M. MELLONI ON THE POLARIZATION OF HEAT. 167 



obscure heat of copper at 400°, to the Umit of /^^^, ah-eady ob- 

 tained by means of the direct heat of flame. It is impossible to 

 attain this limit with the heat of a vessel filled with boihng water, 

 because the mica exerts upon it an action too strongly absorbent 



as we have shown when considering the results of his observations. Mr. Forbes 

 appears to attribute the application of the reflector to the thermo-electrical piles 

 to M. Nobili. Another physicist, M. Despretz, says, in the last edition of his 

 Traife de Physique, that the thermomultiplier which I empluy is due entirely 

 to M. Nobili, and that I have only rendered its indications regular. Perhaps I 

 may be here allowed to state the real facts. 



The first idea of measuring temperatures by thermo-electrical currents is due 

 to M. Becquerel. His object being to estimate high degrees of heat, he con- 

 structed his electrical thermometer of wires of platina and palladium, which he 

 put in communication with a multiplying galvanometer, made according to the 

 principles of Poggendorf. A few years later M. Nobili proposed the employ- 

 ment of thermo-electricity, for the production of a tliermoscope of contact supe- 

 rior in sensibility to that of the late M. Fourier, which consists of a common 

 thermometer, around which is tied a small bag of flexible skin, filled with mer- 

 cury. For this purpose he made use of bismuth and antimony, which develope 

 the maximum thermo-electrical effect ; of these substances he formed a pile, 

 which he immersed almost entirely in a wooden cylindrical box, containing li- 

 quid mastic, so as to leave exposed only the superior alternate contacts, which 

 were polished and reduced into the same plane ; two bars of copper passing 

 through the sides served to establish the communications with the two ends of 

 an astatic galvanometer. The box was held in the hand, and the bodies, 

 whose diflTerences of temperature were sought, were touched with the uncovered 

 face of the pile. The elements of this pile were twelve in nvnnber, (six pairs) 

 folded over rectangularly, and in a contrary direction at the two extremities, 

 in order to prevent the contact of the intermediate parts when they were sol- 

 dered together. Their section was from forty to fifty square millimetres, and 

 the diameter of the box from two to three inches. Upon the instrument thus 

 constructed, I commenced my labours to convert it into a thermoscope of radi- 

 ation. Having observed in some preliminary trials, that the action upon the 

 multiplier depended much more upon the number than upon the bulk of the 

 elements, and likewise, that the thermo-electrical currents never acquired, 

 within certain limits, the tension necessary for traversing non-metallic bodies ; 

 I gave the elements the form of small flat bars, from thirty to forty times 

 lighter than M. Nobili's, and kept them insulated in their whole length, except 

 at the extremities where the solder was placed, by small bands of paper ; I in- 

 creased the number of them considerably, and fixed them by the middle upon 

 an operculum adapted to a transversal ring seven or eight lines in diameter, and 

 low enough for the two extremities and a great part of the rest to be perfectly 

 free. I then covered all the salient parts of the pile with lamp-black, and sur- 

 rounded them with cylindrical tubes, or conical reflectors, according as my ob- 

 ject was to appreciate the action of a small pencil of parallel rays, or to collect 

 the divergent heat proceeding from the walls of a room, or any other large 

 distant surface. Lastly, I gave it the form and the proportions of the thermo- 

 multipliers so skilfully constructed by M. Gouijon, and which are now to be 

 fovuid in the principal collections of philosophical instruments, both at Paris and 

 in foreign countries. 



The advantage obtained by diminishing considerably the transversal section 

 of the elements, is not only that of being able to introduce a larger number 

 into a very small space, and thus to increase the tension of the electric current 

 which is to traverse the long wire of the galvanometer, but it is specially useful 

 in preventing the formation of the reUuning currents which took place in the 



