450 Prof. Vlucker and T>T. Beer on the Magnetic Axes of Crystals, 



cube, the side of which measured 11 millims. The magnetic 

 axis of the crj^stal, that is, the middle Hue between the two optic 

 axes which enclosed an angle of 75° with the plane of most emi- 

 nent cleavage, was perpendicular to opposite sides A and B of the 

 cube. The other surfaces were marked with the letters C, D, 

 E and F. The cube could be fixed in a cork with a square 

 opening, the cork being fastened in a brass ring and suspended 

 from one ann of a balance. Tlie large half-anchors were laid 

 upon the large electro-magnets ; and to obtain the strongest 

 action, their rounded edges were brought within a few milli- 

 metres of each other. Close over these adjacent edges the crystal 

 of sulphate of iron was equilibrated and then the magnetism 

 excited. The weight necessary to raise the cube was obtained 

 by shaking sand upon the opposite scale-pan until the attraction 

 was overcome^ and weighing the sand afterwards. The diflferent 

 surfaces (with the exception of A) were in this manner one after 

 the other separated from the adjacent half-anchors. The follow- 

 ing table contains the amount of attractive force exerted in each 

 particular case, and expressed in grammes : — 



Surface in contact. Attraction. 



B 5-37 



•78 



^ \4-80 



^ \5-43 



E 4-90 



' \5-46 



B 5-42 



" The last experiment is a repetition of the first, by which it 

 was proved that the action of the circuit was constant. The 

 double determinations made with the same surfaces resting on the 

 magnet show, by their coincidence, the accuracy of the method 

 of weighing. The deviations consequent on changing one sur- 

 face for another are not so much to be attributed to the imper- 

 fection of the cube, as to the fact of its setting itself differently 

 upon the anchors, which was a consequence of the action of the 

 magnetic axis. The mean attraction parallel to the latter amounts, 

 according to this, to 



5"395 grms., 



and the mean attraction perpendicular thereto, to 



5"17 grms., 

 which shows a balance of nearly -^r in favour of the axis. 



" A second series of experiments gave au exactly similar result. 



