OF RADIANT HEAT THROUGH DIFFERENT BODIES. 55 



longer equally successful when for the salt we substitute alum or any 

 other diaphanous substance. But as recourse might be had to sup- 

 posed differences between the conducting, the absorptive, or the emis- 

 sive powers of these bodies, it seems advisable first to prove the refrac- 

 tion of the nonluminous rays without using lenses. 



With this view I place, at a certain distance from the thermoelectric 

 pile and out of the direction of its axis, a plate of copper heated to 390° 

 by an alcoholic lamp, or, what is still better, a vessel filled with water 

 in a state of ebullition. The pile being lodged at the bottom of a me- 

 tallic tube blackened inside, the rays of nonluminous heat emitted from 

 the vessel in a direction oblique to the axis cannot reach the thermosco- 

 pic body, and the index of the galvanometer remains perfectly at rest. 

 Matters being now in this state, I take a prism of rock salt and fix it at 

 the mouth of the tube with its axis placed vertically and its refractive 

 angle turned towards the angle formed by a line drawn from the source 

 to the extremitj' of the tube. ( See Plate I. fig. 2.) A considerable de- 

 viation is immediately perceived in the galvanometer. The rays of heat 

 are therefore conveyed into the tube by the action of the prism. 



To show that the effect is really due to the refraction and not to the 

 heat of the salt it will be sufficient to turn the angle of refraction in a 

 contrary direction ; for as soon as this is done the needle falls again to 

 zero, notwithstanding the presence of the prism. The experiment is no 

 less successful with the heat of the lamp, or that of the incandescent 

 platina. Calorific rays of every kind are therefore, like luminous rays, 

 susceptible of refraction. 



But on the principle of analogy, as each species of light, so Avill each 

 species of heat possess a different refrangibility. Hence it is evident that 

 if the prism be left in its position and the radiant source changed it 

 would become necessary at tlie same time to change the angle formed 

 by the axis of the pile with the direction of the rays, in order to obtain 

 the desired effect on the galvanometer. If however we attempt to ve- 

 rify this conjecture we obtain no decisive result. This is easily con- 

 ceived when we reflect that the aperture of the tube has a certain dia- 

 meter and that it is placed quite close to the prism, so that the rays 

 refracted at angles differing but very little from each other can ahvays 

 reach the pile though no change should be made in the inclination of 

 the axis of the tube. 



But there is another process by means of which, if we cannot exactly 

 measure the refrangibility of each, species of calorific rays, we prove at 

 least that the angle of refraction varies with the measure of the radiating 

 source. I took a graduated circle ABC (Plate I. fig. 3.) 22 inches 

 in diameter carrying a ruler C D as a moveable radius. At the extremity 

 of this ruler I placed a thermoelectric pile M composed of fifteen pairs 

 disposed in one line perpendicular to the plane of the circle. 



