ORDER IV. PRISMATIC EPSOM-SALT. 4 



the particles are very delicate, the lustre becomes 

 pearly. Pulverulent. 



OBSERVATIONS. 



1. In the combinations occurring among the crystals of 

 this species, some of the faces of the pyramid P (I) are very 

 often irregularly enlarged at the expence of others. Since 

 this enlargement sometimes takes place in the alternating 

 faces of the pyramid, it has been supposed by several crys- 

 tallographers, and among others by Messrs HAUY and 

 WEISS, to follow a certain constant rule. Considering the 

 vertical prism as rectangular, and completing what otherwise 

 might have been wanting in the forms, they have thus re- 

 presented the series of crystallisation of prismatic Epsom- 

 salt as being hemi-pyramidal with inclined faces, a supposi- 

 tion rendered sufficiently improbable, as far as regards the 

 pyramidal system, if we only attend to the position of the 

 single perfect face of cleavage. Professor MITSCHERLICH, 

 however, has lately observed, that of some secondary faces, 

 as *, *, and , , which belong to (Pr) 3 and (Pr) 3 , only the 

 alternating ones appear in the combinations, as represented 

 in Fig. 1G8. ; and according to this observation, the Cha- 

 racter of Combinations has been stated above as hemi-pris- 

 matic with inclined faces.* 



2. According to VOGEL, the prismatic Epsom-salt con- 

 sists in its natural state of 



Water 48-0. 



Sulphuric Acid 33-0. 



Magnesia 18'0. 



It is Mg s' 2 4- 14 Aq, or 16-6 Magnesia, 32-2 Sulphuric 

 acid, and 51'2 Water, according to MITSCHERLICH. It 

 dissolves very easily in water, deliquesces before the 



* Professor Mitscherlich has kindly communicated to me 

 this interesting fact, which he has likewise observed in pris- 

 matic Vitriol-salt, and in the crystals of sulphate of nickel. H. 



