M. MELLONI ON THE POLARIZATION OF HEAT. 145 



SO that the effects observed represent the sum of the actions 

 exerted upon the thermoscopic instrument by the two portions 

 of heat which always co-exist in the phasnomena dependent upon 

 the passage of calorific rays by diathermanous substances, viz. the 

 immediate transmission and the conductivity. The latter, in 

 altering the absolute value of the index of polarization, would 

 have allowed the equality in the proportion of heat polarized by 

 sources of every description to subsist, could it have operated in 

 these different cases with the same intensity ; for all the calorific 

 rays being equally polarizable, it is evident that the continuance 

 of the action of heating cannot disturb the continuance of the 

 effect due to the polarization. But the diathermancy of mica 

 being analogous to that of glass, the quantity of heat which it 

 absorbs, and consequently its proper heating, varies with the 

 temperature of the source, and thus alters, by this variation of 

 the perturbing cause, the constant effect produced by the prin- 

 cipal cause. 



In a new series of experiments which has appeared in t^e 

 last volume of the Edinburgh Philosophical Transactions, Mr. 

 Forbes has partly avoided the different incidence of the calorific 

 rays upon the piles by placing the sources at a distance, always 

 the same, but about three times greater than that which he had 

 adopted in his previous researches. The results obtained have 

 a nearer approach to equality. In fact, the index of polarization 

 of the same system of piles inclined about 34° upon the axis of 

 radiation was /„% to -^-^-^ for the Argand lamp, -J-^^ for incandes- 

 cent platinum, -^J-^ for copper heated to 400°, y%% for the iron 

 crucible filled with mercury at 280°, and y%% for the vessel full 

 of boiling water*. 



But the perturbation due to the heating of the piles still re- 

 mained, and the existence of this cause of error, which Mr. 

 Forbes allowed in the arrangement of his apparatus, is alone 

 amply sufficient to account for the differences observed mthout 

 its being necessary to suppose that different species of heat un- 

 dergo, in a parity of circumstances, degrees of polarization so 

 various. It might even be demonstrated that the influence of 



and fourth observation. But instead of the two equahties, we find increasing 

 quantities, which prove with the clearest evidence the progressive action of the 

 beatin;^ of the piles. 



• Trans, of the K. S. of Edin., vol. xiii. part ii., Researches on Heat, sucond 

 aeries, p. 14. London and Edinburgh Philosophical Magazine, vol. xii. p. 551. 



