Third Series, — Variable Polarizahility of Heat. 101 



low, I obtained a higher degree of polarization than I have 

 ever done before or since. The ordinary proportion between 

 the indications with I and K parallel and crossed^ is with in- 

 candescent platinum 100 to 26 or 27. In this case it was 

 100 ; 20 ; and when the heat was lifted by an interposed plate 

 of thin glass, it rose as high as 100 : 13. 



The general results obtained in the way above described 

 are stated in the following table, in which I have included the 

 numbers for mercury heated to 410°, and for boiling water 

 taken from the second series* ; those experiments not having 

 been repeated because the use of a lens is in those cases of 

 little avail. 



Polarizing Plates I and K. 



Source of heat. Rays out of 100 polarized. 



Argand lamp 78 



Locatelli lamp 75 to 77 



Incandescent platinum (usually) 74 to 76 



Incandescent platinum, with glass '06 inchl 



thick, interposed, 6 to 7 per cent, more, or J 



Alcohol flame 78 



Brass heated to about 700° 66'6 



Ditto, with a plate of mica '016 inch thick "i 



interposed, (between ^K and B) J 



Mercury in a crucible at 410° 48 



Boiling water 44 



I presume that it will be conceded, that the experiments 

 now cited, incontrovertibly establish the unequal polarizability 

 of heat from different sources. Yet, I confess, I should have 

 felt uneasy, could I have thrown no light upon the cause of 

 the discrepancy between M. Melloni's results and my own. 

 This I believe, that I am able completely and satisfactorily 

 to do, allowing him every credit for the perfect exactitude of 

 his experiments. For the sake of clearness, I will state the 

 course by which I myself arrived at this result. 



It occurred to me, that it would be satisfactory for the 

 further and independent confirmation of the conclusions just 

 given (which were then only partially obtained), to examine 

 the index of polarization (by which I mean the per centage 

 of the heat stopped in the crossed position of the polarizing 

 and analysing plates) deducible for different sorts of heat, 

 from a series of experiments made wholly without reference 

 to this question, I mean those on depolarization, considered 

 in another section of this paper, and which, it will be seen by 



* vol. xii. p. 551. 



