THE MEASURES AND THE NATURE OF HEAT. 505 



[belief was founded, failed on repetition by Powell and Nobili. And even 

 M. Melloni, with his thermomultiplier, was unable to detect the existence 

 of polarization in 1833. In the following year Prof. Forbes took up the 

 subject, and completely succeeded. The characteristic of polarization of 

 light is the exhibition of a reference to sides relative to its path. Thus light 

 which passes directly through one slice of tourmaline is of the same intensity 

 in whatever way the tourmaline be presented to it. But if a second tour- 

 maline be applied, the intensity of the ray which merges from it, depends 

 on its situation relative to the first ; thus proving that the light which had 

 passed through the first tourmaline differed from common light in having 

 acquired a property connected with direction perpendicular to its path. 

 Prof. Forbes showed that the same is equally true of heat. In the first 

 place, he found that two tourmalines, with their axes crossed, stop more 

 heat than when they are parallel, even if the source of heat is brass not 

 luminous. In the next place he discovered an apparatus which facilitated 

 the exhibition of the different facts connected with polarization. It con- 

 sists of plates of mica split very thin by the application of sudden heat. 

 These placed at an angle of about 45, formed an excellent polarizer, and 

 enabled him to detect polarization with the greatest facility. By this means 

 he established the more delicate facts of depolarization and circular pola- 

 rization. A plate of mica being placed between the polarizer and ana- 

 lyzer, when in a crossed position, restored to the heat its capacity of being 

 transmitted through the latter, so that under certain circumstances, the 

 interposition of the plate actually increased the quantity of heat which 

 passed through the apparatus. Prof. Forbes discovered circular polari- 

 zation in 1836, thus establishing the complete analogy between heat and 

 light. There are, however, some points which appear to present an obstacle 

 to basing our theory of heat directly on the corresponding theory of light. 

 In the first place, Prof. Forbes shows that non-luminous heat is less pola- 

 rizable at the same angle than luminous. In the next place, most bodies 

 are found to absorb the less refrangible rays in excess, so that the mean 

 refrangibility is increased by transmission through them. Smoked rock 

 salt, or mica, was found by Melloni* not to possess this property, conse- 

 quently it was argued that the state of the surface produces the effect ; 

 and accordingly with a roughened surface Prof . Forbes t found the quantity 

 of dark heat transmitted to be in a threefold proportion to that which was 

 transmitted from a glass lamp. If, however, the surface was regularly 

 scratched, or if it had a grating before it, heat of every kind was trans- 

 mitted in the same proportion. This last fact may probably tend to recon- 

 cile the theories of heat and light, for I have proved J that interference, 

 whether by means of a prism or by Franenhofer's gratings, when they 

 are regular, produces no effect in adding to, or diminishing the quantity of 

 light. The total amount received on a screen is in exact proportion to the 

 amount of surface left uncovered, the effect of interference being merely a 

 displacement of its position. 



* Comptes Rendus, Sep. 3, 1839. 



f Proceedings of the Royal Soc. of Edin. p. 281. Ed. Tr. vol. xv. 

 t On the Aggregate Effect of Interference, Camb. Tr. vol. vii. part ii. On the 

 Absolute Intensity of Interfering Light, Ed. Tr. xv. 315. 



