168 M. MELLONI ON THE POLARIZATION OF HEAT. 



to allow it to traverse a considerable number of laminae, while 

 maintaining a sufficient intensity ; but, happily, this experiment 

 is not necessary to prove that calorific rays proceeding from dif- 

 ferent sources have an equal aptitude for polarization. It is suf- 

 ficient to show that, under the action of a given number of la- 

 mina:, placed at a determined incUnation, every species of heat, 

 when rendered parallel by means of a rock-salt lens, and separated 

 from the rays produced by the variable heating of the polarizing 

 piles, gives indices of polarization sensibly equal. Piles composed 

 of a small number of elements which furnish a sufficiently abun- 

 dant transmission of heat from any source whatever, may be very 

 advantageously employed for this pm'pose. 



The indices of polarization are easily calculated from the data 

 of observation, by means of the table which furnishes the ratios 

 between the forces, and the deviations of the magnetic needles 

 of the galvanometer ; but if we would be independent of this 

 table, and show by the simple inspection of the movements 

 of the galvanometrical index, the equal polai'ization of radiant 

 heat thrown off by soiu'ces of different temperatures, incandes- 

 cent platina, and copper at 400°, for example, an artifice must 

 be employed analogous to that recently described when speak- 

 ing of the calorific rays transmitted by different species of bodies 

 exposed to the radiation of flame. 



After observing the greatest calorific effect obtainable by 

 means of the heat derived from copper at 400°, we must again 

 take incandescent platina, and interpose one or more plates of 



interior of M. Nobili's pile, and destroyed a part of the effect produced. The 

 mastic which covered one of the faces of his pile was also a great inconveni- 

 ence, for it hindered the exterior thermometrical variations from communicating 

 with equal rapidity with all the metaliic parts, the consequence of which was, 

 that deviations of from oO° to 40° were produced, during whole hours, solely by 

 the difference of temperature between the mastic and the ambient air. At last, 

 by substituting polished metal for wood in the construction of the envelopes, 

 the instrument was secured from the exterior calorific radiations, and the ob- 

 server thereby enabled to approach it, without the apprehension that the heat 

 of his own body would injure his experiments. 



The greater part of the alterations that I made in the thermo-electrical pile, 

 are described in a note published by M. Nobili himself, who so far recognised 

 their importance as to say, " In future I shall combine a second pile of this 

 kind with ?«// first thermoimillipUer." (BihUollicque Uiiiverselle, vol. xliv., 

 p. 233.) But from that time the original pile of contact was really of little 

 importance, which was the reason that M. Nobili thought it just and proper tc 

 add my name to his when the electrical thermometer actually in use, that is, 

 the thermomultiplier /or the measurement of i-adiant heat, was presented to the 

 Institut, Scptembc) 5th, 1^31. 



