COLD. 
excites the sensation of iieat : but when the 
same hand is placed in the water at tlie 
temperature of oO”, it is deprived of calo- 
ric, because the surrounding medium is far 
below its temperature, and thus the sensa- 
tion of cold is produced. But from the 
left, placed in the water at 30°, caloric is 
abstracted, which gives the sensation of 
cold, and the same hand placed in the wa- 
ter at 50", receives caloric, and this enter- 
ing the body, excites the sensation of heat. 
Thus the term cold is expressive of the re- 
lative temperature of two bodies. There 
have, however, been persons who would 
account for the phenomena of cold by the 
existence of frigorific particles, supposed 
to be floating in the air, and by mixing vlith 
liquid bodies convert them to solids, and 
there are facts which seem to support this 
doctrine. 
Nothing appears at first sight more di- 
rectly contradictory to the common opinion 
of cold being only relative, and only a nega- 
tive term implying the abstraction of heat, 
tlian the facts which shew the apparent 
radiation, absorption, and reflexion of cold ; 
the evidence of which stands on the same 
ground as the corresponding motions of 
heat, namely, on the rise or fall of the ther- 
mometer. If the rise of the liquor on the 
scale of a thermometer, whose bulb is placed 
in the focus of a mirror, be considered 
as a proof of the propulsion of certain ca- 
lorific rays from a distant heated surface, 
and their subsequent reflexion according to 
the laws of catoptrics ; the sinking of the 
same thermometer liquor under similar cir- 
cumstances of position, when the surfece 
which before was sensibly hotter than the 
atmosphere is now sensibly colder, would 
seem from a parity of reasoning to indicate 
the propulsion and reflexion of frigorific 
rays. Nor can we consider tiiis question as 
at all detennined, though an ingenious hy- 
pothesis has been advanced by, M. Prevost, 
which goes a considerable way to reconcile 
the apparent contradiction of the doctrine 
of the unity of heat and cold. 
It is singular- that the reflexion of cold 
should have been accidentally discovered, 
and decidedly announced about the year 
1667, by the members of the Florentine 
Academy del Cimento, without any further- 
prosecution of so curious a fact. The ex- 
per-iment is the following : a mass of ice of 
aborrt 50016. was set some distance before 
a concave glass mirror, and the bulb of a 
spirit thermometer put in the focus to ti-y 
whether, cold would bp reflected. Imme- 
diately the spirit of the thermometer- began 
to sink, and fell several degrees. To pr-ove 
tlrat this was not merely owing to the con- 
tiguity of the ice, the smface of the mirror 
was covered with a cloth to preverrt the 
reflexion, and tire thermometer again rose. 
No further inlerence is drawn from this ex- 
periment, and the author of it seemed even 
to doubt of the reality of the reflexion, and 
to be disposed to impute it to some other 
unknown cause. This experiment was re- 
peated in a nntch more accirrate way by M. 
Pictet. The apparatus which he used was 
the same as that before described, as em- 
ployed for- the reflection of heat ; that is, two 
tirr mirrors placed directly opposite each 
other- at some distance, in the focus of one of 
which was placed the bulb of a very sensi- 
ble thermometer, and irr the other, the ves- 
sel intended to produce the heat or cold. 
In this instance, this latter was a mattress 
full of snow : the rrrirrors were separated to 
the distarrce of lOi feet. At the instant 
the mattress was placed in one focus, the 
thermometer in the opposite focus began 
to sink, and descended several degrees. 
Whert stationary, nitrous acid was poured 
on the snow, which pt-oduced a cold of 
much greater irrteirsity, and the tlrenrrorne- 
ter in consequence immediately descended 
several degrees lower. When taken out 
of the focus, it again rose to tire common 
temperature. 
Mr. Leslie also found, not only the same 
effect in this experiment, but that the ac- 
tion of a cold-radiating surface upon the tin 
reflector produced exactly the same pro- 
portional effect upon the differential ther- 
mometer as the hot radiating surface, only 
in the opposite direction of the scale. The 
differential thennometer, which is always 
at zero when both bulbs are equally heated, 
is beautifully calculated to shew this strik- 
ing experiment. Thus, if the difference of 
temperature between the heat-radiating 
substance and the atmosphere be 60 de- 
gree's, and if this raises the thermometer 45 
degrees, the same difference between the 
cold-radiating substance and the atmo- 
sphere will sink the thermometer 45 de- 
grees, and so in proportion ; so that a cold 
of 16 degrees will sink the thermometer 12 
degrees ; for 60 ; 45 16 : 12. 
Great degrees of cold are produced by 
mixing together those substances which dis- 
solve rapidly. The reason of this will' ap- 
pear by recollecting what has been said of 
the absorption of caloric when a solid 
body is converted into a fluid. Mixtures 
