356 RADIANT HEAT 



menon really produced in the experiments on dipolarization, if the 

 mica be of a suitable thickness. The direct experiment with rhombs 

 of rock-salt has been already mentioned also. The author here gives 

 a detailed account of them, and the laws of the phenomena deducible, 

 in which the precise analogy with those of light is preserved. 



Equal Polar izability of Heat from different sources: Melloni. 



Melloni' s second memoir on the Polarization of Heat appears to be 

 founded on the second part of his communication to the Royal Academy 

 of Sciences in January, 183G. It is printed in the Ann. de China. , lxv, 

 May, 1S37, and the translation in Taylor's Scientific Memoirs, Part VI. 



The principal points of these extensive researches may be reduced 

 to the following- heads : 



(1.) Referring to Professor Forbes' s researches, first series, Melloni 

 contends that the differences of polarizability in the heat from 

 different sources there exhibited are in fact due to differences of 

 secondary radiation from the heating of the mica piles, and subse- 

 quently appeals to Forbes's second series, in which he conceives the 

 approach to equality is much nearer, as this source of error was more 

 avoided. 



At lower temperatures of the source, he observes that mica transmits 

 less heat in proportion, and therefore absorbs more; thus the secondary 

 radiation is greater, and the apparent difference in the two positions, 

 or index of polarization, is less. 



(2.) He remarks that Professor Forbes had found the heat from a 

 dark source, after transmission through glass, to become as polarizable 

 as that from incandescent platina, whereas he considers that the glass 

 plate absorbed the greater part of those rays which otherwise would 

 have heated the piles, and that thus the apparent polarization was 

 increased. 



(3.) Melloni describes his apparatus, and the precautions for avoiding 

 secondary radiation, &c, employing piles of split mica, and throwing 

 parallel rays on them by means of a rock-salt lens, having its principal 

 focus at the source of heat. 



He then enters upon the details of his results, in several series, 

 with piles of different numbers of laminaj, and at different inclinations 

 to the axis, (the source of heat being a lamp,) giving in each case the 

 calorific transmissions in the rectangular positions, or proportions of 

 heat polarized. These are comprised in a series of eight tables, from 

 which the author derives the following conclusions: 



I. The proportion of heat polarized increases as the inclination of 

 the piles is diminished. 



II. It attains a maximum at a certain inclination. 



III. This inclination is greater as the number of laminaa is increased. 

 He points out the close agreement of these results with the phe- 

 nomena of light according to Brewster and Biot. 



(4.) The author pursues a further series of experiments on polar- 

 ization by reflection, and arrives at the conclusion that the angle of 



