of Mineral Species. 77 



of the solution remaining on the surface of the hemi-prismatic 

 crystals. Change of temperature is the only agent upon which, 

 in both cases, the change of the position of particles within the 

 solid mass depends. 



The isomorphism of zinc and magnium, is remarkably dis- 

 tinct in the regular forms, with all their peculiarities, and in the 

 cleavage, of their sulphates. But it extends even to the pheno- 

 mena, described above of sulphate of zinc. They both give 

 exactly the same results. 



The specific gravity of the hemi-prismatic species has not 

 been ascertained. It is very probable that it does not mate- 

 rially differ from that of the prismatic species, as the change 

 from one to the other takes place without producing a consider- 

 able change in the appearance of the shape of the crystals. 

 When arragonite is exposed to heat, it becomes opaque, and 

 splits violently into multitudes of small particles, previous to its 

 giving off any of its carbonic acid. It is highly probable that 

 it is thus transformed into common calcareous spar, which re- 

 quires more space to exist in than arragonite, nearly in the ratio 

 of 29 to 27, their contents of carbonate of lime being equal, and 

 no attention given to the accidental and variable contents of 

 carbonate of strontia. Perhaps the separation of the particles is 

 assisted by the unequal expansion of the rhombohedral indivi- 

 duals in the direction of their axis, and perpendicular upon it. 



I must mention here another example of the formation of 

 crystals in the place of a solid mass, consisting of the same che- 

 mical ingredients, as a product of the power of crystallization, 

 though the substance in which it occurs, is not comprised within 

 the generally received idea of a mineral. M. BEUDANT, I be- 

 lieve, first called the attention of naturalists to the fact, that the 

 whitish coat with which barley-sugar is covered, when it is kept 

 for some time, shews a fibrous structure, the direction of the 



