Magneto-optic Properties of Crystals. 7 



both sides of these lines, angles equal to the angle included by 

 A and B; the first being the case if the crystal be a positive 

 one, the last if a negative one. Thence it follows, that if the 

 crystal, by any kind of horizontal suspension, should point to 

 the poles of a magnet, it is a positive one; if it should point 

 equatorially, it is a negative one*." 



In experimenting with this crystal, we have found the great- 

 est care to be necessary. Its diamagnetic force is so weak, 

 that the slightest local impurity, contracted by handling or 

 otherwise, is sufficient to derange its action. The crystals as 

 they come from the mineralogist are unfit for exact experi- 

 ment. We have found it necessary to boil those which we 

 have used in muriatic acid, and to scour them afterwards with 

 fine white sand, reduced to powder in a mortar. These pre- 

 cautions taken, we have been unable to obtain the results de- 

 scribed by M. Pliicker. We have examined five specimens 

 of topaz from Saxony, the axial dimension of some of them 

 exceeding the dimension perpendicular thereto by one-half; 

 the axis, notwithstanding, stands in all cases from pole to pole. 

 Two specimens of Brazilian topaz, the one of an amber colour, 

 the other almost as clear as distilled water, give the same re- 

 sults ; the axes of the crystals stand from pole to pole, and 

 turning round makes no difference. On a first examination, 

 some of the crystals exhibited an action similar to that de- 

 scribed by M. Pliicker ; after boiling and scouring, these 

 irregularities disappeared, and they one and all stood axial. 



One crystal in particular caused us considerable embarrass- 

 ment. Its action was irregular, and the irregularity remained 

 after the adoption of the methods described to ensure purity. 

 A splinter from one of its sides was found to be attracted, 

 a splinter from the side opposite was found to be repelled. 

 To the naked eye the crystal appeared clean and clear. On 

 examination, however, under a powerful microscope, the side 

 of the crystal from which the magnetic splinter was taken was 

 found dotted with small black particles imbedded in its mass; 

 the other side of the crystal was perfectly transparent. On 

 cleaving away the impurities, the irregularity vanished, and 

 the crystal stood as the others. 



In the letter quoted, diopside is pronounced by M. Pliicker 

 to be a positive crystal. On examination with circular polar- 

 ized light, as recommended by Dovefj we find the crystal to 

 be negative. The same method pronounces topaz positive, 

 instead of negative, as affirmed by M. Pliicker. The speci- 

 mens we have examined in this way are from Brazil and 



* Phil. Mag.,vol.xxxiv. p. 450. 



t Poggendorff's Annalen, vol. xl. pp. 457, 482. 



