358 RADIANT HEAT. 



nature of the heat obtained by the intervention of diathermanons sub- 

 stances are not the same as those between heat from luminous and 

 dark sources. And further, in the experiments mentioned with ra- 

 diations from different sources, no numerical results are stated. On 

 this point we shall presently notice some more detailed researches of 

 Professor Forbes. 



The sixth point, on the subject of depolarization, is confessedly one 

 of the most delicate in the whole inquiry; but for the same reasons as 

 before, the effect of secondary radiation cannot be referred to as ca- 

 pable of having produced the differences observed. 



Unequal Polarizability of Heat from different sources : Forbes. 



Professor Forbes' s third series of Researches of Heat appears in 

 vol. xiv of the Edinburgh Transactions, having been read before the 

 Royal Society of Edinburgh, April 16, 1838. It is also printed in 

 the London and Edinburgh Journal of Science, vol. xiii. 



In the first section the author discusses the variable polarizability 

 of the different kinds of heat. The establishment of this fact was his 

 object in one portion of his second memoir. But these investigations 

 having been objected to by some, and opposite results (as we have 

 seen) obtained by Melloni, the author now repeated the inquiry with 

 every precaution. He rendered the rays parallel by a rock-salt lens, 

 as Melloni had done, and operated at a sufficient distance from the 

 pile; still the differences in the rectangular positions, when different- 

 sorts of heat were employed, were as unequal as formerly. 



Having varied the experiments in every possible way, he still 

 comes to the same conclusion as before, and gives the following results: 



Source of heat. Lays out of 100 polarized. 



Argand lamp 78 



Locatelli lamp 75 to 77 



Incandescent platina 74 to 76 



Incandescent platina with glass screen 80 to 82 



Alcohol flame 78 



Brass, at 700° 66.6 



Brass, with mica screen 80 



Mercury in crucible, at 450° 48 



Boiling water 44 



Melloni' s opposite result of apparent uniform polarizability, the 

 author then shows, must necessarily arise from the use of mica piles, 

 consisting of a number of distinct plates superposed. Such a thick- 

 ness 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; whereas Mr. Forbes's results were obtained by the 

 use of mica split by heat, (as before described,) which includes so 

 many surfaces within a very small thickness that the polarized heat 

 is comparatively unaltered in its character. He shows directly that 

 these piles transmit heat from a lamp sifted by glass, and from brass 



