Prof. Forbes on the Refraction and Polarization of Heat. 135 



tirely by means of an instrument of great delicacy— the 

 thermo-multiplier of MM. Nobili and Melloni, — I shall pre- 

 mise some account of its application to the investigation of 

 some more familiar modes of action. 



§ 1. Miscellaneous Experiments. 



2. We could hardly quote a stronger proof of the rapid 

 and unexpected advances which enlarged theory may produce 

 in practice, than by referring to the employment of thermo- 

 electric action, discovered a few years since by Seebeck, to 

 the measurement of heat, with a degree of accuracy and fa- 

 cility which, perhaps, no thermometer has ever attained. 

 Such is the principle of the thermo-multiplier of Nobili and 

 Melloni. It is well known, that when two metals (and espe- 

 cially bismuth and antimony) are soldered together, and the 

 point of union heated, an electric current is established from 

 the one metal to the other, which may be carried off by wires, 

 and caused to act upon a delicate galvanometer or multiplier, 

 the needle of which serves as an index ; the galvanometer 

 consisting, of course, of a magnetic needle, nearly freed from 

 the influence of the earth's magnetism, and so conrlected with 

 the wire which transmits the electricity, that the mutual in- 

 fluence of the magnetism and the electricity shall (by the law 

 of CErsted) be a maximum. 



3. It will readily be conceived, that, if a series of alternat- 

 ing bars of bismuth and antimony be placed parallel to each 

 other, and the extremities alternately soldered together, when 

 all the extremities facing one way are heated (as by the radi- 

 ant influence of a lamp), whilst the others remain at the tem- 

 perature of the apartment, the effects produced in a single 

 pair, such as we first supposed, will be produced at each junc- 

 tion, and that the intensity of the whole effect will be greater, 

 just as in the voltaic pile. At one time it appeared doubtful 

 how far electricity, of such small tension as is thus produced, 

 could be so reinforced ; but the instrument in question seems 

 to prove the practicability of it. About thirty pairs are em- 

 ployed, and so delicately are they made, that the ends which 

 exhibit one set of junctions are contained within a superficial 

 area of four tenths of an inch square. 



4. The wires, from the extremity of the first and last ele- 

 ment (just as in the voltaic battery), convey the electricity to 

 the multiplier, which consists of a flattened coil of silver-wire, 

 covered with silk, the coils of the wire being parallel to the 

 quiescent position of an astatic magnetic needle, which is per- 

 pendicular to the magnetic meridian. The deviations are 

 measured in the usual manner, on a divided circle; upon 



