of Mineral Species. 79 



their form, in passing from one state of aggregation into another. 

 The reverse also very frequently takes place. Crystals efflo- 

 resce by losing their water, and are converted into a loose mass 

 of a pulverulent consistency, which retains the original shape, 

 but readily gives way to the pressure of the finger, and falls 

 into powder. Prismatic glauber-salt, prismatic natron-salt and 

 others, are familiar examples of this change. Many more 

 might be quoted of the numerous cases of what chemists call 

 spontaneous decompositions, depending upon loss of water, oxi- 

 dation, &c. Among a great many facts of a similar nature, ob- 

 served by Professor MITSCHERLICH, during my stay in Berlin in 

 the winter of 1825, I shall mention here a very interesting one, 

 in which a crystallized substance was formed within another, by 

 the application of heat, and a loss of water thereby occasioned. 

 He exposed crystals of hemi-prismatic vitriol-salt, the ordinary 

 hydrous protosulphate of iron, immersed in alcohol, to a degree 

 of temperature equal to the boiling point of that liquid. De- 

 composition ensued, though the external shape of the crystals 

 remained unchanged. On being taken out of the liquid, and 

 broken, each of them was found hollow, and presented a geode 

 of bright crystals, deposited on the planes of the original ones. 

 The crystals had the form of low eight-sided prisms, belonging 

 to the prismatic system, and were proved by analysis to contain 

 exactly half the quantity of water which is required in the mix- 

 ture of the original species. 



III. Changes in Minerals containing Copper. 



Mineralogists are very generally acquainted with the crystals 

 from Chessy in France, having the form of blue copper, but con- 

 sisting of fibrous masses of malachite. Such varieties are found 

 in that locality, as well as perfect homogeneous crystals ; but 



