10 M. MELLON! ON THE FREE TUANSMISSIOX 



natural state when the radiation is stopped ; or that tlie heat, which is 

 supposed to be diffused through the material points of the screen, is but 

 common caloric obej'ing the iviiown laws of equilibrium. In the first case 

 we sliould be only attempting to explain the very cause of the transmis- 

 sion, and the hypothesis, true or false, does not at all invalidate the fact 

 which we are desirous to establish. In the second case, this heat, when it 

 has reached the interior of the body, must take some time to issue from 

 it ; besides, this time must vary with the thickness of the screen, and its 

 powers of conduction and emission. But let us intercept the calorific 

 communication in our apparatus ; let us remove the diaphanous screen 

 from its stand, and expo-se it for some moments to the free radiation of 

 the lamp on the other side of the diaphragm : if the supposition be true, 

 the internal molecules of the glass will instantaneously acquire some 

 heat. In order to see whether this heat really exists, let us replace the 

 screen on its stand before the pile, still leaving the calorific communi- 

 cation with the lamp intercepted. The further surface of the plate of 

 glass will, according to the hypothesis, immediately begin to emit to- 

 wards the pile that caloric which reaches it successively from within, 

 and the index of the galvanometer must lose its equilibrium. But what- 

 ever be the nature or the thickness of the screen with which this expe- 

 riment is performed, we never obtain tlie slightest indication of a move- 

 ment in the magnetic needle. It is therefore completely demonstrated that 

 the deviations of the galvanometer exhibited in the experiments made 

 with the diaphanous screens are not to be attributed, in the least degree, 

 either to the external or the internal heat of the screen itself, but solely 

 and exclusively to free transmission. Thus, whenever, in consequence 

 of the radiant heat of the source being made to fall on a screen, a 

 deviation of the galvanometer is perceptible, we may rest assured that 

 the whole of the eiFect produced is to be ascribed to the rays of heat 

 immediately transmitted through it, in the same manner as luminous 

 rays. 



Before I conclude these preliminary considerations, it is necessary to 

 remark, 1st, that galvanometers of very great sensibility, such as must 

 be used for the thermomultiplier, do not directly indicate quantities 

 less than half-degrees ; 2ndly, that the ratios of the degrees of the gal- 

 vanometer and the forces of deviation are unknown. But it is often 

 useful to have the fractions below the half-degree, and in certain cir- 

 cumstances it is absolutely indispensable to know the ratios of the seve- 

 ral degrees of calorific action which move the magnetic needles to dif- 

 ferent distances from their primitive position. 



To find the fractions sought, we have only to take the means of a 

 certain number of observations. As to the ratio of the deviations and 

 the forces, it is difficult and, in the present state of the science, perhaps 

 impossible to determine it generally. But electric piles, such as those 



