344 On the Rotation of the Plane of 
i Hi mre, bs lutea a 
XXXIV.—On* the Rotation of the*Plane of Polarization of 
® Heat by Magnetism; by MM.°F. pe wa Provosrayve and 
P, Desains.* gu : ; 
Suorrry after the brilliant discovery of Prof. Faraday of the 
rotation of the plane of polarization of light by magnetism, M. 
Wartmann announcedt that he had tried the same experiment 
with radiant heat. He employed the heat of a lamp, which he 
partially polarized by making it pass through two piles of mica 
crossed at right angles. The electro-magnets and a cylinder of 
rock-salt were placed between these piles, and consequently very 
near the thermo-electric apparatus. The galvanometer, on the - 
contrary, to be preserved from the action of the electro-magnets, 
was removed to a great distance; but the result was a considera- 
ble increase in the length of the circuit, and a diminution of sen- 
sitiveness, 
Notwithstanding all these inconveniences, which he clearly 
_ pointed out, and which he was not able to overcome, M. Wart- 
mann thought he observed that the needle of the galvanometer, 
after having attained a fixed deviation under the influence of the 
adiation not intercepted by the piles of mica, was again displaced 
and took a fixed position different from the first when the current 
was established, which seemed to indicate a rotation of the plane 
of polarization of heat. sd 
At Paris, some persons having vainly attempted to reproduce 
these phenomena, we have considered that it would be useful to 
revert to these experiments, and to point out a method which 
permits of making them succeed with facility. 
e have introduced into M. Wartmann’s process three prinei- 
pal modifications :—Ist, we employ solar heat; 2ndly, we take 
for polarizing apparatus two prisms of achromatic spar; 3rdly, and 
this appears to us indispensable, instead of placing the principal 
sections at 90°, we arrange them so that they make an angle of 
nearly 45°. 
The employment of spars and solar light permits of removing 
the electro-magnets to a great distance from the thermo-electric 
pile. With respect to the arrangement of the prisms, the law of 
Malus shows all the advantages which it presents. In fact, let 
us take for unity the deviation which the solar ray transmitted 
through the principal parallel sections would produce. ‘The de- 
viation, when the prisms form an angle of 45°, will be cos? 45° 
3a. If the current is set in action, and it produces a rotation of 
ies pa a mali 
x, From the Annales de Chimie et de Physique, October, 1849,—cited from. Phil 
4) XXXv, 481. ay? 
fe teh May 6th, 1846, No. 644. eae 
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