76 INFLUENCE OF FORCE ON PART i. 



A sphere of amorphous substance freely suspended 

 under magnetic influence is indifferent, that is to say it 

 has no tendency to set one way more than another ; but 

 a sphere cut out of a crystal whether magnetic or dia- 

 magnetic, is more powerfully attracted or repelled in 

 one direction than in any other, which shows a connection 

 between the magnetic forces and crystalline structure. 



Crystals of carbonate of iron and carbonate of lime 

 are isomorphous, that is, they have exactly the same 

 c^stalline form, but the carbonate of iron being highly 

 magnetic is most powerfully attracted in the direction 

 of its greatest optical axis which therefore sets axially, 

 that is, in the line of magnetic force ; while the principal 

 optic axis of the carbonate of lime, which is diamagnetic, 

 is most powerfully repelled and therefore sets equato- 

 rially. In both cases the antithetic forces follow the 

 same law of decrease in intensity from the greatest 

 optical axis to the least. 



A bar of soft iron sets with its longest dimensions 

 axially, but a bar of highly compressed iron-dust, whose 

 shortest dimensions coincide with the line of pressure, sets 

 equatorially, because it is most powerfully attracted in 

 the line of greatest density. A bar of bismuth sets 

 equatorially, but a bar of highly compressed bismuth 

 dust, whose shortest dimensions coincide with the line 

 of pressure, sets with its length axially, because it is 

 most strongly repelled in the direction of its greatest 

 density. Hence the action of magnets upon matter is 

 most powerful in the line of maximum density, the force 

 being attractive or repulsive according to the kind of 

 magnetism possessed by the atoms. It follows there- 

 fore that the density is greatest in the line of the 

 principal optical axis, and gradually decreases to the 

 least optical axis, where it is a minimum. 



The position which crystals take with regard to the 

 magnetic force depends also upon their natural joints of 



