40 



WRITINGS OF JAMES 8MITHS0N. 



I have a very small, but very regular, crystal of the forjoa. 

 of Fig. 1. 



Fig.l 



Mg.2. 



mp = 90° 

 mt =90° 

 am = 135' 

 mb = 135"= 

 cb = 125° 

 gb = 144° 

 dm= 116° 

 fm = 153° 



By mensuration the faces a and m appear to form together 

 an angle of about 135°, and the faces c and b an angle of 

 about 125°. 



It is said in the account above quoted, that the primitive 

 form of this matter is a rectangular tetraedral prism, but 

 no proofs of this have been offered ; nor have the dimen- 

 sions of this prism been given, a circumstance of the first 



