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 extremity 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 will 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 the 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 always 
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 aruler 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. 
