276 Prof. Tyndall on the Nature of the Force by which 



directed, but actually excited in the bismuth by the magnet. 

 Such induced currents would, according to known laws, have a 

 direction opposed to those of the inducing magnet, and hence 

 would produce the phsenomena of repulsion. To carry out the 

 assumption here made, M. Weber is obliged to suppose that the 

 molecules of diamagnetic bodies are surrounded by channels, in 

 which the induced molecular currents, once excited, continue to 

 flow without resistance. 



This theory, notwithstanding its great beauty, is so extremely 

 artificial, that I imagine tlie general conviction of its truth can- 

 not be very strong ; but there is one conclusion flowing from it 

 which appears to me to be in direct opposition to experimental 

 facts. The conclusion is, " that the magnetism of two iron par- 

 ticles in the line of magnetization is increased by their reciprocal 

 action ; but that, on the contrary, the diamagnetism of two bismuth 

 particles lying in this direction is diminished by their reciprocal 

 action." The reciprocal action of the particles varies inversely 

 as the cube of the distance between them : at a distance expressed 

 by the number 1, for example, the enfeeblement is eight times 

 what it would be at the distance 2. 



The conclusion, as regards the iron, is undoubtedly correct ; 

 but I believe experiment proves that the mutual action of dia- 

 magnetic molecules, when caused to approach each other, in- 

 creases their repulsive action. I have had massive iron moulds 

 made and coated with copper by the voltaic current ; into these 

 fine bismuth powder has been introduced and submitted to pow- 

 erful hydraulic pressure. No sensible fact can, I think, be more 

 certain, than that the particles of this dust are brought into 

 closer proximity along the line in which the pressure is exerted, 

 and this is the line of strongest diamagnelization. If a portion 

 of the compressed mass be placed upon the end of a torsion 

 beam and the amount of repulsion measured, it will be found 

 that the repulsion is a maximum when the line of magnetization 

 coincides with the line of compression ; or, in other words, with 

 that line in which the particles are packed most closely together : 

 if the bisumth were fixed, and the magnet moveable, the former 

 would repel the latter with a maximum force with the line of 

 compression parallel to the direction of magnetization : it is a 

 stronger diamagnet in this direction than in any other. Cubes 

 of bismuth, which, in virtue of their crystallization, possessed a 

 line of minimum magnetization, have been placed in those moulds 

 and pressed closely together in the direction of the said line : 

 the approximation of the particles thus effected has converted 

 the direction spoken of from one of mininunn into one of max- 

 imiim magnetization. It would be difficult for me to say how 

 many diamagnetic bodies I have submitted to compression, some 



