Homogeneous Bodies; voith illustrative Experiments. 19 



67. When any point is suddenly made pretty hot, without 

 elevating the temperature of the opposite side, which can easily 

 be done when a cylinder is employed of more than two inches 

 diameter, the electric force is very considerable, and will de- 

 flect the needle to an angle of 20° or 30° ; and by dexterously 

 turning it the other side up before the returning needle ar- 

 rives a" the magnetic meridian, another impulse is given, and 

 the angle increased on the other side. Two or three turns of 

 the cylinder in this way will cause the needle to sweep a con- 

 siderable arc ; but the arc over which the needle passes will 

 be very much increased if the needle be followed up by the 

 active sides of the cylinder, still keeping the one parallel to 

 the other. 



Remarks on the preceding Experiment. 



68. There is a peculiarity in the phaenomena displayed in 

 this experiment which has not been observed in any of the 

 rest:— When a cylinder of antimony is cavernous on one side, 

 I have shown (57) that the electric current invariably flows 

 over the same parts of the surface ; but in cylinders of uniform 

 density on every side of the axis, the law of thermo-magnetic 

 action is very different, and the route of the electric current 

 over the surface of the metal entirely depends upon the situa- 

 tion of the point of heat. 



69. When a cylindrical bar of antimony is uniformly dense 

 on every side of its axis, it will invariably present a regular 

 crystalline form at every transverse fracture. The general 

 contour of the section is that of a series of exceedingly thin, 

 concentric crystalline lamina>, of which the whole face of the 

 fracture seems to be composed from the centre to the surface 

 of the cylinder. Aided by a magnifier, the eye is enabled to 

 trace apparent radiating veins, which by close inspection are 

 observed to separate the lamina; into distinct parcels or tall 

 narrow bundles, with their edges inclined to each other at 

 various angles, both salient and re-entering : and the apparent 

 veins, which are frequently nothing more than an angle at 

 which two bundles of lamina; unite, give to the fracture a 

 beautiful glittering appearance. Some of those radiant veins, 

 however, are absolutely the flat facets of lamina', or more fre- 

 quently the sloping edges of bundles of them, which have a 

 brilliancy far superior to any other jwirt of the fracture. The 

 general position of the lamina-, however, is, that their flat sur- 

 faces are presented to the axis of the cylinder: and although 

 there are certainly objections to this position being uniformly 

 determined by the crystalline lamina^ because of several ol 

 the piles or bundles being posited at various angles, yet the 



1) 2 major 



