258 J. D, Dana on Pseudomorphism. 



Bed Copper Ore to Malachite. — Combination with the car- 

 bonic acid of the atmosphere and moisture. 



Galena to Sulphate of Lead. — A simple oxidation of the 

 sulphur and lead. 



Pyrites to Brorcn Iron Ore. — By the decomposition of water 

 the sulphur forms sulphuretted hydrogen, and the iron is 

 oxidized ; water then combines with the oxide formed. ( Vide 

 p. 248.) 



Pyrites to Bed Oxide of Iron. — Probably the same as the 

 last, except that the oxide formed does not combine with 

 water. 



Carbonate of Iron to Brown Iron Ore. — From contact with 

 moisture, the surface of the carbonate of iron is dissolved ; 

 and then, on subsequent evaporation, the carbonic acid passes 

 off, the iron becomes peroxidized, and, by uniting with other 

 portions of water, forms the resulting brown iron ore. This 

 process, repeated for a period of time, changes the crystals 

 throughout. 



The evaporation of chalybeate waters, containing carbo- 

 nate of iron in solution, illustrates this process ; carbonic 

 acid escapes, as above described, and the iron falls as a 

 hydrated peroxide, or brown iron ore. It is a common labo- 

 ratory experiment. 



Galena to Minium. — Similar to the change of pyrites to 

 brown iron ore, except that the resulting oxide does not 

 combine with water. A hydrated oxide of lead has not been 

 observed in nature. 



Galena to Carbonate of Lead. — Similar to the last, except 

 that the oxide of lead combines with carbonic acid. 



Carbonate of Lead to Pyromorphite. — Phosphates so fre- 

 quently result during animal decomposition, that we deem it 

 quite probable that this pseudomorph might have received its 

 phosphoric acid from this source. 



Felspar to Kaolin. — This change is attributed to the dis- 

 position of the potash to combine with water and carbonic 

 acid. By this means the equilibrium of the compound is 

 disturbed. New attractions then begin to work among the 

 remaining elements, and kaolin is the result ; at the same 

 time, those elements capable of solution passing off which do 

 not enter into the combination. 



