OF RADIANT HEAT THROUGH DIFFERENT BODIES. 41 



ing the heat acquired by the screens ; in the second case also it remains 

 unmoved, although in this case the plates (blackened or opake) are sub- 

 mitted to the actual radiation of the source itself. In the third case the 

 index of the galvanometer leaves its position of equilibrium and describes 

 arcs of greater or less extent according to the quality and thickness of 

 the screen. But the time which it takes to reach the extremity of 

 these arcs is invariable, and equal to that which it takes to describe a 

 deviation of 30° when there is no screen interposed. 



This third proof, though indirect, is nevertheless the most convincing, 

 and possesses the additional advantage of showing, as it were, palpably, 

 that the manner in which radiant heat is transmitted in the interior of 

 diathermanous substances is altogether analogous to that. in which light 

 is propagated through transparent media whether solid or fluid. For in 

 respect to the latter we perceive no appretiable difference between the 

 times which the luminous rays take to pass through layers of any quality 

 and thickness whatsoever. 



The analogy between the transmission of light and that of radiant heat 

 is rendered still more striking if, by shaking or otherwise, a motion is 

 produced in the mass of the screen submitted to the experiment. I have 

 passed the different parts of a large square of glass rapidly before the 

 narrow aperture of the metallic plate through which the calorific rays 

 that strike the surface of the pile are transmitted. By means of a bow 

 I made it vibrate ; it emitted sounds more or less acute : the index of 

 the galvanometer pointed invariably to the same degree of its scale. I 

 found the deviation of the magnetic needle equally invariable when I 

 measured the intensity of the calorific radiation through a layer of 

 acidulated water, at first still, but afterwards set in motion by agitators 

 or traversed by a strong electric current. 



Here then, though under different forms, the fact observed in the 

 experiments of Pictet and Saussure when we agitate the mass of air in- 

 terposed betwen the reflectors is reproduced; namely, the impossibility 

 of altering by these means the direction or the intensity of the luminous 

 or the calorific rays passing through atmospheric air or any other dia- 

 phanous medium. 



These different considerations seem to me well calculated to dispel every 

 shade of doubt that may yet be entertained as to the immediate trans- 

 hiission of radiant heat by diathermanous bodies, whether solid or liquid. 

 But (to return to the four sources) we have already observed that in 

 our method of proceeding it is necessary to operate uniformly under 

 the influence of a radiation equal to 30° of my thermomultiplier. Now 

 to effect this with sources of various temperatures they must be brought 

 more or less close to the thermoelectric pile until we have obtained the 

 galvanoinetric indication required, and such is the way in which we have 

 proceeded in all our experiments of transmission. The same screen 



