OF UADIANT HEAT THROUGH DIFFERENT BODIES. 59 



interior of one continuous medium. This transmission Ave have ex- 

 amined, and, as we have just seen, it presents nothing contrary to its ana- 

 logy with the transmission of light through coloured media. There is 

 however a particular case in which two homogeneous screens act in so 

 singular a manner in respect to light that it must be interesting to know 

 whether something analogous does not take place in respect to caloric. 



The optical phsenomena presented by most of the slices of tour- 

 maline cut parallel to the axis of crystallization are universally known. 

 If these slices are placed one over the other and their axes laid in the 

 same direction, they transmit light in considerable quantities. But if 

 they be laid at right angles to one another, the light is totally intercepted. 

 Do these phaenomena, arising, as is well known, from the polarization of 

 the light in the interior of the slices, take place in respect to calorific 

 rays also; or, in other words, is radiant heat capable of being polarized 

 in its passage through touniialine ? 



In order to ascertain this I have taken two square plates of the same 

 dimensions. I have made an aperture in the centre of each. This 

 aperture was likewise a square having its sides parallel to those of the 

 plate and each equal to the least breadth of the two polarizing slices. 

 I then took some soft wax and attached a tourmaline to each aperture, 

 holding the axis of the former parallel to one of the sides of the latter. 

 These two plates being laid one over the other, it evidently depended on 

 one of the sides of the one plate being placed parallel or perpendicular 

 to a side of the other whether the light was to be transmitted or inter- 

 cepted. Yet this pair of plates being placed vertically on the stand of 

 my thermoelectric apparatus and exposed to the radiation of a lamp or 

 incandescent platina, uniformly produced the same calorific transmission, 

 whatever might be the relative direction of the sides of each plate. 



That this fact might be put beyond the reach of doubt the galvano- 

 metric index was carried to the 18th or 20th degree, and the calorific 

 communication now established was suffered to remain while we placed 

 one of the plates on each of its sides in succession. The flame or the 

 incandescent platina was then observed to appear and disappear alter- 

 nately Avhile the magnetic needle continued invariably at the same point 

 of deviation. 



This experiment was repeated many times with several tourmalines, 

 and the angle formed by tlie intersection of their axes varied. The re- 

 sult was in all cases the same. Tiie quantity of calorific rays trans- 

 mitted through the two polarizing slices is then independent of the re- 

 spective directions given to their axes of crystallization; that is to say, 

 the heat radiating from terrestrial sources is not polarized in its passage 

 through tourmalines*. 



* Tills result seems oj)i)()sc(l to the experiments of M. Bcrard on the polari- 

 zation of reflected heat ; but, ignorant as wc arc of the nature of the relations 



