Professor Powell on the Influence of Colour on Heat. 335 



through the plain glass must have been, in part at least, a direct 

 continuance of radiation through the glass. 



Finding in the subsequent luminous cases that this greater 

 effect through the plain glass increases in relation to that through 

 the coated glass, M. de La Roche infers that this is only an ex- 

 tension of the same phenomenon as in the non-luminous cases, and 

 thence adopts the general conclusion of increasing transmission. 

 On this point I would first remark, that, to whatever cause 

 the former phenomenon be owing, and if it be a direct transmis- 

 sion of simple radiant heat, it must not be confounded with the 

 subsequent phenomena, and the explanation of it must not be 

 extended to the latter effects. 



The experiments given in ray paper in the Philosophical 

 Transactions 1825, are, I think, sufficient to prove that, in the 

 instances of luminous bodies, if the same partial transmission 

 still continue, yet the experimental conclusion above established 

 must still be admitted with respect to the principal heating ef- 

 fects, and there still remains the same distinction between the 

 two heatinoj agents to be maintained. 



It is by no means improbable, or incompatible with any thing 

 I have advanced, that there may be a small direct transmission 

 of simple heat through glass at high temperatures ; or, on the 

 other hand, it may with equal probability be conceived, that 

 even from hot bodies which are non-luminous to our organs of 

 sight, a certain degree of light may emanate, and being trans- 

 missible through glass, may exert its heating influence on a 

 black surface like other light. 



In the next place, I would observe, that the supposition it- 

 self, that the effect through the blackened glass ought ^06^ equal 

 to, or greater than, that through a plain, does not appear to me 

 by any means a necessary one. It is indeed from particular in- 

 stances, in which, from the nature of the surfaces employed, the 

 absorption and subsequent radiation were greater than when the 

 screen presented a plain surface, that Sir J. Leslie has mainly 

 established the doctrine of the effect of screens ; but, without at 

 all interfering with that doctrine, it is very possible to conceive 

 a screen so constituted as to be incapable of radiating heat on 

 one side in the same degree that it acquires it on the other ; it 

 is possible that some peculiarities of surface may produce greater 



