Prof. Forbes on the Refraction and Polarization of Heat. 207 



Another series on a different day gave the following quantities 

 per cent. 91, 82, 94. Mean of the whole, 86*4< : 100. 



21. Having obtained these decisive results, I proceeded to 

 operate with other sources of heat, and with different tourma- 

 lines. Anxious to avoid the interposition of glass, I had a 

 pair of tourmalines of large size cut without any support. But 

 the best kind will not bear this, and they polarized imperfectly. 

 Only fifteen sixteenths (approximately) of the light in the 

 bright position was stopped in the dark, whilst with the tour- 

 malines A and B every vestige of the brightest gas flame was 

 excluded. With these tourmalines (which may be called C 

 and D) I verified the general conclusions. I was unable to 

 get sufficient effect from non-luminous heat to verify the law 

 in that case. 



22. I had two very fine tourmalines cut and mounted on ex- 

 tremely thin glass. These we may call E and F, With them 

 I was enabled to extend and verify the law of polarization even 

 to the case of non-luminous heated brass, (whose temperature 

 when warmed by alcohol, M. Melloni estimates at 390° cent. 

 = 734° Fahr.) And it is worthy of observation that among 

 twenty-nine pairs of comparative observations, made with three 

 sets of tourmalines, and heated from the following sources, Ar- 

 gand lamp, simple oil lamp, platinum rendered incandescent 

 by alcohol, and non-luminous hot brass, there was only one 

 which did not give positive indications of polarization. The 

 effect, however, with non-luminous heat is extremely feeble, and 

 the percentage very small, because it is with great difficulty that 

 we can obtain results at all with the interposition of two plates 

 of glass, and two of tourmaline (however thin), and a large 

 portion of heat which reaches the pile is derived from conduc- 

 tion, and therefore diminishes the proportion of polarization. 



2S. It is very important to observe, that in this and all simi- 

 lar cases, the effect of conduction or the secondary radiation of 

 heat from screens always tends to disguise, and never to produce 

 the differences of which we are in search ; that is, so long as 

 the means of alternate observations are taken in the way we 

 have described. 



24. The following are the general results of my experiments 

 on tourmaline. 



Source of Heat. No. of Comparisons. Proportions of Heat polarized by 

 AandB EandF Tourmalines A and B EandF* 



Argand lamp, 3 16l U 



Oil lamp, 7 3 14percent. 11 (§ 



Incandescent platinum, 4 3 15 ... 12 j u 



Brass at 700°, 7 (1 negative) 3 J £- 



* It appears that the axes of E and F were not precisely crossed in these 

 experiments. 



