1826.] Scientific Notices — Mineralogy. 31S 



Madrid, and a pupil of M. Thenard, in honour of whom he has 

 named it. 



The forms of the crystals are easily ascertainable, but the 

 planes are too uneven to admit of accurate measurement ; the 

 planes obtained by fracture are, however, even, and the primary 

 lorm of the crystal is determinable with considerable accuracy ; 

 the cleavage is threefold, and in one direction the laminae are 

 perfectly smooth and brilliant. The primary form (fig. 1), indi- 



Fig. I. Fig. 2. Fig. 3. 



Z71 



cated by cleavage, is a right prism with a rhombic base, the- 

 angles of which are nearly 125° and 55°; taking the mean of 

 several measurements, the height of the lateral planes is to that 

 of the terminal as 13 to 15 ; the cleavage is most distinct in the:: 

 direction of the base. 



There are two varieties of the crystal ; first, the octahedrofiv 

 (fig. 2). It is formed by a decrement of two rows of molecules 

 in height, on the edges of the bases of the primary prism. The 

 octahedron is symmetrical, and very flat in the direction of the 

 small diagonal of the bases of the primary prism. Its vertical 

 section in the direction of the greater diagonal of the base is a 

 slightly acute rhomb, the smaller angle of which coincides with the 

 summit of the crystal. 



The second variety (fig. 3) is the preceding crystal, with the 

 summits replaced by a rhombic plane parallel to the bases of the 

 primary form. 



The crystals would probably be doubly refractive, but they 

 are not suflaciently transparent to admit of this point being; 

 determined. The specific gravity is nearly the same as that of 

 glauberite, viz. about 2*73. 



The chemical characters of the crystals are, that, when exposed 

 to the air, they become opaque, and the surface is covered with 

 a powder which is readily removed. According, however, to 

 M. Casaseca, this is not owing to the loss of water, but to the 

 absorption of a small quantity, for the salt is perfectly anhydrous^ 

 losing scarcely any weight by exposure to a strong heat ; and 

 this little is probably derived from the shght efflorescence at the 

 surface already noticed. Thenardite is perfectly soluble ia 

 water ; the solution when saturated is slightly alkaline. Whea 

 put into dilute sulphuric acid, it effervesces, owing to the evo- 

 lution of carbonic acid gas. Examined by the usual re-ageuts^ 



