Magneto-optic Properties of Crystals. 9 



deportment. After the washing and scouring process, the 

 finest and most transparent crystals we could procure con- 

 firmed the first experiments of M. PI ticker, and therefore 

 contradict, the new modification of his law. It is almost in- 

 credible how slight an impurity is sufficient to disturb the 

 action of this crystal. A specimen with smaller crystals at- 

 tached to it, or growing through it, is suspicious and ought 

 to be rejected. Clear isolated crystals are alone suitable. 

 We must remark that a fine cube, with faces half an inch 

 square, suspended with the optic axis horizontal, showed no 

 directive action ; either one or the other of the diagonals set 

 itself from pole to pole, though the axis ran parallel to four of 

 the faces. 



As far as it has been practicable, we have cut and cloven, 

 and examined the optical properties of the crystals which have 

 passed through our hands ourselves, testing, in every possible 

 case, the results of others by actual experiment. Most of the 

 crystals in Brewster's list have been gone through in this 

 way. Iceland spar, quartz, mica, arragonite, diopside, lepi- 

 dolite, topaz, saltpetre, sugar, sulphate of zinc, sulphate of 

 magnesia, and others have been examined and verified. In 

 two cases, however, our results differed from the list, these 

 being sulphate of nickel and borax. A prism of sulphate of 

 nickel was suspended from its end between the poles ; on ex- 

 citing the magnet it took up a determinate position. When 

 it came to rest, a line parallel to the magnetic axis was 

 marked thereon, and a plate taken from the crystal parallel to 

 this line and to the axis of the prism. Such a plate, ground 

 thin, exhibited in the polariscope a pair of very beautiful 

 ring systems. The ring systems of borax were found in a 

 similar manner. The middle line, therefore, in both cases 

 stood equatorial, and, according to the list, would contradict 

 the law of M. Piiicker, for both are there set down as positive. 

 A careful examination with circular polarized light \ed us to 

 the opposite conclusion. We thought it worth while to send 

 specimens of each to Berlin, so as to have them examined by 

 Professor Dove, the author of the method by which we ex- 

 amined them. The crystals have been returned to us with 

 a note certifying that they are negative, and thus confirm- 

 ing our observations. This certificate has reached us in the 

 form of a private note, but we believe Professor Dove will 

 not charge us with imprudence for thus availing ourselves of 

 such a high authority as his opinion confessedly is in optical 

 matters. 



Yellow Ferrocyanide of Potassium. — This crystal does not 

 stand in the list of Brewster, and we have sought for it in 



