180 Prof. Forbes's Researches on Heat. 



whole figure shows the exact relative proportions of the plates, 

 the exact distances and dimensions of the mass of liquid, on a 

 scale reduced from those actually employed. 



158. The lines a, «, a, represent the copper-plates found 

 by the experiments contained in the first table (No. 8), and 

 those marked b, b, b, the plates from the second table 

 (No. 9). It is almost needless to remark, that the plates, 

 though shown separately in the figure for the sake of the com- 

 parison, must be considered as having occupied precisely the 

 same position in actual experiment. 



[To be continued.] 



XXIV. Researches on Heat. Third Series. § 1. On the un- 

 equally Polarizable Nature of different Kinds of Heat. 

 § 2. On the Depolarization of Heat. § 3. On the Refran- 

 gibility of Heat. By James D. Forbes, Esq.., F.R.SS. 

 Li. 4* E., Professor qf Natural Philosophy in the University 

 of Edinburgh. 



[Continued from p. 113 and concluded.] 

 § 3. On the Refrangibility qf Heat. 

 CINCE the admirable discovery by M. Melloni of the 

 ^ power of rock-salt to transmit and refract heat of every 

 kind, one of the most obvious and important questions(formerly 

 intractable) of which it seemed to offer the means of solution, 

 was the accurate determination of the refrangibility of heat 

 from various sources, luminous or non-luminous. Such a 

 determination is of the first consequence to the formation of 

 a just theory of heat, and a detection of the subtle bond by 

 which it is connected with the comparatively familiar modifi- 

 cations of light. 



Such experiments have not been wanting. M. Melloni, 

 in his second memoir on radiant heat, in the Annales de Chimie 

 for April 1834, has described the apparatus which he em- 

 ployed, and which is figured in Plate III. of that volume*. It 

 consists of a thermo-electric pile, constructed of a single ver- 

 tical row of elements, so as to be exposed to a very narrow 

 beam of heat. It was made to move on a sector of a circle, 

 at whose centre was placed a prism, by which the beam of 

 heat was refracted from its primitive direction a b into that 

 cd, (see next page), and therefore produced a maximum 

 effect on the galvanometer when the pile was at d. The 

 other parts maintaining the same positions, it is evident that 

 the pile must be moved into the position d\ if the source of 



[* A translation of Melloni's second reemoir on radiant lieat will be 

 found in the Scientific Memoirs, vol. i. p. 39. — Edit.] 



