38 M. MELLONI ON THE TRANSMISSION OF RADIANT HEAT. 
If we suppose the action of sulphuric acid analogous;to that of water; 
but not so energetic, we shall see the reason why, with the prism of acid, 
the maximum takes place in the.orange. . In. short,’ the very glass.of 
which the common prisms are made.must, operate in, a) similar manner, 
and cause in each ray a loss inversely.proportioned:to its refrangibility. 
Therefore, if we employed in the. construction of the common prism a 
substance less active than common. glass, the losses sustained by the 
less refrangible rays.would be diminished in a greater ratio ; so that they 
would gain on the more refrangible rays, and the maximum would pass 
in a direction opposite to the preceding, that is, frem the violet to the 
red. 
This is exactly the result obtained by Herschel, Englefield, and See- 
beck by operating on prisms of flint glass; for the maximum was trans- 
ferred to the obscure space quite close,to the,last red stripe of the 
spectrum. 
Let us compare these, effects with the numbers which. represent the 
ealorific transmissions. We shall find that.the maximum of heat, in 
passing from the yellow, where it is found when we use a prism of water, 
departs from it always in the.same direction in proportion as the sub- 
stances of the prisms substituted for the water.are more diathermanous. 
It passes a little out of the spectrum when, instead of, crown, we em- 
ploy flint glass. . Admitting then the.correetness of such a-theory, the 
line of greatest heat must pass quite, beyond the colours mto a,space 
far distant from the red limit if we employ rock salt, a substance. pos- 
sessing a far greater diathermancy as compared with, flint glass than 
flint glass does as compared with crown. I tried the experiment; it was 
completely successful. I found that the maximum of temperature in the 
spectrum derived from the prism of. salt was thrown into the dark space 
as far at least from the last band asthe blue is (in an opposite direction) 
from the red.. At the moment. I cannot assign more exact measures ; 
for in the first place I operated with very small prisms, and when I sub; 
sequently obtained larger pieces the; season..did not.allow me to re- 
consider and study the.result more. nicely... But the effeet-has been so 
marked in the experiment which I: made, and. so invariable in several 
suecessive repetitions, that I look upon it as decisive, and; have not the 
least doubt as to the removal of the maximum of temperature.to.the 
last band of the red rays in the spectrum produced with rock salt *. 
The distribution of the degrees of temperature in the solar spectrum 
* I have since obtained the same results with five prisms of rock salt whose 
angles of refraction vary between 30° and 70°. These prisms have been made 
out of several pieces taken from the mines of Cordona, Wieliecza, and Vicq: 
they have been cut in different directions relatively to the axis of crystallization. 
I shall give the numerical data in a work in which it is intended to treat spe- 
cially of the analysis of the caloric solar rays. 
