Magneto-optic Properties of Crystals. 23 



to its axis ; topaz perpendicular to its axis. The planes of 

 cleavage, therefore, stand in both cases equatorial, thus agree- 

 ing with sulphate of zinc and sulphate of magnesia*. 



Where do these facts point? A moment's speculation will 

 perhaps be allowed us here. May we not suppose these cry- 

 stals to be composed of layers indefinitely thin, laid side by 

 side, within the range of cohesion, which holds them together, 

 but yet not in absolute contact? This seems to be no strained 

 idea; for expansion and contraction by heat and cold compel 

 us to assume that the particles of matter in general do not 

 touch each other; that there are unfilled spaces between them. 

 In such crystals as we have described, these spaces may be 

 considered as alternating with the plates which compose the 

 crystal. From this point of view it seems very natural that 

 the magnetic laminae should set themselves axial, and the dia- 

 magnetic equatorial ; for in crossing transverse to the cleavage, 

 the respective forces would encounter the obstacle presented 

 by the intervening spaces; while in the direction parallel to 

 the cleavages no such obstacle existsf. 



We have a very fine description of sand-paper here. The 

 sand or emery on the surface is magnetic, while the paper 

 itself is comparatively indifferent. By cutting a number of 

 stripes of this paper, an inch long and a quarter of an inch 

 wide, and gumming them together so as to form a parallelo- 

 piped, we have a model of magnetic crystals which cleave 

 parallel to their axes ; the layer of sand representing the mag- 

 netic crystalline plate, and the paper the intermediate space 

 between two plates. For such a model one position only is 

 possible between the poles, the axial. If, however, the paral- 

 lelopiped be built up of squares, equal in area to the cross 

 section of the model just described, by laying square upon 

 square until the pile reaches the height of an inch, we have a 

 model of those magnetic crystals which cleave perpendicular 

 to their axes. Such a model, although its length is four times 

 its thickness, and the whole strongly magnetic, will, on closing 

 the circuit, recede from the poles as if repelled, and take up 



* Topaz possesses other cleavages, but for the sake of simplicity we have 

 not introduced them; more especially as they do not appear to vitiate the 

 action of the one introduced, which is by far the most complete. 



■\ In these speculations we have made use of the commonly received 

 notion of matter. Mr. Faraday, for reasons derived from electric conduc- 

 tibility, and from certain anomalies with regard to the combinations of 

 potassium and other bodies, considers this notion erroneous. Nothing, 

 however, could be easier than to translate the above into a language agree- 

 ing with the views of Mr. Faraday. The intervals of space between the 

 laminae would then become intervals of weaker force, and the result of 

 our reasoning would be the same as before. 



