"336 M. MELLONI ON THE POLARIZATION OF HEAT. 



We were already aware that the rays immediately transmitted through 

 bodies differing in their nature pass in very different proportions through 

 a given plate of a diathermanous substance* : we were also aware that 

 those rays are differently absorbed by the surfaces of certain opake 

 bodies f. To these distinctive characteristics we are now enabled to add, 

 that they undergo in the same system of tourmalines an apparent vari- 

 able polarization. 



We see, in fact, that of every hundred rays of heat transmitted by 

 the tourmalines when they are placed with their axes parallel, about 22 

 are made to disappear by merely crossing the axes. This proportion suf- 

 fers no very decided change in the rays transmitted by the common, the 

 red, the orange, the yellow, the blue, the indigo, and the violet glass ; 

 but is reduced to -r-hr or -rhy when we employ green or opake-black 

 glass ; and when we employ sulphate of lime, yellow amber, water either 

 pure or saturated with salts, and alum, the quantity of heat polarized 

 amounts successively to ^Vo■, -rtrV; nV-iD tVtj> and ^Vcr• 



It is a fact worthy of remark, that the character derived from the in- 

 dex of polarization leads to the same consequences that we have deduced 

 from the experiments of transmission. Indeed, the latter analytical 

 process had authorized us to admit that the colouring matter introduced 

 into the composition of coloured glass merely extinguishes a part of the 

 calorific stream transmitted by the colourless glass, without sensibly af- 

 fecting the proportions which the groups of rays composing that stream 

 bear to each other in respect to quantity ; so that the effect produced 

 by that matter relatively to radiant heat is analogous to that which 

 would be produced relatively to light by bro-\\n or blackish substances 

 diluted in a liquid having no chemical influence on those substances ;{:. 

 Now, since the proportion of heat polarized by the tourmalines vai'ies 

 with the quality of the calorific rays transmitted by the different screens, 

 the constancy of this proportion between the rays which issue from the 

 coloured and those which issue from the uncoloured glass clearly shows, 

 as in the experiments of transmission, that the colouring matters do not 

 affect the composition of the calorific stream transmitted by the glass. 

 True, the green and the opake-black glasses furnish a very marked ex- 

 ception; but the experiments of transmission furnish an exception 

 completely analogous §. 



* Jnn. de Chim. et de Phys., torn. Iv. p. 384. 



+ Id., p. ,388. 



: /(/., p. 381. 



§ The same consequences are derived from the experiments of refraction. 

 With this view, one of the faces of the refracting angle of a rock-salt prism is 

 covered with a plate of coloured glass, and the distribution of temperature in the 

 bands of the spectrum produced by exposing this system to the light of the sun 

 is then observed. I; we change the colour of the glass, we not only find the 



