Properties 
of Heat. 
=o 
PLATE 
as shewn at a, Fig. 9. By ae er the aperture P, 
@CLXXxix, 80.as to allow the rays which diverge from it to traverse’ 
Fig. 9. 
Fig. 10. 
676 
at E, and the a) P raised so as to be seen through 
the upper stratum of fluid, it will appear of a:perfect- 
ly circular form. Let the heated iron be now suspend- 
ed, asin the Figure. In.a few seconds the upper stra- 
tum of fluid will expand, as the heat penetrates the mass, 
and the circular aperture will. have an elliptical form, 
the fluid strata at different , the aperture will 
have the appearance shewn at 6, c, becoming perfectl 
circular at the point.c pe gt the heat has oe ne 
These appearances are the necessary consequence 
Rese of the heat downwards, which diminishes 
the density, and consequently the #efracting power of 
the upper strata. A very curious phenomenon now 
takes place upon removing the hot iron GH. The up- 
ost stratum mn, Fig. 9. which was formerly the 
ottest and the least dense, now gives out its heat to 
the superincumbent air, and to a certain depth c, Fig. 
10. the fluid diminishes in density, while from ¢ to f 
its density increases. If, in this situation, we again ex- 
amine the circular aperture, it will be found to be ex- 
tremely elliptical at a, Fig. 10. having its larger axis ho- 
aizontal. The ellipticity will gradually diminish to-: 
‘wards c, where its form will be circular, and below c it’ 
will again become elliptical, the larger axis of the el- 
lipse being now in a vertical direction. In these 
experiments the heat propagated down the sides of the 
vessel produced no effect, and if it had produced any, 
"it could have been easily distinguished from that which 
Fig, 10. 
No, 25 
Transmis- 
sion of heat 
through 
gases. 
Rumford’s 
experi- 
ments. 
was occasioned immediately below the hot iron. 
Now as all these changes are capable of being accu- 
rately-measured by a micrometer, we are furnished with 
the means not only of ascertaining the relative conduct- 
ing powers of all transparent bodies, but also of deter- 
mining the curve of density, whether the variation of 
refractive power is produced by heat, by pressure, or 
by the mutual penetration of two different fluids, The 
optical properties which were developed during these 
experiments, will be explained in our article Op- 
‘rics, Similar results may be obtained in a manner very 
different, but equally satisfactory. Let AB, Fig. 10. 
No. 2. be a yessel having a plate of annealed glass, 
whose section is MN, placed horizontally near the sur- 
face mn of the fluid, and supported by a glass pillar P. 
When this vessel is properly exposed to polarised light, 
no appearance will be seen through the edges MN of 
the me ; but ifa heated iron is suspended, asin Fig. 8. 
the descent of the heat towards the surface MN, and its 
propagation through the glass plate, will be marked by 
beautiful fringes of light parallel to MN. Hence we 
have another method, and a very direct one, of measu- 
ring the conducting powers either of transparent or opaque 
bodies, by the Chromatic thermometer, See Grass, vol. 
x. p. 821; Tuermometen; and the Phil. Trans. 1816, 
p. 108. 
We have but few remarks to make on the manner in 
which heat is transmitted through gases. From their 
physical constitution, especially from the facility with 
which their particles move among each other, we might 
infer that heat would be conveyed through them, more 
inthe way that it is through fluids, by the action of 
currents, than from particle to particle, as in solids. 
We know also that air, like liquids, is expanded by heat, 
and that this expansion causes it to become specifi- 
cally lighter, and thus forms ascending currents of 
warm, and descending currents of cold air, Rum- 
ford endeavoured to prove by experiment, that ‘the 
passage of heat through air was-entirely brought about 
by these currents, and that it was in itself a perfect 
HEAT. 
non-conductor. 
his 
at least, effected by 
Berthollet has indeed advanced some considerations in Berthol 
posite to'that of Rum-~ opinic 
favour of an opinion the very op 
ford’s, that air is lon unceenbatoaanioastineaienn 
heat, we must make a few pore eR re cooling of bedi 
ies. This process depends upon the combination of 
pantie upon the presence of those currents, which we 
ve described as existing in fluids and gases. Professor 
Leslie, in the course of his researches, to which we 
have so often alluded, made some experiments on the experi 
cooling of water contained: in vessels that had diffe- ™e's- 
rently radiating surfaces. He found that a globe of 
polished metal, when filled with boiling water, cooled 
to acertain degree in 156 minutes, while under the 
same circumstances, €: that the globe was .coated 
with lamp black, the water required only half that 
time for cooling. In this experiment, however, the 
cooling depends partly upon radiation, and partly upon 
the conducting power of the substance of which the yes« 
sel is formed ; and Professor Leslie attempted to estimate 
the relative effect of these two operations. From pre- 
vious trials, he knew. what quantity of heat would have 
been abstracted by radiation alone ; and thus:it was, 
easy to calculate what portion of the heat was removed 
by radiation, and what by the conducting power of 
the vessel. The general result is, that the effect of ra« 
diation is more powerful than that arising from the 
conducting power ; for the lamp black, which is known 
to be a bad conductor, increases ‘the cooling of the wa- 
ter by augmenting the radiation. As m be pre- 
dicted, from what has been already related, when the 
globe was immersed in water, the cooling went. on at 
the same rate, whether the surface was , or was 
covered with the black paint. In this case the process 
depended entirely on the internal currents that were 
formed in the water, while the surface of the vessel 
has no effect upon it. Inquiry, p. 268. 316. 
Both Professor Leslie and Count Rumford performed 
a number of experiments, similar to the one mentioned 
above, on the action of the surfaces of vessels in pro- 
moting or retarding the escape of heat from bodies con- 
tained in them. We arrive at many conclusions which 
are singular and important ; but they so obviously fol~ 
low from the facts which have been already noticed 
respecting the radiation of heat, that it will not be neces« 
sary to dwell very long upon them. In Professor Leslie’s 
experiments, it was always found that the cooling of 
water in metallic vessels was accelerated by coating the 
vessel with paper, paint, isinglass, or other similar sub« 
stance ; and while the coating was comparatively thin, 
this acceleration appeared to be in proportion to its 
thickness. So that although in these cases the conduct 
ing power must have been injured, yet this defect was 
more than counteracted by an increase of radiation. It 
can, however, scarcely be doubted, that, were we to 
go beyond a certain limit, we should find, that, by aug- 
menting the thickness of the coating, the diminution of 
