M. MELLONI ON THE POLARIZATION OF HEAT. 3i5 



we look at those lights through our systems of tourmalines with their 

 axes first in the parallel and then in the perpendicular position, all the 

 coloured images which present themselves with sufficient vividness and 

 brilliancy in the first case, completely disappear in the second ; or if any 

 of them remains, it is but an excessively feeble gleam*. The rays of 

 platina in a state of incandescence, and of copper heated to 400°, repre- 

 sent, relatively to radiant heat, the coloured alcohol flames ; and the rays 

 of a Locatelli lamp, transmitted through water, glass, or alum, are, in 

 respect to this heat, no more than the lights of different colours which 

 we perceive through the coloured glasses. Now we have seen that the 

 effect of the tourmalines on the several species of rays, so far from being 

 equal as in the case of light, presents differences so strongly marked, 

 that sometimes tlie heat passes in a sensibly equal quantity in all posi- 

 tions of the axes of crj'stallization, and at others it is almost completely 

 intercepted when the axes of the two plates are perpendicular to each 

 other. 



The phaenomena of calorific polarization, in a variable proportion, 

 produced by a system of tourmalines which polarizes the luminous rays 

 of all colours equally, are analogous to the extremely marked differences 

 of absorption which the various species of calorific rays undergo in their 

 passage through a sufficiently thin plate of glass, rock crystal, water, al- 

 cohol, and almost all perfectly diaphanous sul)stances, the absorbent 

 force of which, within these limits of thickness, if there be any, is the 

 same for all sorts of luminovis raysf . Among those actions which vary 



* M. Biot possesses a prism cut out of a tourmaline of a light violet- red co- 

 lour, which not only does not completely extinguish the common sheaf, as is 

 done by the tourmalines that are too thin or too lightly coloured, but colours the 

 sheaf while it weakens it ; so that the two images of a minute object seen through 

 this prism are sensibly white and of equal intensity near the apex of the refrin- 

 gent angle ; but, in proportion as the eye moves towards the thickest part, the 

 ordinary image is observed to decrease in its intensity and to take at the same 

 time a red tint which becomes gradually deeper, while the extraordinary image 

 never presents itself in any other colour than a slight tinge of the shade which 

 belongs to the tourmaline. It appears then that two flakes of this particular kind 

 of tourmaline would not act alike on all the coloured rays, and that in the per- 

 pendicular position of the axes they would still be permeable to red light. Per- 

 haps the red rays themselves would be finally extinguished if the two flakes 

 were of a certain thickness. However, tliis case is but an exception ; for all 

 the other tourmalines constantly produce the effect we have just announced ; 

 that is to say, that they extinguish indiscriminately and equally all the luminous 

 rays whatever be their colour, provided they are made to pass through the flakes 

 when their axes are at right angles to each other. 



t The experiments on the different degrees in which coloured and uncoioured 

 media absorb the light and heat of the solar spectrum {Ann. de C/iim. et de Pliijs., 

 Dec. 1835, p. 402 ; Scientific Memoirs, vol. i. part i.) afford a still more striking 

 example of the same kind ; for the differences then present themselves in pairs 

 of calorific and luminous rays, which being isolated and, so to speak, purified by 

 by the force of refraction, seem to constitute that species of light and heat which 

 are most identical. And here I shall take the opportunity to correct a strange 



