1864. | Srawarr on Radiant Light and Heat, 595 
Towards the latter part of last century Professor Pictet, of Geneva, 
performed a curious experiment which appeared to prove the reflexion 
of cold. He placed two polished metallic concave reflectors facing 
one another, as in Figure 5, and while in the focus of the one he 
put a thermometer, in that of the other he placed a lump of ice, the 
effect of which was to lower the temperature of the thermometer. 
At first sight this result would seem to indicate that cold is some- 
thing more than a mere negation, and indeed that it is an influence 
susceptible of radiation and reflexion in the same manner as heat. It 
soon, however, occurred to Professor Pierre Prevost, of Geneva, that 
this was by no means a necessary conclusion, and this sagacious 
reasoner proposed, as an explanation of the experiment, the theory of 
Exchanges, or, as he termed it, a movable equilibrium. In order to 
comprehend this definition, let us suppose that we have a room walled 
in on every side, and that its walls, including the floor and ceiling, 
have the temperature of 60° Fahrenheit. Now in whatever part of 
such a room a thermometer is placed, it will ultimately attain the 
temperature of the walls ; that is to say, it will indicate 60° Fahrenheit, 
after which the mercury will neither rise nor fall, and there will be 
an equilibrium of temperature. When this has happened one of two 
things must be taking place. Hither the thermometer does not give 
out radiant heat, or if it does it receives back continually just as much 
as it gives out. Prevost supposes the latter to be the case, and that 
all bodies even of the same temperature are continually giving out 
heat to one another, and receiving in return just as much as they give 
out. Therefore, according to his theory, a body will fall in temperature 
when it radiates or gives out more heat than it absorbs, its temperature 
will be constant when the radiation and absorption are equal, and will 
rise when the absorption is greater than the radiation. 
Let us now see how this idea will explain Pictet’s experiments. 
By this theory, although the bulb p is of the same temperature as the 
reflector uF, yet there is a constant interchange of heat between them. 
Hence rays of heat will leave the bulb in the direction p£ and pF, 
and these will finally be reflected upon a. The body a is however 
giving out rays in return, which finally fall upon p. But since a is 
colder than p, the rays which leave p and fall upon A are more intense 
than those which leave a and fall upon p. The consequence of this 
will be that on the whole there will be a transference of heat from D 
to a, and the temperature of the thermometer will fall. And this 
transference from p to a will be just as much intensified by the 
reflectors as that from a to p would have been had a been hotter 
instead of colder than p. . 
Thus we see how easily the observation of Pictet is accounted for 
by the principle of exchanges—a principle which soon exhibited all 
the marks of a true theory by explaining facts as they were elicited by 
experiment, and at the same time by suggesting new truths, in which 
latter aspect more especially it has been of very signal service to 
science. But it is not our intention to give the history of this problem ; 
let us rather remark its more interesting and important applications. 
Let us for this purpose imagine a red-hot chamber having the tem- 
