PLUCKER ON THE THEORY OF DIAMAGNETISM. 357 



but which are often found to contain an admixture of iron. I 

 will here refer only to tinstone and wernerite, in which the 

 phaenomena spoken of only exhibit themselves when the sub- 

 stance is magnetic, but then in such a high degree that the 

 first-mentioned crystal is directed by the magnetism of the 

 earth. 



37. When the foreign magnetic substances are irregularly 

 distributed through the mass of a crystallized body, for example, 

 through a crystal of tourmaline where the unequal distribution 

 is indicated by a difference of colour, the action of the magnet 

 will necessarily be distributed and a direction may be given to 

 the crystal, which, without it, would show no direction what- 

 ever*. 



Finally, a trace of iron, which appears under circumstances 

 which deserve mentioning, may give rise to error. To cite a 

 distinct case, let a plate or prism of rock crystal be taken, both 

 of which, suspended between the poles, turn in consequence of 

 the ordinary diamagnetic action into the equatorial position. 

 If the length be shortened so as to annul the effect of form, the 

 shortening being effected by a hammer, we generally find a trace 

 of iron attached to the crystal through contact with the hammer. 

 This iron will not be at all detected, if we bruise the crystal 

 close to a powerful magnetic pole, but when placed at a suf- 

 ficient distance, the magnetic attraction of the iron overcomes 

 the diamagnetic repulsion of the mass. This follows from the 

 law deduced in 5 f. Without this law, we must of necessity 

 conclude from the motion of the crystal between the poles, that 

 in the case of the prism the optic axis is attracted, while in the 

 case of a plate cut perpendicular to the optic axis, repulsion of 

 the latter takes place. At different times I have fallen into 

 this two-fold error ; the first time, considering our knowledge of 

 the subject, I was certainly not to blame. The above phaeno- 



* There is the strongest reason for supposing these remarks applicable to 

 the crystals of calc-spar examined by M. IMiicker, on which he bases his con- 

 clusions that there are diamagnetic crystals of this class whose optic axes are 

 attracted, and magnetic specimens whose optic axes are repelled. See Phil. 

 Mag.— J. T. 



t The law in No. 5 is at variance with the above fact, but the contradiction 

 is doubtless due to an error in printing. — J. T. 



