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



The facts observed by M. Matteucci seem to me to be a simple 

 corroboration of this deduction*. 



The third objectiou is directed against an imaginary case, " si 

 I'ou cousidere les plans de clivage et la ligne de compression 

 comme jouissant des meme proprietes." It must be evident that 

 a crystal like bismuth, possessing a number of cleavages of un- 

 equal values, cannot be compared in all respects with a body 

 which has suffered pressure in one direction only. I have no 

 doubt whatever, that, by a proper application of force in differ- 

 ent directions, a compressed mass might be caused to imitate to 

 perfection every one of the actions exhibited by crystallized bis- 

 muth. Indeed I would go further, and say, that I shall be 

 happy to undertake to reproduce, with bismuth powder, the 

 deportment of any diamagnetic crystal whatever that M. Mat- 

 teucci may think proper to name. 



In looking further over M. Matteucci's instructive book, I 

 find another point alluded to in a manner which tempts me to 

 make a few i-emarks in anticipation of a fuller examination of the 

 subject. The point refers to the reciprocal action of the particles 

 of magnetic and diamagnetic bodies. It is easy to see, that if 

 the attraction of a bar of iron varies simply as the nvimber of 

 the particles attracted, then, inasmuch as the weight of the body 

 varies in the same ratio, and the moment of inertia as the weight, 

 the times of oscillation of two masses of the same length, but 

 possessing different numbers of attracting particles, must be the 

 same. Coulomb indeed mixed iron filings with wax, so as to 

 remove the particles out of the sphere of their mutual inductive 

 action, and proved that when needles of equal lengths, but of 

 different diameters, were formed from the same mixture, the 

 duration of an oscillation was the same for all. From this he 

 inferred that the attractive force is simply proportional to the 

 number of ferruginous particles ; but this could not be the case 

 if these particles exerted any sensible reciprocal action, either 

 tending to augment or diminish the induction due to the direct 

 action of the magnet. On account of such a mutual action, two 

 bars of soft iron, of the same length, and of different diameters, 

 have not the same time of oscillation. 



In examining the question whether the particles of diamag- 

 netic bodies exert a similar reciprocal action, M. Matteucci fills 

 quills of the same length, and of different diameters, with pow- 

 dered bismuth, and finds that there is no difference between the 

 duration of an oscillation of the thick ones and the slender ones ; 

 from this he infers that there can be no reciprocal action among 

 the particles of the bismuth. 



* For a more complete examination of this subject see the Appendix to 

 this paper.— J. T., May 1855. 



