278 Mr Haidinger on the Parasitic Formation of Minerals , 



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 rhom- 

 bohedral individuals in the direction of their axis, and perpen- 

 dicular 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. M. Beudant, I believe, first called the at- 

 tention of naturalists to the fact, that the whitish coat with 

 which Barley-sugar is covered, when it is kept for some time, 

 shows a fibrous structure, the direction of the fibres being per- 

 pendicular to the surface of the specimens. When the decom- 

 position, which here only affects the form and arrangement of 

 particles, is allowed to proceed farther, crystals of sugar-candy 

 are formed in the space formerly occupied by a homogeneous 

 mass which presented the most perfect conchoidal fracture, 

 and not a trace of crystalline structure. * 



II. Changes dependent upon the presence of Water. 



Hauy^s Chaux sulfatee epigene, is a substance familiar to 



every mineralogist. His view of it is perfectly correct : it 



was Anhydrite, and is changed into Gypsum, by combining 



with a portion of water. The original cleavage planes, still 



* Barley-sugar has no double refraction, and has the same general relation 

 to sugar that melted quartz (which has no double refraction) has to crystal- 

 lized quartz. When by the attraction of moisture from the air it is gra- 

 dually dissolved, the particles of sugar liberated from their constrained 

 position form doubly refracting crystals of sugar, or the whitish fibrous 

 coat mentioned in the text. Hence we are here presented with the sin- 

 gular fact of a substance gradually passing from a simply refracting mass 

 to a doubly refracting crystal. — Editor. 



