M. MELLOXI OX THE POLARIZATION OF HEAT. 169 



glass in the course of the rays transmitted by the piles with par- 

 allel planes of refraction, in order considerably to diminish the 

 more intense energy of the calorific radiation, and render it 

 equal to that from the feeble source, when the planes of refrac- 

 tion of the piles are also parallel. We afterwards dispose these 

 planes perpendicularly, and the index of the galvanometer de- 

 scends pi-ecisely the same quantity in both cases. 



Sir D. Brewster found that to arrive at the limit of the obli- 

 quity at which the polarization of light becomes complete, by 

 means of refraction, the number of laminae requisite diminishes 

 as their refractive power increases. The refrangibillty of each 

 coloured ray that enters into the composition of white light di- 

 minishes gradually from the violet to the red ; therefore, for a 

 certain series of laminfe disposed at a determinate inclination, 

 inferior to the angle which is the limit of complete polarization, 

 the quantity of light polarized will be greater for the violet rays 

 than for the blue, greater for the blue than the green, &c. 



Analogy induces us to believe that similar phsenomena occur 

 with respect to the different species of calorific rays which we 

 have frequently compared to light of various colours. But, as 

 we have just seen, these variations entirely escape the existing 

 resources of calorimetiy. Nor can this circumstance occasion 

 much surprise if we consider, I. that in the case of light the 

 differences between the quantities polarized by glass or mica, 

 acting at a given incidence upon violet and red, which are the 

 rays of the greatest and least refrangibillty, do not much exceed 

 the hundredth part of the entire quantity, even in the most 

 favourable circumstances ; II. that these small variations w^ould 

 probably not have been discovered and measured, if the criterion 

 of coloration, which enables the eye immediately to distinguish 

 luminous rays of different refrangibillty, had been wanting in 

 light as well as in heat; III. that the differences of refraction 

 of the divers rays of heat proceeding from terrestrial sources are 

 very small, and only exceed the amount of the analogous varia- 

 tions of light by a scarcely sensible quantity*; IV. that we can 

 never opei'ate alone upon one sort of calorific rays, since eveiy 

 direct flux of heat contains several species, which pass, in groups 

 more or less complex, through the piles of mica and other la- 

 minae interposed, and consequently give a species of interme- 



* Vol. Iv. p. 308, of the Annales de Chim. et de Pliys. [or Scient. Mem. 

 vol. i. p. .5(5. Edit.] 



