RADIANT HEAT. 355 



"by the second, the dynamical effect, or arc, moved over by the initial 

 disturbing action, is reduced to the final or statical effect, and thence 

 to the true measure of heat. Several peculiarities attendant on the 

 use of the galvanometer are likewise discussed. 



In section 2 the observations formerly published on the polarizing 

 action of tourmaline are confirmed, including the case where heat, 

 entirely unaccompanied by light, was employed. In this case, the 

 author allows, the greatest difficulty was to be encountered. 



The third section treats of the laws of the polarization of heat by 

 refraction or transmission. Professor Forbes expressly observes that 

 his former results were not held out as numerically precise; and with 

 reference to Melloni's conclusion, ' ; that all kinds of heat are equally 

 polarizable at the same incidence," he confirms his former view of 

 the incorrectness of this inference by a great number of experiments, 

 which show that the heat from non-luminous sources is less polariza- 

 ble by a given plate of mica, at a given angle of incidence, than that 

 accompanied by light. 



These experiments were performed with plates of mica, prepared in 

 a way discovered by himself, to which reference is made (though with- 

 out describing the process) in a paper before quoted in the London 

 and Edinburgh Journal of Science, March, 1836. The method con- 

 sists in applying sudden heat to a thick plate of mica, which splits 

 into an infinity of extremely thin films, so thin as to be incapable of 

 retaining heat; these form polarizing piles of great energy. With 

 one pair of such plates the author obtained the following per centages 

 of heat stopped, when the planes of refraction of the two plates were 

 in the rectangular position: 



Source of heat. Kays out of 100 polarized. 



Argand lamp 72 to 74 



Incandescent platina 72 



Brass about 700° 63 



Brass with glass screen 72 



Mercury in crucible at 410° 48 



Boiling water 44 



These observations were repeatedly made, and verified by others 

 with other pairs of plates. The results agree with the analogy of 

 light; those lowest in the scale being the cases of the least refran- 

 gible rays. 



In the fourth section the law of polarization by reflection is dis- 

 cussed. A number of reflecting surfaces were tried, and split mica 

 was preferred. The amount of polarization by reflection at a given 

 angle is shown to vary with the source of heat; and it is probable 

 that the kinds of heat do not rank in the same order when the angle 

 is changed. This is the case with light. The change of the plane 

 of polarization by subsequent reflection is similar to that which occurs 

 when light is used. 



The circular polarization of heat by total internal reflection is dis- 

 cussed in the fifth section. This, as before remarked, is a pheno- 



