side, and put it in the place of the transparent plate, taking care to turn its blackened surface to the lamp. The needle remains stationary, although the caloric rays continually fall on the anterior surface. It will be found immoveable also, if we employ a plate of copper coated on both sides with black colouring matter, or a thin flake of wood, or even a sheet of paper. Thus, though we should suppose the screen to be diaphanous, exceedingly thin, an excellent conductor of caloric, and possessing great powers of absorption and emission, the utmost elevation of temperature that can be acquired during the experiment would not furnish rays sufficiently strong to move the index of the galvanometer.
One is surprised at first to see caloric rays capable of giving a deviation of 30° fail to produce any effect when they are absorbed by the screen, which must necessarily send its acquired heat upon the apparatus. But our surprise ceases when we reflect that this heat is sent equally in all directions by every point of the heated screen, and therefore that the portion of total radiation which reaches the apparatus is but a very small fraction.
We shall see hereafter, that the anterior surface of the pile does not measure six square centimetres. With these data, if we suppose even that the thirty degrees of heat are completely absorbed by the screen, and afterwards dispersed through space, we find that the quantity of the rays which reach the thermoscopic body does not amount to the six-hundredth part of the whole. But the galvanometer that I use is capable, at the most, of marking only the 150th part of the force which moves the needle to 30°. Thus, even though the instrument were capable of discovering the presence of a heat four times as feeble, there would be no perceptible action.
The experiments which I have been describing seem to me to leave no doubt whatsoever as to the truth of the proposition just now enunciated; namely, that in my mode of operating the deviation of the galvanometer proceeds entirely from the heat instantaneously transmitted through the screen. These proofs, though so conclusive to my mind, seem however not to have been equally convincing to others; for I have heard some persons say, "We grant that the deviation of 21° obtained through the screen does not arise from the caloric propagated by conduction from the anterior to the other surface, but it may be maintained that it is caused by a heat instantaneously diffused, in the same manner as light, over all the points of the glass." Before we admit such a mode of transmission, it seems to me that we ought to demonstrate its existence by some decisive experiment. But supposing it true, then we must also suppose one of these two things,—either that the molecules of the glass acquire from the action of the source such modifications that they themselves become so many calorific centres, and return to their
