OF RADIANT HEAT THROUGH DIFFERENT BODIES. * 
sults obtained by means of the thermomultiplier*, and a source whose 
radiation is much weakened by distance. 
The apparatus is disposed in the following manner. A thermoelectric 
pile of thirty pairs is closed at one end and enveloped, at the other, in 
a small tube blackened inside to prevent reflection. At a certain distance 
there is placed a large metallic diaphragm, with an aperture at the 
centre equal to the section of the pile. On the other side, in the same 
line, there is a lighted lamp, which is brought more or less close, until 
the needle which serves as the index of the galvanometer, marks an 
elevation of 30°. The radiation is afterwards intercepted by a screen 
of polished metal placed between the lamp and the diaphragm, and the 
needle returns to zero. Then there is placed on the other side of the 
diaphragm a stand, with a ‘plate of glass fixed on it, and the whole ap~ 
paratus is moved gently until it is brought midway between the pile and 
the calorific source. 
This being done, the opake screen is removed; the rays passing 
through the glass fall on the pile, and immediately cause the galvano- 
meter to move. In 5° or 6§ it is driven through an are of nearly 215, 
but it afterwards returns nearly to zero, oscillates in an are of greater 
or less extent, and at last settles definitively at 21°. This last deviation 
decidedly marks the whole effect; for it is useless to continue the 
experiment for 15° or 20°. There is no longer any perceptible move- 
ment. 
The time which the needle takes to attain its position of steady equi- 
librium is a minute and a half+. When the experiment is repeated 
* For the description of this instrument see the number of the Annales de 
Chimie for October 1831. 
+ Although the velocity with which radiant heat is propagated is unknown, 
we are nevertheless pretty certain, since the experiments of Saussure and Pictet, 
that this agent traverses spaces of from fifty to sixty feet in a time altogether 
inappretiable. It might be asked, therefore, why does not our apparatus in- 
stantaneously indicate the presence and the intensity of the rays emitted by the 
source? To this I answer, 1st, that the index of the galvanometer deviates at 
the very instant when tixe calorific communications are established, and we have 
just seen that in five or six seconds it describes almost the whole are of devia- 
tion. Ifa few seconds more are required to mark the entire action steadily, it 
is because the great conducting power of the bismuth and the antimony, and 
the great powers of absorption and emission belonging to their blackened sur- 
faces, render the lapse of a certain interval necessary, in order that a balance 
may take place between the rays which enter the pile and those which leave it 
or are extinguished within its interior. But the time required for the definitive 
equilibrium is much greater when common thermometers are used. If, for 
instance, one of Rumford’s most delicate thermoscopes, having the ball black- 
ened, and a metallic cover perforated on the side towards the source of heat, be 
submitted to the action of calorific radiation, it will be found that the time re- 
quisite to mark the whole effect is four or five times more than that required 
by the thermomultiplier. This delay is the consequence of the obstructions en- 
countered by the conductible heat in its passage through the glass, and in its 
