J. D. Dana on Cohesive Attraction. 



373 



9. Similar parts of crystals, with a single class of exceptions, 

 are similarly modified. "Now as the similar parts are those, simi- 

 larly situated as regards like axes, it follows that 



IX. 



»/ 



taneously undergo this variation as regards the attraction. 



10. In the excepted cases just alluded to, only half the similar 

 parts are modified alike. In the cube of boracite, only half the 

 angles have similar secondary planes (fig. 15) ; in pyrites it is usual 

 to find only one of the two beveling 

 planes in fig. 8, on each edge, as in 

 fig. 14; it is an alternate one through- 

 out, so that the form is still symmet- 

 rical, and this is uniformly true. The 

 right and left handed quartz are other 



Fig. 14. 



Fig. 15. 



examples. Consequently, 



if a molecule on opposite sides of 



Fig. 16. 



a pole undergo a different amount of variation of 

 this takes place symmetrically with regard to all the poles. 



11. In the formation of a cube with truncated edges, the cube 

 is not finished out on the edges. There is therefore a diminution 

 of the force of attraction in the line of the primary axes, since 

 these axes fail of completing the cube. Hence, 



XI. If the state of the attraction which produces aprymm y vmuo 

 or prism is considered its normal state, when secondary planes 

 are produced there is a decrease of force in the direction of the 

 principal axes, and this decrease is in some simple ratio. 



12. In the enlarging cube, the mole- 

 cules are added in planes of increasing 

 breadth, as in this way only would the 

 form continue to be a cube. If we 

 consider the case, we find that the cen- 

 tral molecule attracts a molecule by 

 each of its poles, and also simultane- 

 ously the added molecules act by their 

 lateral axes to complete the plane (fig. 

 1). We observe in the sectional view 

 (through the centre) in fig. 16, when 

 A by its axes unites with four B's, the 

 Bs, simultaneously, while in the act of union, umte * th B^B 

 &, B' ; and thus the square form is retained. This is a simple 



statement of the process. - ' nc i _.. _ rt 



If now when the B's are uniting, their lateral axes d no ^act 



at the same time, then the forming cube will have the edges trim- 



