OF RADIANT HEAT THROUGH DIFFERENT BODIES. 'J 



The method consists in observing the thermometer as in the preceding 

 cases; tliat 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, the imme- 

 diate radiation being intercepted, it is clear that the elevation of the tem- 

 perature at tlie other side must be attributed only to the conducting power 

 of the layei-s. Should the elevation be now found less than it v,as 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 ^^•ater 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 ; 1 st, 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 intei-ception 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 tlie 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. Tlie effect of conduction must therefore be limited to this 

 latter half. But as the screen, when blackened, stops the whole radiation, 



* Jonrnnl fie Phijsiquc, ^-c, par Delametherie, 1812. — Delaroche, Oisc/Ta- 

 iioiis siir le Calorique rayontianl . 



J$ 2 



