186 Dr. Tyndall on Diamagnetism and Magnecrystallic Action. 



to answer the following questions : — How is it possible that a 

 greater differential action can be exhibited by a cube of pow- 

 dered bismuth than by the crystal itself? What is it that 

 causes the magnecrystallic axis of the crystal to forsake its usual 

 position and to set equatorial when the mass is compressed in 

 the direction of the said axis ? He must further assume a cry- 

 stalline structure on the part of wax, flour, shale, and the pith 

 of fresh rolls. 



With regard to the experiment with the cherry-tree bark, '\ 

 have a bar of chemically pure bismuth which does not contain a 

 trace of magnetism, and which exhibits the precise phsenomena 

 observed ^vith the bark. These phsenomena do not therefore 

 necessitate the hypothesis of two conflicting forces, the one or 

 the other of which predominates according as the poles of the 

 magnet are more or less distant. I have already commenced an 

 investigation in which the deportment of the bark and other 

 phsenomena of an analogous nature will be more fully discussed. 



Eveiy physicist who has occupied himself experimentally with 

 electro-magnetic attractions must have been struck with the 

 great and speedy diminution of the force by which soft iron i» 

 attracted in the immediate neighbourhood of the poles. In ex- 

 periments with spheres of soft iron, I have usually found that 

 a distance of j^Qih. of an inch between the sphere and the mag- 

 net is sufficient to reduce the force with which the former is 

 attracted to -ij^th. of the attraction exerted when the sphere is 

 in contact. To any one acquainted with this fact, and aware, 

 at the same time, of the comparative sluggishness with which a 

 bismuth ball moves in obedience to the repulsive force even 

 when close to the poles, a law the exact reverse of that affirmed 

 by PlUcker must appear exceedingly probable. 



The bismuth balls were placed upon the torsion-balance ; on 

 the top of one of them a particle of an iron filing was fixed, and 

 with this compound mass the space opposite to a core excited 

 by a current of 50^ was sounded. The beam was bi-ought by 

 gentle pushing into various positions, sometimes close to the 

 magnet, sometimes distant. The position of equilibrium for the 

 beam when the core was unexcited was always zero. When the 

 beam was pushed to a distance of 4° (about /^jths of an inch) 

 from the core end, on exciting the magnet it receded still fur- 

 ther and rested against a stop at 9° distant. When the current 

 was interrupted the beam left the stop and approached the 

 magnet ; but if, before it had attained the third or fourth degree, 

 the circuit was closed, the beam was driven back and rested 

 against the stop as before. 



