44 M. MELLONI ON THE IMMEDIATE TRANSMISSION 



of the same flake arose from an internal or external action of the screen. 

 Nay, more ; the ordinary properties of the caloric seemed to lead to the 

 far more probable consequence that the interception was entirely super- 

 ficial ; or in other words, that as the same plate of glass successively 

 exposed to the radiations from several sources gave different calorific 

 transmissions, it was natural to suppose that the heat was first stopped 

 at the external surface in a proportion varying with the temperature of 

 the source, and subsequently propagated inwards according to the known 

 laws of conductibility. But the experiments which I have just men- 

 tioned seem to me to demonstrate clearly that the calorific rays from 

 different sources are more or less quickly extinguished in the very interior 

 of the mass. 



Thus the molecules of glass act upon radiant heat with a real absorp- 

 tive force, the activity of which is greater in proportion as the tempe- 

 rature of the source is lower. It will perhaps be now asked whether this 

 kind of action be common to all diaphanous substances or peculiar to 

 glass only. 



To determine this, it is not necessary to repeat on all the bodies those 

 experiments which we have made on diiferent thicknesses of glass ; 

 for, the law of Delaroche being once established, it will follow that the 

 substance of which the flake is composed operates on the rays of heat 

 with an absorptive force inversely as the temperature of the source : and 

 as this force acts from all points of the mass, it is clear that the differ- 

 ence between one transmission and another must decrease with the thick- 

 ness of the screen. The question is therefore reduced to this ; whether 

 all bodies more or less transparent act upon heat radiating from different 

 sources in a manner analogous to that which we have observed in one 

 only of our flakes of glass. 



I have registered in the following table the quantities of heat imme- 

 diately transmitted from each of the four sources through plates of dif- 

 ferent kinds reduced to the common thickness of 2"""*6. The transmis- 

 sions are expressed in hundredth parts of the incident quantity. They 

 are uniformly measured, like the preceding, under the action of a radia- 

 tion of the same force derived from each source of heat. 



