CALORIC. 
traced beyond the violet ray. He had 
found, however, tliat all the coloured rays 
of the spectrum have the power of heating : 
it may be questioned, therefore, whether 
there be any rays which do not warm. The 
coloured rays must either have the property 
of exciting heat as rays of hght, or they must 
derive that property from a mixture of rays 
of caloric. If the first of these suppositions 
were true, light ought to excite heat in all 
cases ; but it has been long known to philo- 
sophers, that the light of the 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 tliese produce a very 
sensible heat. Here then are rays of light 
which do not produce heat : rays, too, com- 
posed of all 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 sun and combustible bodies, 
since they excite 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. Hooke, and afterwards by 
Scheele, who separated the two species 
from each other by a very simple method. 
If a glass mirror be held before a fire, it re- 
flects the rays of light, but not the rays of 
caloric; a metallic mirror, on the other 
hand, reflects both. The glass mirror be- 
comes hot; the metallic mirror does not 
alter its temperature. If a plate of glass 
be suddenly interposed between a glowing 
fire and the face, it intercepts completely 
the warming power of the fire, without 
causing any sensible diminution of its bril- 
liancy ; consequently it intercepts the rays 
of caloric, but allows the rays of light to 
pass. If thb glass be allowed to remain in 
its station till its temperature has reached 
its maximum, in that situation it ceases to 
intercept the rays of caloric, but allows them 
to pass as freely as the rays of light. This 
curious fact, which shews us that glass only 
intercepts the rays^)f caloric till it be satu- 
rated with them, was discovered by Dr. 
Robison. These facts are sufficient to 
convince us, that the rays of light and of 
caloric are different, and that the coloured 
rays derive their heating power from the 
rays of caloric which they contain. Thus it 
appears that solar light is composed of three 
sets of rays, the colorific, the calorific, and 
the deoxidizing. The rays of caloric are 
refracted by transparent bodies just as the 
rays of light. We see too, that, like the 
rays of light, they differ in their refrangi- 
bility ; that some of them are as refrangible 
as the violet rays; but that the greater 
number of them are less refrangible than 
the red rays. Whether they are transmitted 
through all transparent bodies has not been 
ascertained ; neither has the difference of 
their refraction in different mediums been 
examined. We are certain, however, that 
they are transmitted and refracted by all 
transparent bodies which have been em- 
ployed as burning-glasses. Dr. Herschel 
has also proved, by experiment, that it is 
not only the caloric emitted by the sun 
which is refrangible ; but likewise the rays 
emitted by common fires, by candles, by 
hot iron, and even by hot water. The rays 
of caloric are reflected by polished 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. M. Pictet also had 
made a .set of very ingenious experiments 
on this subject, about the year 1790, which 
led to the same conclusion. 
All the phenomena concur to shew, that 
the rays of caloric move with a veiy con- 
siderable velocity, though the rate has not 
been ascertained in a satisfactoiy manner. 
Some experiments of Mr. Leslie would lead 
us to conclude, that they move with the 
same velocity as sound. The following ex- 
periment ofM. Pictet indicates a very con- 
siderable velocity. He placed two concave 
mirrors at the distance of 69 feet from eacrh 
other ; the one of tin,, the other of plaster 
gilt, and 18 inches in diameter. Into the 
focus of this last mirror he put an air ther- 
mometer, and a hot bullet of iron into that 
of the other, A few inches from the face of 
the tin mirror there was plated a thick 
screen, which was removed as soon as the 
bullet reached the focus. The thermometer 
rose the instant the screen was removed 
without any perceptible interval, conse- 
quently the time wliich caloric takes in 
moving 69 feet is too minute to be measured. 
The velocity of caloric, if it is equal to that 
of light, would prove that its particles must 
be equally minute. Therefore, neither the 
addition of caloric nor its abstraction can 
sensibly affect the weight of bodies. 
Caloric agrees with light in another pro- 
perty no less peculiar : its particles ai-e 
never found cohering together in masses; 
and whenever they are forcibly accumu- 
lated, they fly off in all directions, and se- 
