38 M. MKLLONI ON THE TRANSMISSION OF RADIANT HEAT. 



If we suppose the action of sulphuric acid analogous to that of water j 

 but not so energetic, we shall see the reason why, w ith tlie 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, fr»m 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 tlie numbers which represent the 

 calorific 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 correctness of such a theory, the 

 line of greatest heat must pass quite beyond the colours into 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 as the 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 laiger pieces the season did nqt allow jne to re- 

 consider and study the result more «icely. But the effect has bepn so 

 marked in the experiment which I made, and so invariable in several 

 successive 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 tlie 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 refractio'i vary between 30° and 70°. These prisms have been made 

 out of several pieces taken from the mines of Cordona, Wieliecza, and Vicq : 

 tliey 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. 



