OF RADIANT HEAT THROUGH DIFFERENT BODIES. 9 



side, and put it in the jDlace of the transparent plate, taking care to turn 

 its blackened surface to the lamp. The needle remains stationary, al- 

 though 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 tlake 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 absoi-ption and emission, the vitmost eleva- 

 tion of temperature that can be acquired during the experiment would 

 not furnish rays sufficiently strong to move the index of the galvano- 

 meter. 



, One is surprised at first to see caloric rays capable of giving a de- 

 viation of 30° fail to produce any eftect when they are absorbed by the 

 screen, which must necessarily send its acquired heat upon the appa- 

 ratus. But our surprise ceases when we reflect that this heat is sent 

 equally in all directions by every point of the heated screen, and there- 

 fore tliat the portion of total radiation which reaches the apparatus is 

 but a veiy 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 

 cajjable, 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 enun- 

 ciated ; namely, that in my mode of operating the deviation of the gal- 

 vanometer 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 liave 

 heard some persons say, " We grant that the deviation of 21° obtained 

 through the screen does not arise from the caloric propagated by con- 

 duction from the anterior to the other surface, but it may be main- 

 tained 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 tiic source such modifications tliat 

 they themselves become so many calorific centres, and return to their 



