OF RADIANT HEAT THROUGH DIFFERENT BODIES. 3 
The method consists in observing the thermometer as in the preceding 
eases; that is, when the caloric rays fall upon it after having passed 
through the plate of glass. We thus obtain a complex measure of the ef- 
fects produced by immediate transmission and by that conducting power 
of the layers to which we have given the name of successive propagation. 
If we know the value of either of these, we have that of the other. Now 
it is easy to determine the influence of the conducting power by repeating 
the experiment after having blackened with Indian ink that surface of the 
plate which is turned towards the calorific source. In this case, theimme- 
diate radiation being intercepted, it is clear that the elevation of the tem- 
perature at the other side must be attributed only to the conducting power 
of the layers. Should the elevation be now found less than it was at first, 
it will be a decisive proof of immediate transmission. And such was 
the fact in almost all the experiments of Delaroche; I say almost all, be- 
cause it was found that the quantity of heat freely transmitted varied 
with the temperatures of the source. For temperatures lower than that of 
boiling water it was nothing, and when an Argand lamp* was employed, 
it was found to be more than half of the whole quantity. 
No doubt can be raised as to the truth of this beautiful discovery of 
Delaroche ; and yet the method which he has employed to measure the 
quantities of heat freely transmitted is by no means exact, especially in 
respect to high temperatures. In order to understand this seeming para- 
dox two things are to be observed; Ist, the difference produced by change 
of surface between the two quantities of heat which penetrate the glass 
by reason of its conducting power; 2nd, the difference produced be- 
tween those two quantities by the total or partial interception of the 
calorific rays. 
It is fully proved by the experiments of Leslie and others, that glass, 
when blackened with Indian ink, absorbs all the rays of heat, though, in 
its natural state, it reflects a certain number of them. The quantity of 
heat which penetrates the screen will therefore be greater in the former 
than in the latter case. However, as polished glass reflects but a very 
small portion of caloric rays, the error arising from a difference in the 
state of the surface will be reduced to a very inconsiderable quantity and 
may be safely disregarded. But the case is different when we examine 
the error produced by the total or partial interception of the caloric ra- 
diation. In some of the experiments of Delaroche one half, at least, of 
the incident rays immediately passed through the screen. Thus it was 
evident that it was the other only which was stopped at the first surface 
of the glass. The effect of conduction must therefore be limited to this 
latter half. Butas the screen, when blackened, stops the whole radiation, 
hel Journal de Physique, §c., par Delametherie, 1812,—Delaroche, Observa- 
tions sur le Calorique rayonnant. 
BQ 
2 
