Observations on Pseudomorphism. 87 



Galena to Sulphate of Lead. — A simple oxydation of the sul- 

 phur and lead. 



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

 sulphur forms sulphuretted hydrogen, and the iron is oxydized; 

 water then combines with the oxyd formed, (see page 78.) 



Pyrites to Red Oxyd of Iron. — Probably the same as the last, 

 except that the oxyd 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 peroxydized, 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 carbonate of 

 iron in solution, illustrates this process; carbonic acid escapes as 

 above described, and the iron falls as a hydrated peroxyd or brown 

 iron ore. It is a common laboratory experiment. 



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

 iron ore, except that the resulting oxyd does not combine with 

 water. A hydrated oxyd of lead has not been observed in nature. 



Galena to Carbo7iate of Lead. — Similar to the last, except 

 that the oxyd of lead combines with carbonic acid. 



Carbonate of Lead to Pyromorphite. — Phosphates so frequent- 

 ly result during animal decomposition, that we deem it quite 

 probable that this pseudomorph might have received its phos- 

 phoric acid from this source. 



Feldspar to Kaolin. — This change is attributed to the disposi- 

 tion 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 ele- 

 ments, and kaolin is the result — at the same time, those elements 

 capable of solution passing oif which do not enter into the com- 

 bination. 



This example affords the general theory for many pseudo- 

 morphs of the more complex mineral species ; — Some one element 

 or more, by the influence of external agents, etiters into new comn 

 binations and is removed ; this disorganizes the original com- 

 pound and leaves the remaining elements free to combine anew ; 

 such as are capable, consequently unite by their affinities either 

 alone or with water, (or other chemical agents present ;) the ex- 

 cess, if any, and soluble, passes off in solution. 



