§08 



Source of Heat. Rays per cent, polarized. 



Argand Lamp 78 



Localelli Lamp, 75 to 77 



Incandescent Platinum (usually), 74 to 76 



Incandescent Platinum, with glass .06 inch thick interposed, 



6 to 7 per cent, more, or . . . . . . 80 to S2 



Alcohol flame, 78 



Brass heated to about 700°, 66.6 



Do. with a plate of Mica .016 inch thick interposed, . . 80 



Mercury in a Crucible at 450% 48 



Boiling Water, 44 



The apparently uniform polarizability of all kinds of heat, as ob- 

 served by M. Melloni, the author shews must necessarily arise from 

 the use of mica bundles, such as he used, consisting of a great num- 

 ber of distinct plates of mica superimposed. Such a tliickness of 

 mica modifies heat from dark sources in such a way as to give the 

 portion which it transmits the same character as to polarizability 

 as luminous heat. The plates used by the author, and split by the 

 action of a hot fire, as explained in a former paper, contain so many 

 surfaces within a very small thickness, that the polarized heat is 

 comparatively unaltered in its character. The author shews that 

 the piles used by him transmit heat from a lamp sifted by glass, 

 and from brass at 700° in nearly equal proportions, whilst mica 

 .016 inch thick transmits five times less of the latter than the 

 former. 



The Second Section relates to the Depolarization of Heat. Pur- 

 suing the methods given in the first series, the author ascertained 

 the proportion of heat depolarized by five difi'erent thicknesses 

 of mica. From the numerical results thus obtained, he deduces 

 the value of the expression in Fresnel's formula of depolari- 



. retardation . , /. . . , .... , 



zation, ror , -, — , either or which quantities bemo: assumed, 



length of wave 



the other becomes known. If the numerator (the difi^erence of 

 paths described within the depolarizing mica plate by the ordinary 

 and extraordinary ray) be assumed to be the same as in light, the 

 length of a wave would come out 3 times as long as for red light, 

 and 4^ times as long as a violet wave. Since, however, this result 

 seems not easily reconcilable with the slight diflFerence of the mean 

 refractive index for heat and light (which is afterwards obtained), 

 the author rather inclines to the supposition, that the energy of 

 double refraction is smaller for heat than for light, or that a greater 

 thickness of the medium is required to produce a given retardation. 



