Conclusion.
I had intended to introduce here some general reflections on the different hypotheses which have been proposed to explain the phænomena of heat, and on the question of the identity of radiant heat and light. But as these two agents are nowhere more intimately united than in the rays of the sun, such considerations should be preceded by a tolerably complete statement of the numerical results obtained by the application of our several processes to solar heat. The experiments however which I have hitherto been able to make with this view are too deficient in number and variety to justify my attempting any statement of the kind. I will therefore not enter, for the present, into any dissertation on the nature of heat, but will conclude with a recapitulation of the principal consequences to which I have been led by my inquiries into the properties of the radiant heat emitted by terrestrial sources, in order that being thus comprehended at a single glance they may be more easily compared with the analogous properties of light.
Radiant heat passes instantaneously, and in greater or less quantities, through a certain class of bodies, solid as well as liquid. This class does not consist exactly of diaphanous substances, since opake plates or plates possessing but a feeble transparency are more diathermanous or permeable to radiant heat than other plates possessing perfect transparency.
There are different species of calorific rays. They are all emitted simultaneously and in different proportions by flame, but in the heat from other sources some of them are always wanting.
Rock salt reduced to a plate and successively exposed to radiations of the same force from different sources always transmits immediately the same quantity of heat. A plate of any other diathermanous substance will, under the same circumstances, transmit quantities less considerable in proportion as the temperature of the source is less elevated: but the differences between one transmission and another decrease as the plate on which we operate is more attenuated. Whence it follows that the calorific rays from different sources are intercepted in a greater or less quantity, not at the surface and in virtue of an absorbent power varying with the temperature of the source, but in the very interior of the plate and in virtue of an absorbent force similar to that which extinguishes certain species of light in a coloured medium.
The same conclusion is attained by considering the losses which the calorific radiation from a source at a high temperature undergoes in passing through the successive elements which constitute a thick plate of any other diathermanous substance than rock salt. For if we imagine the plate divided info several equal layers, and determine by experiment what ratio the quantity lost bears to the quantity incident upon each of the layers, we find that the loss thus calculated decreases rapidly
