Formation of Compound or Twin Crystals. 293 



theory, we may account for numerous other facts connected with 

 crystals. 



We perceive from the theory some reason for the fact that dissim- 

 ilar faces of a crystal are unlike in their lustre and cleavage, and at 

 times in hardness and color, they owing their peculiarities to the ac- 

 tion of dissimilar axes. 



We understand why crystalhzations in veins are usually fibrous, 

 the power of attraction in the sides of the vein causing the addition 

 of particles principally in lines perpendicular to these sides. For 

 the same reason cubes and crystals generally, occur lengthened in 

 the direction of one or more of their axes, and are often vari- 

 ously distorted. The attraction in the direction of an axis may be 

 augmented by that residing in the rock supporting the crystal, and 

 thus give rise to long and slender forms, or the attraction may be di- 

 minished by a similar cause and produce unusually short crystals. 

 The kind of pole (whether north or south,) that attaches itself to 

 the rock will depend on the polarity of the rock at the commence- 

 ment of the crystallization. On the same principle, we understand 

 why the introduction of a solid into a liquid about to crystallize, 

 may commence or accelerate the process. 



From it also seems to be at least in part apparent, the cause of 

 the curious acoustical phenomenon noticed by Savart, in connection 

 with Crystals of Quartz,* that is, that the tone and acoustic figures 

 obtained by striking three alternate faces of the terminating pyramid, 

 differ from those obtained on the other three, a fact which enabled 

 him to determine the primary planes of the crystal. In one in- 

 stance, the crystal was struck in the direction of the Crystallogenic 

 axes, in the other, at points equidistant between them. This ought 

 not to have occurred had there existed in the crystal four crystallo- 

 genic axes — those of the Hexahedral prism. Fig. 8. 



By the theory is also aflTorded a probable explication of the facts 

 arranged under the head of Isomorphism and Plesiomorphism. The 

 ability of one element or compound to replace another in a series of 

 combinations, without changing the primary form, depends on their 

 similar crystallogenic relations, (or possibly electric relations) incon- 

 sequence of which the same arrangement of the axes takes place in 

 the different compounds. 



* See Brewster's Ed. Journal, Vol. I, new series, p. 144, in an article entitled 

 Researches on the Elasticity of regularly crystallized bodies. By JM. Felix Savart. 



