178 M. MELLONI ON THE POLAHIZATION OF HEAT. 



which immediately traversed the laminae. In each of the sources 

 employed, the heating of the piles, and, consequently, the quan- 

 tity of proper heat which they radiate upon the thermoscopic 

 body, does not alter in the two positions which are successively 

 given to the principal section of the interposed lamina. How- 

 ever, the calorific absorption of the mica, whence this heating is 

 derived, varies with the quality of the incident rays, and becomes 

 strong in proportion to the intransmissibihty by the system, of 

 the heat supplied from the source. Besides, we shall see that 

 all the calorific rays undergo the same effect of depolarization, 

 and consequently give the same difference between the two por- 

 tions of heat that immediately traverse the system, when the 

 principal section is parallel, and afterwards inclined the same 

 [angular] quantity upon the plane of primitive polarization. 

 But it is evident that by adding a given number to two different 

 quantities, they must necessarily approach to equahty, and that 

 in a proportion corresponding to the largeness of the number 

 added. Wherefore the heat fi'om sources at low temperatures, 

 that is, heat from sources whose rays are not very transmissible 

 by mica, undergoing a greater absorption, must have produced, 

 in Mr. Forbes's experiments, a depolarization smaller in appear- 

 ance than the heat from sources at elevated temperatures, whose 

 rays communicate less heat to the system. 



I demonstrate the equality of the depolarization of heat of 

 eveiy kind by means perfectly analogous to those which I employ 

 to prove the equality of their polarization. 



If the subject under investigation be heterogeneous calorific 

 fluxes transmitted by different bodies submitted to the radiation 

 of flame, I take those endowed with the most opposite diather- 

 mancy, which, combined separately with the system of depolar- 

 ization, transmit equal quantities of heat, when the principal 

 section of my circular lamina is parallel or perpendicidar to the 

 plane of primitive polarization, and in each case I incline the 

 principal section 45° upon this plane ; the progress of the gal- 

 vanomctrical index is precisely the same for both experiments. 



If we desire to verify this equahty relatively to the heat emit- 

 ted by different sources, the maximum transmission obtained 

 with the source at a low temperature is to be first observed, and 

 then glass laminre, more or less thick, are to be interposed upon 

 the exterior passage of the radiation from the source at an ele- 

 vated temperature, until the effect of the minimum transmission 



