372 



J. D. Dana on Cohesive Attraction. 



netic polarity, and its varying in direction 

 with the changes of temperature. There are evidences of other 

 modifications in the condition of the attracting force, which we 

 now consider. 



7. The same species of matter often presents a variety of forms 

 in its crystals, built up on a fundamental type. For example, 

 when the type is a cube (fig. 6), the species may occur as cubes; 

 or as cubes with the edges truncated (fig. 7) ; or with the edges 

 beveled (fig. 8) ; or with the angles replaced (figs. 9, 10), and so 



Fig. 6 



Fig. 7. 



Fig. 8. 



Fig. 9 



Fig. W. 



on. 



If then a certain state of the attraction in a molecule will 

 produce the primary cube, some variation from this state is ne- 

 cessary to produce another form, and a different variation for 

 every different secondary plane. Consequently, 



)f cohesion in molecules of 

 force j but admits of variations of 



s. Lhe secondary planes of crystals, as related to the axes 

 the fundamental form, have fixed simple ratios. The plane trim 



* 



of 



Fig. 11. 

















j i i 







--"l 





( J 







ibh 













1 1 



























1 















i 







Fig. 12. 



rt 









Tl 













__ 



1 















i — 















1 







I 1 













— * • * \**&* -*--*-;• v^moi uiaues i a.5 ill li^. o 



1:2 (fig. 12), 1:3, 1:4, 2:3, and other 



eating the edge (fig. 7) referred to the axes, has the simple ratio 

 of 1 : 1 (fig. 11). Other planes (as in fig. 8) may have the ratio 



FiK- 13. 



simple ratios. Planes on the angles (figs. 

 9, 10) referred to the three axes, may have 

 the ratios (indicating their positions) 1:1:1 



(fig. 13), 1:2:2, 1:3:3, 1:2:4,2:3:6, 

 3:5: 15, and so on. Hence, — 



fill The variations which the attrac- 

 tion of cohesion undergoes, take place ac- 

 cording to some simple ratio. 



