296 Formation of Compound or Twin Crystals. 



The expansion of water previous to freezing admits of an easy 

 explanation. In the fluid state the molecules would assume that 

 compact position which the attraction of gravitation and a general 

 mutual attraction would give them. But at 40°, F., commencing 

 to act in obedience to the axes of attraction, which they are just as- 

 suming at the same time with their ellipticity, they would combine 

 by their opposite poles merely, (see Figs. 9 and 2,) and conse- 

 quently a greater space would be required to contain them. 



The facts in Optics connected with the Crystalline Solids are per- 

 fectly consistent with the theory adopted. When the molecules of 

 a crystal are spheres, a solid all of whose sections are circles, there 

 is no axis of double refraction ; when the same are ellipsoids of rev- 

 olution, a solid with its vertical section an ellipse but its horizontal a 

 circle, there is one axis of double refraction. Such is the case in 

 Right Square Prisms and the Rhombohedrons. When they are el- 

 lipsoids not of revolution, in which both the two sections, the verti- 

 cal and horizontal, are ellipses, there are two axes of double refrac- 

 tion. This is the case in the right and oblique, rectangular, rhom- 

 bic and rhomboidal prisms. 



The application of this theory to Natural Philosophy might be 

 continued to considerable length. This subject, however, and also 

 the bearing of the principles on facts in Chemistry, I defer for the 

 present. They may form the subject of a future communication. 



Continuation on the Formation of Compound Crystals ; by 

 James D. Dana. 



Read before the Yale Nat. Hist. Society, April 6th, 1836. 



In the communication on the subject of twin crystals, read before 

 this Society at its last session, the structure of the stellated groups 

 of crystals and doubly compounded forms generally, were stated to 

 be owing to a repetition of the composition on one or more of the 

 similar parts of a molecule. This is true"; still the situation of the 

 molecules in these compound nuclei, is in some instances not imme- 

 diately apparent. I therefore propose to explain a few of them, 

 particularly those which occur as Compounds of Right Rhombic 

 Prisms, they being among the most complex in their internal struc- 

 ture, although not perhaps as complicated as some others in their 

 exterior. I would also make some additional remarks on the form- 

 ation of geniculated crystals, which in the former communication 



