258 MOLECULAR ARRANGEMENT. 



pulsion, forms a circular figure traversed" by xadiL 

 When a plate of gypsum is used instead of glass the 

 figure is found to be elliptical, and the great axis of the 

 ellipse forms a right angle with the principal crystallo- 

 graphic axis, which proves that the electricity distributes 

 itself more easily in a direction perpendicular to the axis 

 than in any other. M. Wiedmann. comes to the con- 

 clusion that crystals which possess a better conductibility 

 in the direction of the principal axis, all belong to the 

 class of negative crystals: while those which have a 

 better conductibility in the direction perpendicular to 

 the axis are positive, which indicates that the direction 

 of best conductibility for electricity is also that according 

 to which light is propagated relatively with greater 

 velocity. 



Tyndale has shown, that if gutta percha which has 

 been rendered fibrous in manufacture is cut so that the 

 fibres are in the direction of this greatest length, or in 

 a direction perpendicular to this greatest length, and 

 placed under the influence of a magnet, they direct 

 themselves equatorially. Ivory cut in the same direc- 

 tion manifests the same conditions, though both these 

 substances are diamagnetic. 



The fibrous structure, and the planes of cleavage, thus 

 determine the magnetic condition of a substance. The 

 special properties presented by crystals, in regard to 

 the action exercised upon them by magnets, is due to 

 a particular mode of grouping their particles. This is 

 also the cause of unequal dilatability, and of unequal 

 condujtibility for heat and for electricity. 



How curiously, therefore, does molecular structure 

 determine the relation of a body to any of the forms of 

 physical force ! 



We still search in the dark, and see but dimly the 

 evidences ; yet it becomes almost a certainty to us, that 

 this stone of granite, with its curious arrangement of 

 felspar, mica, and quartz, presents its peculiar condition 



