B4 Oil the CrysiuUiiie Furm of Iodine. 



Whether the Dewey lite be a true chemical compound, or a mechan- 

 ical mixture of the hydrate of magnesia with silica, I will not pre- 

 tend to decide. The only difference between it and the precious 

 serpentine in chemical constitution, is, that it contains one proportional 

 more of water, and is free from the accidental ingredients of lime, 

 alumine and manganese, so common in serpentine. 



On a comparison of the mineral described by Mr. Tyson, with the 

 Deweylite, the only want of coincidence is seen to lie in the property 

 of hardness and in the composition. His mineral is possessed of an 

 hardness slightly superior to ours, and contains rather more siHca and 

 water. The difference however in our results, is not greater than 

 what is found in the analysis of specimens of Serpentine from differ- 

 ent localities. 



Art. XVIl. — On the Crystalline Form of Iodine ; by Lieut. W. W. 

 Mather, Assistant Professor of Chemistry and Mineralogy at the 

 U. S. Military Academy. 



Iodine has been observed crystallized in rhomboids, rhomboidal 

 tables, and elongated octahedrons, by Sir Humphry Davy, Gay- 

 Lussac, M. A. Plisson, and others ; but we have never seen any 

 account of the angles at which the faces of the crystals incline to 

 each other.* Before seeing M. A. Plisson's notice that iodine could 

 be obtained crystallized, by " exposing ioduretted hydriodic acid," 

 Lieut. Hopkins and myself had observed it in the chemical labora- 

 tory at West Point, in a bottle of hydriodic acid, that had been for 

 one or two years exposed to a limited access of air, by the stopper 

 not fitting very closely. Some of the crystals were y\ of ^^ '""^^^ i^^ 

 length, having the lustre of the specular oxide of iron from Elba. 

 The form was generally that of an octahedron with a rhombic base, 

 having the acute lateral solid angles replaced by tangent planes . The 



*Dr. Wollaston, in the Annals of Philosophy, Vol. V, p. 237, describes iodine as 

 forming octahedral crystals, whose axes are to each other in the proportion of the 

 numbers 2, 3, and 4: also in rhombic plates, bevelled at their edges by two narrow 

 planes, inclined to each other under an angle of 120° 30', from the frequent occur- 

 rence of which he remarks, that some crystallographers may be disposed to regard 

 this rhombic plate (of which the acute angle is about 53°) as a modification of a 

 rhombic prism, whose diagonals are 2 and 4, and its height 3 :— the modification of 

 the octahedron being derivable from either M'ith equal facility. — Ed. 



