330 M. MELLONI ON THE POLARIZATION OF HEAT, 



lorific radiations will be obtained by the mere inspection of the two arcs 

 of impulsion which they successively describe on the galvanometer. 

 The time required by the needles to arrive at the extremity of these 

 arcs is from ten to twelve seconds : they do not remain steady until after 

 an interval of from eighty to a hundred seconds. Now the sources of 



ployed. That branch of the pile which was surrounded with ice was carefully 

 dried and then left exposed to the free action of the air, the other remaining 

 still plunged in the water successively raised to different temperatures. In this 

 instance the intensities of the electric currents became proportional to the ex- 

 cess of the temperature of the water in the vessel over that of the surrounding 

 air ; for the conducting powers of the bismuth and the antimony in minute 

 bars are so very feeble that the heat communicated by the water to one of the 

 faces can scarcely reach the other in any such quantity as to excite in it an ap- 

 preciable elevation of temperature. 



Although these experiments were repeated with equal success in different at- 

 mospheric temperatures, 1 did not yet consider them perfectly satisfactory. In 

 fact, the pile received by contact the differences of temperature which produce 

 the electric currents, and in the ordinary mode of using the thermomultiplier 

 the differences of temperature arise from the action of radiation. It became 

 necessary therefore to demonstrate, by means of radiant heat, that which had 

 been proved by means of heat communicated by contact. After the preceding 

 experiments, the only question now remaining was this : " Whether calorific 

 rays produce in ihermoscopic substances equal dilatations, when they excite in the 

 thermomultiplier equal currents of electricity, whatever may he the intensities 

 and origin of those rays or the modifications they may have undergone in conse- 

 quence of trans7nission, reflection, or refraction^ In order to ascertain how far 

 this question might be correctly answered in the affirmative, I engaged Mr. 

 Bunten to construct an air thermoscope having its reservoirs made of thin cop- 

 per, and its dknensions and mounting, as nearly as possible, the same as those 

 of the pile of the thermomultiplier. The communications between the reser- 

 voirs were established by means of a glass tube, which at first descended very 

 obliquely on each side to the horizon, and then took a horizontal direction in 

 the intermediate part containing tire liquid index : the extreme faces inclosed 

 in the metallic ajjpendages were, as well as the interior of these appendages, 

 covered with lamp-black : the instrument was lastly furnished with a stem of 

 the same thickness as that of the pile, so that the same stand might serve for 

 either. I now opened the two tubes of the pile of the thermomultiplier, and 

 brouglit tlie radiation of a different source to bear on each of its faces; for in- 

 stance, that of the Locatelli on the one, and that of copper heated to 400° on 

 the other ; I then placed the weaker of the two sources more or less near imtil 

 the index of the galvanometer stood fixed at zero. After having thus obtained 

 two opposite calorific actions producing electric currents of the same force, I 

 removed the pile and put the air thermoscope in its place. Tlie same immobi- 

 lity was exhibited by the liquid index. This delicate experiment was repeated 

 with the greatest care on different species of radiations; at first on those that 

 were direct and rendered more or less intense by a suitable approximation of 

 the sources ; then on those that were transmitted through plates of different 

 kinds or concentrated by means of lenses ; and lastly upon the heat given back 

 by reflectors. The result was always the same ; namely, that all calorific radia- 

 tions whatsoever, if they excite in the pile equal currents of electricity or equal 

 electromagnetic actions, will also produce equal dilatations or temperatures in 

 the air thermoscope. (For further details see the report of M. Biot in the xivth 

 volume of Memoires de I'Academie des Sciences.) 



