22 Mr. W. Sturgeon on the Ther7no-magnetism of 



the plane of the neutral lines. The same explanation will 

 apply to this figure as to the cylinder, fig. 13. {65.) 



78. It is not necessary that the point of heat be exactly in 

 the edge of the base to produce the greatest effect, for the 

 direction of the electric force is still the same, and quite as 

 energetic, when the point of heat is at some short distance 

 from the base. Neither is it necessary that any point be made 

 very hot, unless it can be done very suddenly ; for the powers 

 excited are decisively exhibited when the selected point of 

 heat is held only for a few moments in the apex of the flame 

 of the spirit-lamp, and the cone immediately applied to the 

 compass-needle, before the heat has time to spread, to any 

 great extent, over the conical surface. 



79. When the apex of the cone of antimony is heated, the 

 electric force is exceedingly feeble, and its direction quite un- 

 certain. In general, the thermo-magnetic forces displayed by 



vheating any point nearer to the apex than to the base are 

 comparatively insignificant, and their directions not easily 

 predicted. 



80. A cone of antimony which had exhibited the phaeno- 

 mena already described, was cut in two by a saw, at about 

 1*5 from the apex, and parallel to the base* The small cone 

 operated precisely as the original one, of which it was a part ; 

 but the energies were by no means so powerful. 



81. The frustum presented phsenomena as if it had been 

 a complete cylinder, and the electric currents were as de- 

 cidedly traced when the point of heat was near to the section 

 as when it was near to the base. 



82. When cylindrical bars or cones of bismuth are ex- 

 perimented with in the manner I have described with an- 

 timony in those shapes, the thermo-magnetic phfenomena are 

 precisely of the same charactei', and are regulated by the same 

 laws ; so that whatever phaenomena be displayed by the one 

 metal will also be displayed by the other, provided the cylin- 

 ders or cones be well cast, and of uniform density on every 

 side of the axis. 



83. In bismuth, however, it sometimes happens that in con- 

 sequence of an irregularity of crystallization, which it is prone 

 to assume, there will be one point, and sometimes two, which 

 when heated will display thermo-magnetic phaenomena very 

 different to those I have before spoken of; but these are irre- 

 gularities which have nothing to do with the general character 

 of the })haenomena, and but seldom occur. 



Si. Observations. — Whatever peculiarities there may be in 

 the crystallization of antimony and bismuth when in masses 



of 



