nyq and Uie rays of caloric are not lire ' 
lime with the rays of light. 
I.O.i examining the other extremity of the j 
Ipectrmn, Dr. llerschel ascertained that no 
lavs' of caloric can tie traced beyond the 
■•ndet ray. He had found, however, as Sen* 
In-bier had done before him, that all the 'co- 
loured rays of the spectrum have the power 
pf heating: it may be questioned therefore 
v tielher there are any rays which do not warm. 
Li 'he coloured rays must either have the pro- 
perty of exciting heat as rays of light, or they 
In ust derive that property from a mixture of 
h ays of caloric. If the first of these supposi- 
tions was true, light ought to excite heat in 
kill cases; but it has been long known to phi- 
losophers that the light of tiie moon does not 
produce the least sensible heat, even when 
{concentrated so strongly as to surpass, in 
{point of illumination, the brightest candles 
or lamps, and yet these produce a very sen- 
sible heat. Here then are rays of light which 
do not produce heat; rays, too, composed of 
jail the Seven prismatic coloured rays. We 
must conclude, from this well-known fact, 
that rays of light do not excite heat ; and 
consequently that the coloured rays from the 
Isun and combustible bodies, since they ex- 
’ cite heat, must consist of a mixture of rays 
; of light and rays of caloric. That this is the 
case was demonstrated long ago by Dr. 
ilooke, and afterwards by Scheele, who se- 
parated the two species from each other by a 
very simple method. If a glass mirror is 
held before a lire, it reflects the rays of light, 
but not the rays of caloric ; a metallic mirror, 
I on the other hand, reflects both. The glass 
mirror becomes hot ; the metallic mirror does 
j not alter its temperature. If a plate of glass is 
[suddenly interposed between a glowing lire 
and the face, it intercepts completely the 
warming power of the fire, without causing 
any sensible diminution of its brilliancy ; con- 
i sequently it intercepts the rays of caloric, 
but allows the rays of light to pass. If the 
{glass is allowed to remain in its station till its 
| temperature has reached its maximum, in 
j that situation it ceases to intercept the rays 
of caloric, but allows them to pass as freely 
j as the rays of light. This curious fact, which 
shews us that glass only intercepts the rays 
! of caloric til! it is saturated with them, was 
i discovered long ago by Dr. Robison, pro- 
j fessor of natural philosophy in the university of 
Edinburgh. These facts are sufficient to con- 
vince us that the rays of light and of caloric 
j are different, and that the coloured rays de- 
rive their heating power from the rays of ca- 
! loric which they contain. Thus it appears 
| that solar light is composed of three sets of 
! rays, the colorific, the calorific, and the de- 
oxidizing. 
The rays of caloric are refracted by trans- 
j parent bodies just as the rays of light. We 
see, too, that, like the rays of light, they differ 
in their retrangibility; that some of them are 
as refrangible as the violet rays, but that the 
j greater number of them are less refrangible 
than the red rays. Whether they are trans- 
mitted through all transparent bodies has not 
been ascertained ; neither has the difference 
of their refraction in different mediums been 
I examined. We are certain, however, that 
! they are transmitted and refracted bv all 
transparent bodies which have been employ- 
I ed as burning-glasses. Dr. Herschel has also 
proved, by experiment, that it is not only the 
Yol. II. 
HAYS. 
caloric emitted by the sun which is refran- f 
gible, but likewise the rays emitted by com- 
mon fires, by candles, by hot iron, and even 
by hot water. 
The rays of caloric are reflected by po- 
lished surfaces in the same manner as the 
rays of light. This was lately proved by 
Herschel ; but it had been demonstrated long 
before by Scheele, who had even ascertained 
that the angle of- their reflection is equal to 
the angle of their incidence. Mr. Pictet 
also had made a set of very ingenious expe- 
riments on this subject, about the year 1790, 
which led to the same conclusion. He placed 
two concave mirrors of tin, of nine inches 
focus, at the distance of twelve feet two inches 
from one another. In the focus of one of 
them he placed a ball of iron two inches in 
diameter, heated so as not to be visible in 
the dark ; in the other was placed the bulb 
of a thermometer. In six minutes the ther- 
mometer rose 22°. A lighted candle, which 
was substituted for the ball of iron, produced 
nearly the same effect. In this case botli 
light and heat appeared to act. In order to 
separate them, he interposed between the 
two mirrors a plate of clear glass. The ther- 
mometer sunk in nine minutes 14°; and when 
the glass was again removed, it rose in seven 
minutes about 12°; yet the light which fell on 
the thermometer did not seem at all dimi- 
nished by the glass. Mr. Pictet therefore 
concluded, that the caloric had been reflected 
by the mirror, and that it had been the cause 
of the rise of flic thermometer. In another 
experiment, a glass matrass was substituted 
for the iron ball, nearly of the same diameter 
with it, and containing 2044 grains of boiling 
water. Two minutes after a thick screen of 
silk, which had been interposed between the 
two mirrors, was removed, the thermometer 
rose from 47-' to 50-f, and descended again 
the moment the matrass was removed from 
the focus. 
