146 M. MELLONI ON THE POLARIZATION OF HEAT. 



the heat acquired by the mica laminae should operate in the 

 direction indicated by Mr. Forbes' experiments; viz. that the 

 action due to the heating of the piles should diminish the appa- 

 rent index of polarization in proportion as the temperature of 

 the source whence the radiation emanates is lower, as the follow- 

 ing will show. 



The heated piles transmit their own heat to the thermoscope ; 

 and if this secondary radiation be sensible, it always alters, as 

 has just been remarked, the effect due to the heat polarized. 

 But does the alteration produced tend to augment the real index 

 of calorific polarization, or does it render it less apparent ? In 

 order to ascertain, I took some paper well blackened upon each 

 side, which, in this state, is absolutely athermanous, but which, 

 as is known, absorbs a large quantity of heat, and also radiates 

 it in abundance. 



I substituted a rectangle of this paper at the pile nearest the 

 source, and concentrated upon it a large quantity of heat by 

 means of a lens of rock salt. The virtual plane of refraction of 

 the blackened paper was parallel to the plane of refraction of 

 the posterior pile. The heat absorbed by the paper, and after- 

 wards radiated upon the pile, heated the leaves of mica, and 

 they threw the heat acquired upon the thermomultiplier placed 

 at a little distance ; the needle of the galvanometer gradually 

 removed from zero in proportion as the mica became heated ; 

 but as the source was at a constant temperature, after five or six 

 minutes, the quantity of heat acquired by the pile became equal 

 to that lost by radiation and contact with the air, and the needle 

 then had a permanent deviation, which in the circumstances 

 under which I experimented was from 25 to 26°*. This being 



* We know that spiders' threads do not burn when exposed to the solar rays 

 concentrated by the action of the strongest lenses. From this isolated fact, 

 some physicists have inferred that the heat acquired by thin plates, {corps 

 minces) under the action of a constant calorific radiation, is in the inverse ratio 

 of their thickness, and that it becomes null or insensible when they are of an 

 extreme tenuity. This proposition cannot be true in all its generality, and is 

 even completely false in several circumstances ; for in the experiment described 

 above, the effect of the heat of the blackened paper upon the thermomultiplier, 

 instead of diminishing, constantly increased in proportion as the paper employ- 

 ed was thinner. It is needless to say that I had previously ascertained that this 

 increase was not produced by an immediate transmission ; which had no appre- 

 ciable value in any of the sheets under experiment. Thus, in cases of this sort, 

 the fact is directly contrary to the opinion I have alluded to ; paper, and ather- 

 manous substances in general, when exposed to a constant source of heat, beco- 

 ming more heated in proportion to their thinness : at least, when once they have 

 attained their state of mobile calorific equilibritun, they radiate by their pos- 



