S26 M. MELLONI ON THE POLARIZATION OF HEAT. 



then to indicate a \evj marked difference in this respect betvAeen the 

 laws of polarization of heat and those of light. 



Are calorific rays really susceptible of polarization ? Are they all 

 equally and completely so ? Such ai'e the questions which I propose 

 to consider in this memoir, endeavouring at the same time to account 

 for the contradictions (more or less obvious) exhibited by the results 

 at which the different observers just quoted have arrived. 



The instrument invariably employed by me in these researches is an 

 excellent thermomultiplier constructed by M. Gourgon. In order to 

 give the reader an idea of its great sensibility, it will be sufficient to 

 state that the natural heat of the hand placed near one of the extre- 

 mities of the tubes with which the pile is furnished, will impel the 

 index to its maximum of deviation when the temperature of the atmo- 

 sphere is below 15". The pile, which has its two terminating faces 

 perfectly symmetrical, consists of thirty pairs (bismuth and antimony,) 

 formed into a bundle measuring eight lines in the diameter of its trans- 

 verse section and ten lines in length * : the tubes or cylindrical appen- 

 dages Avhich envelop its two faces are nearly of the same breadth as 

 the pile, but three times as long. The astatic system of the galvano- 

 meter, which consists of two needles very powerfully magnetized, mea- 

 suring C'^'i? in diameter and 53'"™ in length, makes but two 

 oscillations a minute. If, however, after the communication with the 

 pile has been interrupted, the system is turned aside 35 or 40 degrees 



* The symmetry, or rather the equality of the two opposite sides of the pile is 

 a condition indispensably necessary, in order to render the observations inde- 

 pendent of the slight changes of temperature that may take place in the sin-- 

 rounding air during the experiments. In fact, if tlie bars of bismuth and anti- 

 mony were stronger, or their solderings less extended, on either of the two 

 sides than on the other, the heating or cooling of the air would no longer be 

 communicated (by contact) with equal promptitude to them both, and the ex- 

 tremities which presented the least mass in proportion to the extent of the sol- 

 dering would be heated or cooled more rapidly than the opposite ends. This 

 circumstance would produce a current which, by its intervention, would disturb 

 the calorific effect of the rays that arc received by the anterior face of the pile 

 Hence it is obvious that witli piles having their opposite faces unequal, exac 

 measures of the calorific radiations are attainable only in that case in which the 

 temperature of the atmosphere undergoes no sensible variation. If it varies, 

 the results will be less accurate in proportion to the greater rapidity of the va- 

 riation and the greater length of time required to make the experiments. The 

 piles [a rayons et ii biseau,] described by Nobili in the 57th volume of the 

 BibUotheque Universellc have not their opposite faces symmetrical ; they are 

 therefore not free from the defect just mentioned. The author himself admits 

 it in the 8th page of the same volume, where, after having given the different 

 rates of calorific transmission obtained in a series of bodies by means of his pile, 

 he adds, that the conditions of temperature which could affect the results, and 

 consequently change Ihem by their variations, are the following: 1st, the tern 

 perature of the soiree ; 2nd, that of the bodies themselves, and particularly tli 

 of the surrounding air. 



