670 
Properties ther, with both ‘their painted sides in contact, and of 
of Heats’ course the bright sides both external, little heat passed 
through them, because the side nearest the canister was 
not adapted to receive caloric, nor the other to radiate 
the little which might have been received; but when 
the varnished sides were placed externally, the quan-~ 
tity of heat that passed through them was considerable, 
because here the varnish enabled the one plate to ab- 
sorb, and the other to radiate caloric with facility. When 
only one plate was used, intermediate effects were pro- 
duced. Several circumstances that were noticed by Pro- 
fessor Leslie, led him to conclude that the rays of heat, 
like those of light, where they proceed from a near ob- 
ject, are sent_off in lines that have a sensible diver- 
gence. The radiation of heat seemed to be entirely 
suspénded, by having the heated body immersed in 
water ; and it appears probable that it cannot exist in 
any medium except air. Inquiry, p. 92. 
e have already alluded to the experiments of Count 
ford’ a 
ooh : Rumford, which were published almost immediately af- 
ments re- ter those of Professor Leslie, and which so generally a- 
ferred to. gree with them, that although they are both numerous 
and ingenious, it will not be necessary to give any minute 
description of them. Like Professor Leslie, Count Rum- 
ford found, that-polished metal radiates heat in a much 
~less degree than a metallic surface, at the same tempera- 
ture, but covered with paint or varnish.; that a covering 
of linen, flannel, paper, or in short of any animal or ve- 
etable substance, tends to promote the emission of 
eat; and he farther found, that the radiation and ab- 
sorption of heat bore a direct ratio to each other, and 
likewise an inverse ratio to the power of reflection. Jn 
one particular indeed, the Count’s experiments had a dif- 
ferent result from Professor Leslie’s ; this latter experi- 
mentalist compared the radiating effect of various me- 
tals, and conceived that they differed from each other in 
this respect, while Rumford could not perceive any 
difference between them ; upon the whole, however, we 
are disposed to regard Professor Leslie’s results as the 
Practicat most correct. Count Rumford, according to his usual 
deductions. custom, deduces some important practical conclusions 
from his experiments, which are the more valuable, be- 
cause some of them are precisely contrary to the previ- 
ous ideas that were entertained upon the subject. In 
putting heated bodies into vessels or tubes, our object 
is sometimes to retain the heat as long as possible ; but 
in others, on the contrary, we are desirous that the heat 
should be quickly dispersed through the contiguous 
air. If we wish to confine the heat, we must emplo 
metal, and have its surface highly polished, a fact fully 
proved by the above experiments, but directly contrary 
to the conclusion that might seem to follow from the 
superior conducting power of a metallic body. . On the 
other hand, if the object be to cool the vessels with their 
eontents, or to transfer their heat to the surrounding 
medium, we must cover them with paint or varnish, or 
with some kind of soft coating, not of a metallic nature, 
As an example of the two cases, we shall mention that 
of tubes eonveying steam, which may, in the. one in- 
stance, be for the purpose of transporting heat from one 
vessel to another, and where, of course, it is an object 
that none should be lost in the passage ; here we must 
use bright metallic tubes, which will radiate as little as 
possible, But if, on the contrary, we introduce steam 
tubes for the purpose of warming an apartment, here 
we wish to promote the radiation as much as possible, 
and we should therefore use the tubes unpolished, and 
varnished, painted, or even rusted. 
bright metallic vessels should be employed, when we 
HEAT. f 
In the same way’ 
wish to preserve the heat in fluids, where they are used F 
for the purposes of cooking or manufactures, while the 
opposite plan is to be followed, when the object is. to 
promote their. cooling. (Pz. Trans. 1804, p. 177, et 
seq.) On the cooling of bodies we shall have occasion 
to aie more at large, in a subsequent part of the ar-— 
ticle. s ‘ ne 
We must now advert to a train of phenomena, con- Radi 
ore with ee radiating power of Tadine a: oh cold. 
rhaps depending upon a different principle, an| 
se nape t new ideas respecting the nature of heat, 
the radiation of cold. By the radiation of cold we 
mean simply to express the fact, that when a cold body. 
is placed in the focus of a concave mirror, a thermome~ 
ter will fall that is suspended in the focus of an oppo 
site mirror. This singular circumstance was first no- Ex 
ticed by the members of the Florentine Academy, and ™e™ 
was very distinctly described by them, alth« { pie 
were somuch surprised at the effect, as almost to doubt 
the accuracy of their own experiment. (Saggi di nat. 
Esper. p. ae) They do not seem to have made any 
attempt to explain it; no farther notice was takenof = 
it at the time, and it appears to have been almost for. Of Piete 
gotten, when Pictet, in the.course of his experiments, ' 
to which we have referred aboye, after having ascer- 
tained the radiation of heat and its reflection by his 
apparatus, placed a vessel filled with ice in one of the 
foci, and observed the thermometer in the other focus 
instantly to sink several degrees ; when the ice was re~ 
moved, the thermometer rose again to its former eleva- 
tion. By adding nitric acid to the snow, and thus 
ducing a more intense cold, he found that the effect on 
the thermometer was augmented. Essay on Fire, - 
§ 69. See our article Coup. + 
The apparent radiation of cold has been since confirm. Of 
ed by Professor Leslie, and what may, at first view, ap- 
r still more remarkable, it seems to be acted upon 
y bodies in the same manner with radiant heat. _It 
is promoted and retarded by the same kind of surfaces 
which promote and retard the radiation of caloric, and 
also in the same proportion. The canister, which had 
been employed in the former experiments, was now 
filled with ice or snow, and ‘its different sides in turn 
exposed to the mirror, the differential thermometer be- 
ing, as before, day rps in the opposite focus. The — 
cold produced by the varnished side was the greatest, 
while that from the uncovered side was the least, the 
glass and the paper being intermediate between the 
two, exactly in the same manner, as when the vessel 
had been filled with boiling water. The results also 
were similar when the thermometer was covered with 
different substances, so as to affect its power of absorb- 
ing heat, and also when the surface of the mirror was 
changed, so as to change its power of producing re- 
flection, For example, when the thermometer was 
coated with a leaf’ of metal it fell less, and when coated 
with varnish, more than in its ordinary state. And 
with respect to the mirror, the reflection of cold was 
most considerable from the bright metallic surface, less 
when a mirror of glass was employed, and still less 
when it was varnished. Here the power of the mirror 
in radiating cold, was exactly in the inverse ratio of its 
reflecting power, just as is the case with the radiation 
of heat. Lastly the interposition of screens of different 
kinds, and with different surfaces, between the ice and 
the mirror, had effects which were precisely analogous 
to those mentioned above; so that under all circum- 
stances, Professor Leslie found the strictest coincidence 
between the two kinds of radiation: (Inguiry, p. 23.) Is 
