184: HISTORY OF THERMOTICS. 



has been discovered by MM. Biot and Melloni that quartz impresses a 

 circular polarization upon heat ; and by Prof. Forbes that rnica, of a 

 certain thickness, produces phenomena such as would be produced l>\ 

 the impression of circular polarization of the supposed transversal vi- 

 brations of radiant heat ; and further, a rhomb of rock-salt, of the shape 

 of the glass rhomb which verified Fresnel's extraordinary anticipation 

 of the circular polarization of light, verified the expectation, founded 

 upon other analogies, of the polarization of heat. By passing polarized 

 heat through various thicknesses of mica, Prof. Forbes has attempted 

 to calculate the length of an undulation for heat. 



These analogies cannot fail to produce a strong disposition to believe 

 that light and heat, essences so closely connected that they can hardly 

 be separated, and thus shown to have so many curious properties in 

 common, are propagated by the same machinery ; and thus we are led 

 to an Undulatory Theory of Heat. 



Yet such a Theory has not yet by any means received full con- 

 firmation. It depends upon the analogy and the connexion of the 

 Theory of Light, and would have little weight if those were removed. 

 For the separation of the rays in double refraction, and the phenomena 

 of periodical intensity, the two classes of facts out of which the Undu- 

 latory Theory of Optics principally grew, have neither of them been 

 detected in thermotical experiments. Prof. Forbes has assumed alter- 

 nations of heat for increasing thicknesses of mica, but in his experiments 

 we find only one maximum. The occurrence of alternate maxima and 

 minima under the like circumstances would exhibit visible waves of 

 heat, as the fringes of shadows do of light, and would thus add much 

 to the evidence of the theory. 



Even if I conceived the Undulatory Theory of Heat to be now 

 established, I should not venture, as yet, to describe its establishment 

 as an event in the history of the Inductive Sciences. It is only at an 

 interval of time after such events have taken place that their history 

 and character can be fully understood, so as to suggest lessons in the 

 Philosophy of Science.] 



Atmoloyical Theories. Hypotheses of the relations of heat and air 

 almost necessarily involve a reference to the forces by which the com- 

 position of bodies is produced, and thus cannot properly be treated of, 

 till we have surveyed the condition of chemical knowledge. But we 

 may say a few words on one such hypothesis ; I mean the hypothesis 

 on the subject of the atmological laws of heat, proposed by Laplace, in 

 the twelfth Book of the Mmtniqne Celeste, and published in 1823. 



