382 On the Theory of the [Nov. 
the rays passing from the body be transmitted (that is to say, ema- 
nated from the interior of the body,) or reflected. If the reflector 
is perfect, the whole current is composed of reflected rays; if it is 
imperfect, it is composed of reflected and transmitted rays. 
The most convenient way of representing to oneself an imper- 
fect reflector is to conceive its surface decomposed into two parts, 
one of which is a perfect reflector, while the other does not reflect 
at all. 
We must here apply the laws of the reflection of light. | In par- 
ticular we must observe that the surface reflects inwards as well as 
outwards. . 
3. Every calorific ray which a body sends by emission or by re~ 
flection, only replaces another ray, which would take the same 
direction if the body were withdrawn.* This is a necessary result 
of the constitution of caloric; for whatever be the direction-of the 
rays emitted or reflected, there is one which follows the same route 
and which the body intercepts. iy 
4. It follows from this, 1. That in a place of uniform tempera- 
ture, a reflector of whatever form does not affect a thermometer 
subjected to its influence. 2. That if it reflect rays emanated from 
a body more or less hot than the place, it will raise or depress re- 
spectively the thermometer subjected to its influence. 
ILI. The application of these principles to the objections detailed 
offers no difficulty. Let us take for an example the first two objec- 
tions stated in the New Edinburgh Encyclopedia. =~ 
1. A hot body, it is said, ought to cool slower beforea large cold 
body than before a small. 
The objector forgets that each of the rays which the cold body 
sends merely replaces the ray which the cold body intercepts. - The 
intercepted ray being hotter than that which comes in its place, it is 
easy to see that the more of these substitutions take place (or, in 
other words, the larger the cold body is) the greater will the cooling 
effect be. 
2, Two bodies, the one with a metallic, the other with a black- 
ened, surface, are presented to a thermometer. It is alleged that 
the blackened body ought to cool the thermometer least, because it 
radiates most. : 
Here the objector has not thought of the portion of radiant heat 
which these bodies give out by reflection. This portion is not 
changed by the change of temperature of the body. It subsists 
quite entire. ‘The portion emitted only is diminished. Therefore. 
by the same diminution of temperature, that one of the bodies 
which emits the most (the blackened surface) ought to radiate least ; 
that one, on the contrary, which is the best reflector (the metallic 
surface) ought to radiate most, which is conformable to experience. 
* It isto be understood that we speak of a hot place, that is to say, where 
caloric radiates. If the intercepting body is of the same temperature with the 
place, the ray which it replaces is equal to itself, If not, this ray or row of 
particles, is more or less abundant in caloric, : 