'The mirrors of tin were now placed at the 
distance of 90 inches from each other; the 
matrass with the boiling water in one of the 
foci, and a very sensible air- thermometer in 
the other, every degree of which was equal 
to about A— of a degree of Fahrenheit. Ex- 
actly in the middle space between the two 
mirrors there was placed a very thin common 
glass mirror, suspended in such a manner 
that either side could b'e turned towards the 
matrass. When the polished side of this 
mirror was turned to the matrass, the thermo- 
meter rose only 0.5°; but when the side cover - 
ed with tin foil, and which had been blackened 
with ink and smoke, was turned towards the 
matrass, the thermometer rose to 3.5°. In 
another experiment, when the polished side 
of the mirror was turned to the matrass, the 
thermometer rose 3°, when the other side 
9.2°. On rubbing off the tin foil, and repeat- 
ing the experiment, the thermometer rose 
18°. On substituting for the glass mirror a 
piece of thin white pasteboard of the same 
dimensions with it, the thermometer rose 10°. 
As the rays of light and of caloric emitted 
by the sun accompany each other, it cannot 
be doubted that they move with the same 
velocity. The rays of caloric, therefore, 
move at the rate of almost 200,000 miles in a 
second. This is confirmed by an experiment 
of Mr. Pictet. He placed two concave mir- 
rors at the distance of 69 feet from each 
other; the one of tin as before, the other of 
4 A 
plaistcr gilt, and IS inches in diameter. Into 
the focus of this last mirror he put an- ■air- 
thermometer, and a hot bullet of iron into 
that of the other. A few inches from the 
face of the tin mirror there was placed a thick 
screen, which was removed as soon as the 
bullet reached the focus. The thermometer 1 
rose the instant the screen was removed, 
without any perceptible interval; conse- 
quently the time which caloric takes in mov - 
ing 69 feet is too minute to be measured. 
We see at once that this must be the case 
when w e recollect that caloric moves at the 
rate of 200,000 miles in a second. 
The velocity of caloric being equal to that 
of light, its particles must be equally minute. 
Therefore neither the addition of caloric nor 
its abstraction can sensibly affect the weight 
of bodies. As thij follows necessarily as a 
consequence from* Dr. Herschel’s experi- 
ments, was it possible to prove by experi- 
ment that caloric affects the weight of bodies, 
the theory founded on Dr. Herschel’s dis- 
coveries would be overturned : but sucli de- 
ductions have been drawn from the experi- 
ments of De Luc, Fordyce, Morveau, and 
Chaussic-r. According to these philosophers, 
bodies become absolutely lighter by being 
heated. The experiment of Fordyce, w hich 
seems to have been made with the greatest 
care, was conducted in the following manner: 
He took a glass globe three inches in dia- 
meter, with a short neck, and weighing 451 
grains ; poured into it 1700 grains of water 
trom the New ltiver, London, and then seal- 
ed it hermetically. . The whole weighed 
215011- grains at the temperature of 32°. It 
was put for twenty minutes into a freezing 
mixture, of snow and salt till some of it was 
frozen; it was then, after being wiped first 
with a dry linen cloth, next with clean 
washed dry leather, immediately weighed, 
and found to be A- of a grain heavier than 
before. This was repeated exactly in the 
same manner five different times ; at each, 
more of the water was frozen, and more weight 
gained. When the whole water was frozen, 
it was.j3g.ths of a grain heavier than it had 
been when fluid. A thermometer applied to 
the globe stood at 10°. When allowed to re- 
main till the thermometer rose to 32 1 , it 
weighed Ag-lhs of a grain more than it did at 
the same temperature when fluid. It will 
be seen afterwards, that ice contains less ca- 
loric than water of the same temperature 
with it. The balance used was nice enough 
to mark - t 6 th part of a grain. 
This subject had attracted the attention of 
Lavoisier, a philosopher distinguished by the 
uncommon accuracy of his researches. Hi* 
experiments, which were published in the 
Memoirs of the French Academy for 1783, 
led [dm to conclude that the weight of bodies 
is not altered by heating or cooling them, 
and consequently that caloric produces no 
sensible change on the weight of bodies. 
Count Rumford’s experiments on the same 
subject, which were made about the year 
1797, are perfectly decisive. He repeated 
the experiment of Dr. Fordyce with the most 
scrupulous caution ; and by a number of the 
most ingenious contrivances, demonstrated, 
that neither the addition nor the abstraction 
of caloric makes any sensible alteration in the 
weight of bodies. 
