REPORT ON RADIANT HEAT. 27 
galvanometer, which, for the most part, do not indicate equal 
increments of force. Two tables are given. By the first, the 
statical deviations of the needle are reduced, so as to be mea- 
sures of the force producing them; by the second, the dynami- 
cal 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 em- 
ployed. 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. Prof. Forbes expressly 
observes, that his former results were not held out as numeri- 
cally 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 infer- 
ence by a great number of experiments, which show that the 
heat from non-luminous sources is less polarizable by a given 
plate of mica, at a given angle of incidence, than that accom- 
panied by light. 
These experiments were performed with plates of mica, pre- 
pared in a way discovered by himself, to which reference is 
made (though without describing the process), in a paper before 
quoted in the London and Edinburgh Journal of Science, March, 
1836. The method consists 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. Rays out of 100 Polarized. 
Parma VOM | Fil) he) ps.) me - (2to 74 
Incandescent platina . . . . . . 72 
Brass about 700° . . . Baresi 
Do. with glass screen. . . . . . 72 
Mercury in crucible at 410° . . . 48 
Bae, SAME oe) toy eaves rience ny A 
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 refrangible rays. 
