heat of the mates above the tem~ 
erature of the room, exactly in the same )Prapeneae = 
te its action, on ‘the thermometer. Hitherto € VES- Experi 
‘sel had been employed with its uncovered side turned ments o 
towards the reflector ; and he next proceeded to contrast the radi 
with this the effect-of the other three sides of the ca- er 
nister, one of which had a-plate of glass cemented to ¢ 
it, another had rhe Pam | on it, and the 
fourth was covered with a varnish of lamp black. The 
effect produced by these different surfaces was very. 
singular, and was, to the experimentalist himself, very 
unexpected. The uncovered side, which had the usual 
polish of a metallic surface, produced an effect upon 
the thermometer equal to 12°, the side to which the 
glass had been applied to 90°, that covered with paper 
to 98°, while the varnished side was equal to 100°. 
From these experiments we arrive at the important 
conclusion, that heat radiates from a polished metallic 
surface with riot quite 3th part of the energy that it does 
from a surface that is covered with some ‘substanice, 
which takes away the effect of the polish. Inquiry . 18, 
An experiment, which may be regarded as the re- 
