88 Dr. Tyndall on the Progress of the Physical Sciences : 



We have already mentioned a variety of notions entertained 

 by physicists as to the origin of thermo-currents. To these 

 M. Magnus adds the discussion of the hypothesis, that the cause 

 is to be sought in the unequal decrease of temperature on both 

 sides of the place heated, and of the notion that they are to be 

 referred to a difference of conductibility for heat on the part of 

 the metals employed. He dissents from both these views ; and 

 proves, in the following manner, that the conductibility of the 

 hard wire was in no way different from that of the soft one. 



From a stout brass wire 2"25 lines in diameter, and which 

 was rendered quite hard by the act of drawing, two pieces each 

 4 feet long were separated. One of these was heated to redness, 

 and thus rendered soft ; both wires were then brought into the 

 tin vessel already described and there subjected to the same tem- 

 perature ; the ends of the wires without the vessel were at such 

 a distance from it, that they retained the same temperature ; to 

 one end of the galvanometer wire before described a bar of anti- 

 mony was attached, and to the end of the other wire a bar of 

 bismuth, both being bevelled off to an edge ; the edge of one of 

 these bars was laid upon the soft wire, and the edge of the other 

 upon the hard wire ; when a difference of temperature existed 

 between the points of contact, a current was exhibited on the 

 galvanometer ; when the temperatures were alike, no current was 

 visible. By finding points of equal temperature in this manner, 

 and by measuring the distance between these points on each 

 wire, their respective conductive powers were ascertained. It 

 was found that the conductibility of both was the same. The 

 conclusion finally arrived at by M. Magnus is, that the currents 

 are produced by the contact of unhomogeneous metals. 



In connexion with this subject, M. Svanberg has laid an inter- 

 esting communication before the Academy of Sciences at Paris, 

 from which we extract the following : — In large masses of bis- 

 muth and antimony the crystalline texture is never in all parts 

 the same, but it is not difficult to find some homogeneous por- 

 tions. From these little bars may be formed, the length of 

 which may be at various inclinations to the planes of crystal- 

 lization. 



Among the planes of cleavage of these two metals in a crystal- 

 lized state, there is one, which, as Mr. Faraday was the first to 

 observe, is distinguished from all others by its superior brilliancy. 

 This plane is perpendicular to the crystallographic axis. Among 

 the other planes there is one which docs not fall far short of the 

 above in point of brightness. Let the bars whose length coin- 

 cides with the intersection of those two planes be named (A), 

 and those bars whose length is perpendicular to the plane of 

 most eminent cleavage be named (B). 



