depending on their Internal Changes* 279 



discoverable in the white, opaque, and faintly glimmering mas- 

 ses, would give no argument of weight for uniting the two spe- 

 cies into one ; and yet considerations of this kind have induced 

 some mineralogists to join blue copper and malachite into one 

 species. These traces are not, however, produced by cleavage, 

 which is the mere tendency of the particles of Anhydrite to se- 

 parate more easily in certain directions than in others; but 

 they are owing to actual fissures in the direction of the planes 

 of cleavage, visible in every fresh or not decomposed variety 

 of the species. On these fissures, and still more distinctly on 

 some larger irregular ones traversing the masses, distinct cry- 

 stals of gypsum are formed. Of the latter, I have seen several 

 specimens from Aussee in Stiria, in the collection of Gratz. 

 The decomposed individuals are much smaller in these than 

 in the varieties from Pesay in Savoy, described by Haiiy. 



The absorption of water from the atmosphere, in saline sub- 

 stances, is usually attended with a solution of the latter in the 

 water so attracted ; that is to say, they deliquesce, and change 

 their form, in passing from one state of aggregation into ano- 

 ther. The reverse also very frequently takes place. Crystals 

 effloresce 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, 

 oxidation, &c. Among a great many facts of a similar nature, 

 observed by Prof. 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 ano- 

 ther, by the application of heat, and a loss of water thereby 

 occasioned. He exposed crystals of the ordinary hydrous 

 protosulphate of iron, immersed in alcohol, to a degree of tem- 

 perature equal to the boiling point of that liquid. Decompo- 

 sition ensued, though the external shape of the crystals re- 

 mained 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. 



