M. MELLOXI ON THE POLARIZATION OF HEAT. 143 



which then, Uke all other liquid and solid bodies in a very atte- 

 nuated state, transmits notable portions of radiant heat emitted 

 from any source whatever.* 



We have noticed some of the results at which Mr. Forbes 

 arrived by the employment of piles of mica. The calorific 

 polarization obtained by means of two piles of ten plates each 

 was far fi-om complete, for it was always less than half, while 

 for light it appeared about y^-. But the circumstance particu- 

 larly worthy the attention of physicists, is the great diiference 

 which he observed in the proportion of heat polarized, according 

 to the nature of the soui-ce : for in similar circumstances the 

 same mica piles gave j%% of polarization for the heat of an Ar- 

 gand lamp, /^^ for a LocatelU lamp, y^y% for a flame of alcohol, 

 j\% for incandescent platinum, -1%% for copper at a temperature 

 of 390 to 400°, y\/^ for an iron crucible heated by mercury at 

 280°, and yf ^ for a vessel filled with boiling water. 



These numbers, compared with the indices of polarization, 

 which vary so greatly with the nature of the calorific rays given 

 by the same system of tourmahnes, might at first sight induce 

 an opinion that the different species of heat are more or less 

 susceptible of polarization. But by examining attentively the 

 manner in which Mr, Forbes obtained his results, it may easily 

 be seen that the numbers which have just been stated do not 

 in the least represent the quantities of heat polarized. In fact, 

 to measure and compare together these quantities of heat, Mr. 

 Forbes has had recourse to the same method which I employed 

 for the purpose of putting beyond doubt the constancy or vai-i- 

 ability of the caloi'ific transmission of various diathermanous 

 substances, in passing from one calorific source to the other, 



* This fact results from a great number of experiments which I have per- 

 formed upon glass, rock crystal, sulphate of lime, mica, water, alcohol, &c. It 

 is intimately connected with the phsenomenon of decreasing transmission pre- 

 sented in general by a given lamina successively exposed to radiant heat from 

 sources at less and less elevated temperatures. It is also in strict relation with 

 the greatly varied proportion of heat which passes through the same body when 

 submitted to emergent calorific rays from different substances. In investigating 

 the analogous effect in relation to light, we find, as I have shown elsewhere, 

 (vol. Iv. p. 361 of these Annals), [Scient. Mem. vol. i. p. 53], that all diapha- 

 nous substances, excepting rock salt, produce precisely analogous effects upon 

 radiant heat to those of coloured media upon light ; and in fact, the coloration 

 which diminishes the transparency of bodies exposed to luminous rays cf 

 several qualities is entirely removed by reducing them into very thin lamina;, 

 so that these tenuous layers then become equally permeable to all kinds of 

 coloured rays. 



K 2 



