s nion of their identity was generally adopted, at the com- 
_ mencement of the last century. Some distinguished 
experimentalists had indeed embraced the contrary doc- 
trine, and some facts were brought forwards in its sup- 
port. Instances perpetually occurred, of high degrees 
of heat being produced, which were not accompanied by 
light ; and, on the other hand, it was found, that there 
were some luminous bodies which were not hot, and 
especially, that the moon’s rays might be concentrated 
by a lens, or mirror, so as to produce a very brilliant 
light, yet that no sensation of heat was excited. These, 
however, and some other analogous facts, were not 
deemed sufficiently conclusive to establish the point. 
Some authors regarded heat and light as the same sub- 
' stance in different states ; while those, who did not ad- 
mit their materiality, conceived that they depended up- 
on the same affection of matter, but differing in its de- 
gree or intensity. Scheele’s experiments seemed to 
prove, that heat and light, when closely united toge- 
ther, as in the wh of the sun, or of a burning bod ; 
may be completely separated, so that the specific effects 
of each may be obtained distinct from the other. We 
have already referred to his observation of the manner 
in which a sheet of glass permits the light to pass 
through it, while it intercepts the transmission of the 
heat ; and he also found, that if rays composed both of 
heat and light, as they were sent off in combustion, be 
received u a glass mirror, the heat is absorbed, 
while the light alone is reflected. 
The distinct nature of light and heat is rendered still 
‘more probable, by some late experiments of Dr Her- 
schel’s, in which he separated them from the state of 
combination in which they exist in the sun’s rays. He 
~was led to observe, that the differently coloured rays 
possess different powers of producing heat; the least 
refrangible rays, those which excite the sensation of 
red, possess the greatest heating power; and the power 
‘diminishes, until we arrive at the violet, or most re- 
frangible rays, which excite the least heat, the inter- 
‘mediate colours possessing an intermediate power in 
this respect. But he not only discovered that the rays 
of heat and of light were thus very differently affected 
‘by the prism ; he farther found, that the effect upon 
the thermometer was most considerable in a point be- 
yond the red-making rays, entirely out of the limits of 
the spectrum, and, of course, in a spot on which no 
part of the luminous ray was received. Beyond this 
int, where the temperature was at its maximum, the 
fans gradually diminished, until it was no longer per- 
ceptible: (Phil. Trans. 1806, p. 286. et seq.) From 
> . these experiments, the results of which were verified 
Englefield’s by Sir H. Englefield, who employed an a of 
speri- er a different kind, we learn, not only that heat is 
emitted in rays from the sun, but that solar heat may 
be obtained ‘separate from light ; and although, like 
light, it is possessed of the power of being refracted, 
‘yet-it possesses this power in a different degree. In Sir 
H. Englefield’s experiments, where the blue rays raised 
the thermometer to 56°, the green to 58°, the yellow 
to 62°, and the red to 72°, the space beyond the pris- 
matic spectrum elevated it to 79°, and it sunk to 72° 
when returned into the red rays: (Journ. of Royal 
crard’s ex. Inst. i. p. 203.) Similar results were obtained by M. Be- 
periments. tard; but he observed, that the maximum of heat was 
at the very extremity of the red rays, when the bulb of 
the thermometer was completely covered with them ; 
and that beyond the red extremity, where Herschel 
found the heat to be a maximum, it was only 1-5th above 
that of the ambient air. See Appenprx to this article. 
~ The radiation of heat .was exhibited in a still more 
meits. 
HEAT. 
667 
decisive manner by Pictet, who seems to have under- Properties 
taken his experiments on the subject at the suggestion of Heat. 
of Saussure. This distinguished naturalist was led to , ~~" 
form some speculations concerning caloric, from certain ae ong 
atmospherical phenomena which he noticed, during his Picte: and 
travels among the Alps, where he conceived that the Seussure. 
communication of heat could not be accomplished by 
the contact of the heated body. He refers to an expe- 
riment of Mariotte’s, which was published in the Me- 
moirs of the Academy of Sciences for the year 1682, in 
which he states, that “the warmth of a fire reflected 
by a burning mirror is sensible at its focus.” Lambert 
also informs us, in his Pyrometrie, that he placed a 
burning body in the focus of a concave mirror, and 
that he was able, by means of it, to inflame another 
body, placed in the focus of an opposite mirror, at the 
distance of above 20 feet. In this experiment, Lam- 
bert distinctly marks the difference between what he 
calls “* Juminous heat” and “ obscure heat,” and attri- 
butes the effect to the latter principle, i. ¢. to heat pro- 
perly so called, in © ition to light. 
aussure repeated the riment in conjunction with : 
Pictet; and, Ta seeder © peeve the feceidudinns of ete 
the action of light, they employed a ball of iron, heat- metallic 
ed to a degree short of what would render it luminous ™ T° 
in the dark. They used tin mirrors, that were placed 
more than 12 feet from each other: and, when the iron 
ball was put into one focus, they suspended a thermo- 
meter in the other, and observed the instrument to be 
very perceptibly affected, more than another thermo- 
meter, equally near the ball, but out of the focus. The 
former was raised from 4° to 144° of Reaumur; the 
latter from 4° to 63° only: (Voyages dans les Alpes, 
§ 926.) . The result of this experiment is easily expli- 
cable, upon the supposition that heat, whatever be its 
nature, radiates in straight lines; that it impinges against 
solids that are opposed to its course; and that, accerd- 
ing to circumstances, it either raises the temperature 
of this body, by being united to it, or is reflected from 
its surface. The heated ball, in this case, emitted rays 
of heat in every direction: those that were contiguous 
to the mirror fell upon it ; but, owing to its polished 
surface, were reflected in straight lines to the other mir- 
ror, and were again reflected from this, according to 
the laws of m ical impulse, into the focus in which 
the thermometer was suspended. This thermometer 
received the effect, both of the rays that were sent off by 
the iron ball, on the side contiguous to it, and of those 
which were on the’ contrary side of the ball, next to the 
mirror ; whereas the thermometer not in the focus, on- 
ly received heat from the side of the ball o to it. 
Pictet’s apparatus is shewn in Plate CCLXXXVIII. 
Fig. 9. Description of Plates. 
. Pictet still further prosecuted. these experiments, Farther ex- 
and varied them in different ways, so as to 1 the periments 
objections that might be urged against the conclusions ° Picts 
which he derived them. In order to separate the 
light from the heat, and to-shew the distinet operations 
of each, he placed a lighted candle in one of the foci, 
and noticed its effects upon a thermometer placed in 
the other focus. He then interposed a plate of glass 
between the candle and the thermometer ; and he found, 
as he had expected, that although the light passed as 
before, a considerable portion of the heat was inters 
cepted in its e from-one mirror to:the other ; in 
this way confirming the: results that had been formerly 
obtained by Scheele. But, in order to remove more 
effectually all suspicion that the effect in this case de- 
pended upon the rays of light, he placed a flask 
of boiling water in the focus, from which we may 
Remarks 
upon the 
experi- 
ments, 
