.' 



J. D. Dana on Cohesive Attraction. 



371 



come thus geniculated simultaneously at both extremities, while 

 they were enlarging, evincing that 



IV". The polarity of molecules may be reversed by extrinsic in- 

 fluence* 



5. Twin crystals have one half of the crystal in a reverse po- 

 sition from the other.f This may be imitated by cutting a crys- 

 tal in halves, and after a semirevolution of one half, applying the 

 parts again together ; an oblique crystal will thus have a reen- 

 tering angle, as in fig. 5. If in fig. Fig. 4. 

 5, A be the position of the nucleal 

 molecule for one half, B must be 

 that for the other. The mode of 

 aggregation for a simple crystal is 

 that shown in fig. 4. The attrac- 

 tion between the two molecules 

 brought them together in either 

 case. But in one the position of » 

 the molecules when about uniting 

 was favorable for the direct union 

 ln fig- 4 ; and in the other, the two 



w ere in opposite positions, yet so close in proximity that union 

 took place by the adjacent poles without allowing of the change 

 °f position necessary for direct union. In the former, the same 

 P°les of the vertical axes, are in the same direction ; and in the 

 la tter, they are in opposite directions. There could not be such 

 an inversion of the molecules, if the axes were a result of the 

 ac t of union. Hence, — 



t V. The axes and polarity of cohesive attraction in solidifica- 

 tion exist before the union of the molecules, instead of being a 

 consequence of that union. 



6. The forms of inequiaxal crystals vary somewhat with a 

 change of temperature ; and at certain temperatures, specific in 

 «ach case, some substances undergo abruptly a total change of 

 {°rm, both as to the direction and relative lengths of the axes. 



V 



Hence 



VI 



: 



definitely 



* 1. The axial lines of cohesive attraction, are noi inaejuu 

 feed in position, but are some way modified in direction and ft 

 h temperature. 



Thus far we have considered the general polar condition of co- 

 hesive attraction in solidification, its liability to a reversion of the 



" See an article by the author, in the American Journal of Science, for 1836, 

 * ' «x, p . 275, and particularly p. 292; also Mineralogy, the chapter on Lrys- 



* We consider here but a single kind of twin crystals, in the briefest manner 

 P" ss 'ble, as the facts are sufficient for our deductions. See farther, Amer. Jour, of 

 ^C'ence, and Mineralogy, as just referred to. 



