164 M. MELLOXI ON THE POLARIZATION OF HEAT. 



tion equal to 1'5*; the angle corresponding to this quantity, 



taken as a tangent, is 56° 19', or 33° 41' reckoning from the 



surface. 



Thus, the angle of complete polarization, by reflection, is very 



nearly the same for both heat and Ughtf. 



Now take a pair of piles, each of twenty laminae, and after 



mounting them properly upon the apparatus of polarization, 

 place a lamina of alum, amber, or black glass, or a layer of 

 water, oil, or some other diathermanous substance, against the 

 opening of the screen which covers the apparatus. The emer- 

 gent rays of the layer added to the system, will then pass through 

 the two packets of mica, which are to receive in succession the 

 two directions adapted for measuring the quantity of heat polar- 

 ized by refraction. Now, in effectuating this experiment it will 

 be found that the index of polarization does not alter in the 

 smallest degree with the nature of the substance interposed, and 

 that its value coincides precisely with the proportion of heat po- 

 lai-ized under the actual incidence of the two piles, when the 

 opening of the screen is free. 



To exhibit this fact with facility, and in a veiy e-\ddent manner, 

 I employ a means which to me appears capable of carrying con- 

 ^^ction even to the most prejudiced mind. I choose two sub- 

 stances endowed with contrary diathermanciesX, that is to say, 

 two bodies which, when exposed to the same calorific flux, admit 



* Biot, Traite de Physique, vol. iv. p. 80. 



t From what precedes, it will easily be conceived that to polarize heat or 

 light by means of refraction, it is nearly always requisite to give the piles a 

 great degree of obliquity upon the incident rays. When the laminae are suffi- 

 ciently numerous we may stop at the inclination at which complete polarization 

 commences, which, in certain cases, allows of placing the piles at incidences 

 nearly appvoaching the perpendicular. However, when the two series of plates 

 consist of very many elements, it is often useful to dispose them, in preference, 

 at the angle of complete polarization by reflection, in order to have an abun- 

 dant transmission of luminous or calorific rays. 



X Experiments have just been commenced at Geneva, upon the quantity of 

 heat radiated by bodies under a serene sky, at difl'erent hours of the day. An 

 account of them may be found in the n»imberfor April, 18.'37, of the Bibliotheque 

 Universclle, in which one of the learned editors of that e.xcellent repertory has 

 discussed the results of those observations under the title ot Diathermansie de. 

 V Atmosphere. The word diathermancy, as I have defined it in my second 

 memoir upon Transmission, (vol. Iv., p. 378 of these Annals) signifies the pro- 

 perty possessed by nearly all diathermanous bodies, of admitting the passage 

 only of certain species of calorific rays. When we wish to denote the quantity 

 of heat transmitted independently of the qualifi/, the term diathermaneity is 

 perhaps preferable, in order to preserve the same termination as the word 

 diaphnneilij, indicating the analogous property in relation to light. 



