186 PHYSIOGRAPHY. 



Epistilbite Epsom Salt. 



1. Epistilbite, according to Dr. BREWSTER, is destitute of the two 

 systems of colored rings, which are visible in Heulandite. The double 

 refraction of the former mineral is also vastly greater than that of the lat- 

 ter ; it also gives the white light of fixed polarization, and exhibits at its 

 edges many orders of colors. 



2. Before the blow-pipe, on charcoal, it froths up, and forms a vesic- 

 ular enamel, but cannot be melted into a globule. In the matrass, it 

 intumesces considerably, and gives off water. Borax dissolves a great 

 quantity of it, and forms a clear globule. It is also soluble in salt of 

 phosphorus, with the exception of a skeleton of silica. With solution of 

 cobalt the enamel becomes blue. It is soluble in concentrated muri- 

 atic acid, with the exception of a fine granular residue of silica. 



3. Analysis. 



By ROSE. 



Silica , . . ' . . 58-59 



Alumina ..... 17*52 



Lime . ... . . 7-56 



Soda ..... 1-78 



Water 14-48 



4. It occurs in amygdaloidal rocks in Iceland and the Faroe Islands, 

 along with Heulandite, and at Poonah in India. 



EPSOM SALT. Prismatic Epsom-Salt. 

 JAMIE SON. 



Primary form. Right rhombic prism. M on M / = 90 

 30'. 



Secondary forms. 



1. The primary, having the terminal edges deeply re- 

 placed, so as to form pyramids at each extremity, and like- 

 wise having the acute lateral edges truncated. 



2. The same form, with the addition of tangent trunca- 

 tions of the obtuse lateral edges, and of the upper edges of 

 the pyramidal terminations. 



Cleavage, perfect parallel to the shorter diagonal of the 

 primary form ; less so, parallel with the faces formed by the 



