Homogeneous Bodies : "with illustrative Experiments. 15 



which at that time was not a little surprising. A reference to 

 fig. 10. will assist the ilkistration of this singular property 

 discovered in the ring of bismuth. 



^9. When the outside of the ring at b was held for a few 

 moments in the point of a well-defined flame of a spirit-lamp, 

 the electric current in every part of the ring was in the direc- 

 tion of the exterior arrow ; but when the inner side of the 

 rim opposite to b was similarly heated, the current proceeded 

 hi the contrary direction, as indicated by the interior arrow. 

 And so directly opposite and close to each other were these 

 two active points, that a transverse section of the metal at the 

 point b, would have embraced them both. I have since that 

 time observed active points simihvrly situated in other pieces 

 of bismuth. 



50. Experiments -mth a solid Prism of Bismuth. — That side 

 of the rectangle (47) which had exhibited /ocaZ electric cur- 

 rents was now cut from the frame, for the purpose of ascer- 

 taining whether pha^nomena similar to those described (47) 

 would be exhibited by this prism when examined as a distinct 

 individual mass. Fig. 11. will represent this prism or bar of 

 bismuth : when heated at b, the former active point, the bar be- 

 came highly magnetic, displaying those energies in a very ex- 

 alted degree. The electric current was traced from the point of 

 heat towards the end c, along that side of the prism which in 

 the figure is hid from view. It proceeded across the end c, as 

 shown by the curved arrow ; and returned to the heated point 

 along the front face of the prism: so that when heated, if the 

 front face a, b, c were placed over and parallel to the needle, 

 and the end c north, the north end of the needle would be 

 deflected towards the east; and by simply turning the prism the 

 other side up, the needle would be deflected towards the west. 

 By thus turning the bar, at suitable times to promote the de- 

 flections, the needle could be made to sweep an arc of 90°. 



51. The other two faces of the prism were nearly neutral, 

 each of them partaking of the opposing currents in the active 

 sides. 



52. The bar was afterwards heated at various parts of its 

 surface, and the direction and energy of the currents ascer- 

 tained by the deflections of the compass-needle. It was ulti- 

 mately found that when the whole of the end c was uniformly 

 liealed, the most powerful current was excited ; its course was 

 still in the same faces of the prism,' but its direction was reversed. 



53. When the end a was uniformly heated, no thcrmo- 

 magnetism was elicited, until, as the heat advanced in the bar, 

 the temperature became disturbed at the point b. Tliis ac- 

 complished, those powers were again roused from their qui- 

 escence, 



