362 PLUCKER ON THE REPULSION OF THE OPTIC AXES 



that half vibrathig in the line of the apices of the poles becomes 

 placed perpendicularly to this line. 



26. The following crystals were very decidedly proved to be 

 those the substance of which was magnetic, and which, in con- 

 sequence of their form, when suspended in the line of the apices 

 of the poles, under the influence of the magnetic attraction of the 

 poles of the electro-magnet, arranged themselves like the tour- 

 maline, in this line, but when raised or lowered, after a rotation 

 of 90°, assumed an equatorial position : — 



1. An opake and perfectly crystallized piece of quartz from 

 Hagen, the longitudinal and axial dimensions of which were 

 10 millim. 



2. A square octahedron of zircon, with truncated angles and 

 edges, from Siberia. 



3. A six-sided crystal of beryl ivom. Siberia, 41 millim. in 

 length and from 11 to 13 millim. in thickness. 



4. Two yellowish-green transparent crystals of emerald, one 

 of which was 27 millim. long and 14 millim. thick, the other 

 much larger, and weighing several hundred grammes. 



5. A black idocrase from Siberia, crystallized in perfect square 

 prisms, the edges truncated, the angles acute, and one of the 

 apices truncated. 



6. A large corundum. 



27. I found two crystals, which were strongly magnetic, and 

 which could not be removed from the axial position, in which 

 the magnetism fixed them, even by the removal of the poles. It 

 is worthy of remark, that both these crystals exhibited magnetic 

 polarity^. It appears to me probable, that by using a more 

 powerful current, which would allow of a greater separation of 

 the poles, these crystals, overcoming the magnetism of the sub- 

 stance, would also have become arranged equatorially, which 

 they would probably have done if I had been able to contract 

 their longitudinal and axial dimensions. 



These two crystals were the following : — 



1. An opake brownish crystal oi pinite from Auvergne, a regu- 

 lar six-sided prism, 12 miUim. long and from 6 to 7 naillini. 

 thick. 



2. A small crystal of sapphire. 



28. After the experiments which have now been detailed, it 



* One of the plates of tourmaline above mentioned was also polar, but 

 slightly so only, and in a direction which did not coincide with the axis. 



