OF RADIANT HEAT THROUGH DIFFERENT BODIES. 9 
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, 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 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 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 effect when they are absorbed by the 
sereen, 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 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 dees 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 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 have 
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 the source such modifications that 
they themselves become so many calorific centres, and return to their 
