OF RADIANT HEAT THROUGH DIFFERENT BODIES. 7 



suits obtained by means of the therniomultiplier*, 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 placetl 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, mai'ks 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 inunediately cause the galvano- 

 meter to move. In 5' or 6^ it is driven through an arc of nearly 21°'5, 

 but it afterwards returns nearly to zero, oscillates in an arc 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 halff. When the experiment is repeated 



* For the description of this instrument see the number of tlie Annates de 

 Chimie for October 1 83 1 . 



t 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 t'!;e calorific communications are established, and we have 

 jnst seen that in five or six seconds it describes almost the whole arc of devia- 

 tion. If a 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 

 tlie 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 tl\e thennonuiltiplicr. Tiiis delay is the consequence of the obstructions eu- 

 counteicd by the conductiblc heat in its passage through the glass, and in its 



