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 superficial; 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 mentioned 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 absorptive force, the activity of which is greater in proportion as the temperature 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 different 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 difference between one transmission and another must decrease with the thickness 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 immediately transmitted from each of the four sources through plates of different kinds reduced to the common thickness of 2mm·6. The transmissions are expressed in hundredth parts of the incident quantity. They are uniformly measured, like the preceding, under the action of a radiation of the same force derived from each source of heat.
