ERI a oem oe = 
- covered with a layer of minium, 
: applied, so.as to 
admit of a degree of polish. It was then found, that 
the coated thermometer, analogous to the polished side 
of the canister, received only about #th as much heat as 
' when the instrument was without the metallic covering. 
The:power of surfaces in absorbing heat, seems there- 
fore to be intimately connected with that of radiating 
it, materially influenced by the nature of the surface, 
much less when consisting of polished metal, and great- 
ly augmented when covered with paper, varnish, or any 
other not-metallic substance. These iments seem- 
ed to indicate, that the er of ‘radiating and of ab- 
_ sorbing heat-was in ‘an inverse ratio to that by which 
heat is reflected from surfaces ; and this point was after 
wards made the subject of direct experiment by Profes« 
sor Leslie. He had already found, that the polished sur- 
face of the canister was a bad radiator, and that the 
coated thermometer, in consequence of its polish, was 
equally unfitted for absorbing ‘heat. The next object 
was to examine, whether these surfaces, which were the 
least fitted to radiate and ‘absorb it, were not’as much 
superior in their reflecting, as they were defective in 
their radiating power. The tin reflector was removed, 
and in its + we was substituted one of ‘glass, while the 
canister of boiling water, and 'the/differential thermo- 
meter, were placed as before in the respective foci. In 
order to produce ‘the greatest action upon the mirror, 
the varnished side of the vessel was opposed to it ; ‘but 
the effect-produced by reflection in this case was not 
considerable. No important alteration was produced, 
by removing the metallic coating from the back’ part of 
the mirror, or by roughening it with emery ; but when 
the anterior surface of the glass was covered with In- 
dian ink, no‘heat was sent of from it. On the contra« 
ry, when the front of the mirror was’ coated with tin- 
foil, the thermometer rose ten times as much as it did 
from the effect of the naked glass. These experiments, 
when compared with each other, lead to the following 
conclusions ; ‘That when the rays of heat strike against 
polished glass, a large portion of them’is absorbed, and 
tends to/raise the temperature of the glass itself; but 
when they impinge against polished metal, few of them 
enter the metal, and nearly the whole are reflected, In- 
(quiry, p. 21. . 
aT Protos Leslie having now fully established the con- 
ive nection between the radiating and the absorbing A gid 
different kinds of surfaves, afterwards proceeded to va 
the’effects in different ways, and to compare them ‘wit 
each other under different modifications. We have 
mentioned above, that the polished ‘side ‘of the canister, 
* when’ opposed to the mirror, produced in the differen- 
tial thermometer ‘a ‘certain rise, which we have called 
to 12; that of the varnished surface being estima- 
ted, as a standard of comparison, at 100. hen the 
tin was rubbed witha small quantity of mercury, an 
effect was ‘produced equal to 14, and when completely 
eoated with it equal to 20. As the brilliancy in this case 
appeats not to have been less than frem the pure tin, 
we must conclude that there is either a different radia- 
ting power attached to different metals, independent- 
ly of their mechanical properties, or that an amal- 
gam ‘of tin and mercury was formed of a soft consist- 
ence. It was afterwards found, that when a metal loses 
its brilliancy by oxidation, the radiating power is increas- 
éd'in the same proportion : a fresh ‘surface of lead rais- 
ed the thermometer only 4th as much’as the lead when 
This effect of oxida- 
HEAT. 
669 
tion perhaps have been icted from the pre. Pr 
ceding éxperiments ; but the tion caused by 
ply’scratching or roughening the metal, seems more 
remarkable. If the perfectly smooth surface produced 
on the thermometer an effect equal to 12, by rubbing it 
with a file or with sand, so as to d its po- 
lish, an elevation took place equal to 26, or rather more 
than‘twice as much asthe former, It was afterwards 
found, that when the tin canister was coated with an 
animal substance, such as jelly, if it was spread wpon it 
only to the thickness of a fine film, it raised the ther- 
mometer to 38 ; but’when laid on so as to form a thick 
coating, the effect was about twice as t, or near 80, 
The effect in this case, was not found, however, to bear 
an exact ratio’ to the thickness of the coating ; nor was 
a difference in the thickness ‘ofthe coating observed to 
have any’ effect, except when an ‘animal substance was 
used, or something of a'similar nature, for metallic coat- 
ings seemed to act merely from their surface. Some 
experiments ‘wete made upon the ‘effect of colour, in 
modifying the radiating power of surfaces ; but the re- 
sults are not sufficiently uniform, to prove any thing 
very decisive on this point. Although the most consi- 
derable effects were produced by lamp-black, yet, as has 
beén mentioned above, writing paper was found to be 
nearly as ‘powerful. We have already had occasion, 
more than once, to refer to the relation which exists be- 
tween the radiating and the reflecting power of bodies, 
a fact which, although contrary to what might have been 
expected upon a transient view of the subject, is yet 
established by numerous and decisive experiments. 
Thus ‘scratching the surface of the mirror, diminishes its 
reflecting power as remarkably as its augments its ra« 
diation ; and also a layer of animal matter spread over 
the face of the canister, diminished its reflecting power 
to about 1-3d, as was determined by the thermometer. 
Inquiry, p. 76. et seq. 
Hee 
—_—_— 
rotessor Leslie was afterwards led to confirm, and ve- Experi- 
ry much extend the views which Scheele originally'sug- ™en's with 
gested respecting the power ‘of certain substances 'to're- 4: 
of 
different 
tain the caloric which falls upon them by radiation, ings 
when they are suchas not tosend it off again by reflec- 
tion. In this ‘case, the body receiving the heat, expe- 
riences an elevation of temperature, until after some 
time it becomes itself a source or eentre of heat, which 
emits it to other bodies. It was in this way that heat 
was separated from light, as we have already related, 
the light passing without interruption through a plate 
of glass, by which the transmission of heat is, for a cer- 
tain time at least, entirely obstructed. When the heat 
is emitted or radiates from its new source, its is 
then found to be varied from that which it originally 
possessed, and to be entirely directed by the condition 
of the surface from en at last oer This pro- 
of heat was illustrated by a series of experiments, 
fn which screens of different Kinds were interposed be- 
tween the mirror and the canister, to which we have so 
often referred : (Inquiry, p. 26, and 17.) A sheet ‘of 
tin was found entirely to intercept the heat, and a plate 
of glass a considerable portion of it; but what consti- 
tutes a curious difference between the effect of heat and 
light is, that the quantity of heat intercepted varies 
greatly, according to the vicinity of the glass to the ra- 
diating body ; more heat being transmitted when the 
glass is near the canister, than when ‘it is more distant 
from it. But, perhaps, some of the most curious of all 
Professor Leslie's experiments were those in which he 
employed two screens of ‘tin, one side of each of which 
was Covered with the black varnish, the other being 
Jeft uncovered. When these tin plates were laid toge- 
