4 Messrs. J. Tyndall and H. Knoblauch on the 



We have examined other crystals of the same form as Ice- 

 land spar, both magnetic and diamagnetic. In all cases, the 

 former act in a manner precisely similar to the magnetic cry- 

 stal already described, while the latter behave as the diamag- 

 netic. The following are examples. 



Nitrate of Soda. — This crystal is exactly of the same form 

 as carbonate of lime, and, like it, diamagnetic. Its deport- 

 ment is in every respect the same. A rhombus cloven from 

 the crystal and suspended horizontally between the poles, sets 

 its longer diagonal axial. Suspending the full crystal between 

 the poles, with its optical axis horizontal, on exciting the mag- 

 net this axis sets itself equatorial. 



Breunnerite. — This is a crystal composed principally of 

 carbonate of lime and carbonate of magnesia, but containing 

 a sufficient quantity of the carbonate of iron to render it mag- 

 netic. Suspended in the magnetic field, the optical axis stands 

 from pole to pole. 



Dolomite. — In this crystal a portion of the lime is replaced 

 by protoxide of iron and protoxide of manganese, which in- 

 gredients render it magnetic. The optical axis stands from 

 pole to pole. 



Carbonate of Iron. — In the cases cited, the substitution of 

 iron for calcium was partial; in the case before us it is com- 

 plete. This crystal differs in nothing, save in the energy of 

 its action, from the magnetic crystals already described. If 

 a full crystal be hung between the poles, with its optical axis 

 horizontal, on closing the circuit and sending a current round 

 the magnet, the said axis turns strongly into the axial line, 

 vibrates through it quickly for a time, and finally comes to 

 rest there. If a thin rhombus be cloven from the crystal and 

 suspended from one of its obtuse angles with its parallel faces 

 vertical, it will set itself exactly equatorial. In this case it 

 is easy to see that the horizontal projection of the optical 

 axis, which passes through the obtuse angle of the crystal, 

 stands axial. Hung from its acute angle, the rhombus takes 

 up an oblique position, making a constant angle with the line 

 joining the poles. To this position, if forcibly removed from 

 it, it will invariably return. The position may be either right 

 or left of the axial line; but the angle of obliquity is always 

 the same, being the angle which the optical axis makes with 

 the face of the rhombus. Hung from the obtuse angle the 

 obliquity is nothing — from the acute angle it is a maximum ; 

 the rhombus is capable of all degrees of obliquity between 

 these extremes, the optical axis in all cases standing exactly 

 from pole to pole. 



Oxide of Iron,— The above phenomena are exhibited even 



