130 Prof. Tyndall on the relation of Diamagnetic Polarity 

 the state of things shown in fig. 6. It is manifest, for the 



Fig. 5. Fig. 6. 



N 



N 



Fis. 7. 



N 



mechanical reasons ah-eady assigned, that the crystal, in both 

 these cases, must recede from the pole. 



Substituting for the sphere of sulphate of iron a sphere of 

 bismuth with its magnccrystallic axis cd, fig. *], perpendi- 

 cular to the strip of card, the 

 bismuth is found to approach the 

 pole when the magnet is excited. 

 The line ab perpendicular to 

 that named the magnecrystalhc 

 axis, has been shown by Mr. Fa- 

 raday to be that of greatest dia- 

 magnetic intensity ; the mass is 

 therefore under the influence of 

 forces precisely similar to those 

 acting on the carbonate of lime 

 in fig. ] . A residual couple, as 

 denoted by the arrows, will act at the extremities of the line ab. 

 The absolute repulsion of a in the field of force here assumed, 

 does not differ much from the absolute attraction of b ; but the 

 latter force acts at the end of a much longer lever, and conse- 

 quently the sphere is drawn towards the excited pole. I cannot 

 help remarking here upon the severe faithfulness with which these 

 results are recorded, and on the inestimable value of such records 

 to scientific progress. The key to their solution being once 

 found, the investigator may proceed confidently to the applica- 

 tion of his principles, without fear of check or perplexity arising 

 from the imperfection of his data. 



In all these cases we have assumed that the magnetic force- 

 diminishes slowly as we recede from the pole, for this is essential 

 to the production of the eflfects. The exact expression of the con- 

 dition is, that the advantage due to the proximity of the part of 

 the mass nearest the pole, must be less than that arising from the 



