and their relation to Crystalline Form and to the Optic Axes. 453 



time is homogeneous and essentially (not by accidental mixture) 

 magnetic, at another time homogeneous and diamagnetic, the 

 attraction of the axis must be changed into repulsion, and vice 

 versa. 



14. As both ends of a thin iron bar become poles, we are 

 hereby furnished with a simple means of representing the de- 

 portment of the crystals in the magnetic field. A prism of 

 tourmaline will be imitated in this way by an indifferent bar of 

 brass at right angles, to which a number of little magnetic bars 

 are fixed. A prism of staurolite will be completely imitated 

 by fixing the little magnetic bars so that they shall be equally 

 inclined to the axis of the brass bar ; the directions of the former 

 indicate the two magnetic axes of the crystal. 



15. The mathematical theory assumes a different attraction of 

 the mass of the ci-ystal in different directions ; and hence the 

 experiments communicated in (7), which show no perceptible 

 difference in this respect, present at first sight a difficulty. But 

 it is easy to see that in the neighbourhood of the poles the rela- 

 tive difference may be weaker or may even disappeai", while at 

 greater distances it may decidedly exhibit itself. Holding fast 

 by the way of representing the matter given in the last paragraph, 

 if we suppose the little magnetic bars shortened until their dimen- 

 sion in the direction of the axis exceeds but little the dimension 

 perpendicular thereto, in the crystal thus imitated the magnetic 

 induction will be in the direction of the axes, but near the poles 

 the common magnetic induction will take place. 



16. Messrs. Tyndall and Knoblauch, in the memoirs referred 

 to, of later date, have arrived at results very nearly similar to 

 those mentioned in (12) and (13) . Their reasoning would naturally 

 have been different had my memoir of December 1849 been 

 known to them. In consequence of my new way of viewing the 

 subject, many of my old opinions must be modified. This point, 

 however, will be more suitably treated after the communication 

 of the experimental investigations which myself and Dr. Beer 

 have made together, 



AMiile in many respects the views of the above-named phy- 

 sicists agree with mine, their fundamental view is different. The 

 theory of Faraday, who first observed the attraction of the mag- 

 netic axes, and defined these axes (magnecrystallic axes) as stand- 

 ing perpendicular to the surfaces of cleavage (in bismuth, anti- 

 mony, arsenic, sulphate of iron, and sulphate of nickel), has been 

 also adopted by them ; l)eing merely thereby modified, that they 

 introduce the opposite deportment of magnetic and diamagnetic 

 crystals as a new portion of tlic subject. To decide how far the 

 pha;uomeua coincide with this view, belongs to the discussion of 

 the experiments. From the first, however, it would appear that 



