170 



The specimen which is here presented is interesting in conse- 

 quence of exhibiting such partial decomposition as permits the for- 

 mation of other ores of lead and silver in quite regular forms. 



Thus we recognize, as covering the surface, beautiful crystals of 

 carbonate of lead, white, transparent, and lustrous, as well as gray. 

 Sulphate of lead, as a crystalline powder, and, more remarkable still, 

 the red antimonial silver in transparent, though minute crystals, as 

 well as in massive form. In the ore there is much bisulphuret of 

 iron and Httle copper pyrites, which aid the decomposition and leave 

 an ochrey deposit, in which the crystallized minerals have formed. In 

 the humid mass, the hydrate of oxide of lead is present, and the 

 formation of carbonate of lead in crystals proceeds by the union of 

 carbonic acid from the air and water with it. The other compounds 

 of lead unite, with acids present, such as sulphurous, to become sul- 

 phuric and silicic acids. 



The formation of the crystals of red silver ore is no more difficult 

 as a study, than is the production of the equally insoluble carbonate 

 and sulphate of lead. It is true that, if we remain embarrassed by 

 the belief that solution is necessary to the jwoduction of perfect crystals, 

 we shall proceed slowly ; and it is better, therefore, to previously con- 

 sider the many instances known, where crystals are formed from their 

 constituents in a divided form, merely suspended, but not dissolved, in 

 water. 



In the present case, the investing, dark, compact covering close on 

 the compound sulphuret, exhibits the first stage of decomposition by 

 oxidation. Further removed, we have oxides forming salts. Finally, 

 in the cavities and on the surface, we see crystals perfected ; this now 

 irregular surface having been naturally buried in a soft adhesive ochre. 



We know that in other cases, where polarization can take place, 

 the transfer of either compounds or elements follows, and where 

 there is sufficient moisture, permitting this Imperfect flow of minute 

 particles to continue, perfect crystals will be built up, witliout solution 

 being necessary. 



The formation of perfect crystals of carbonate of lead, in artificial 

 processes of manufacturing, does not take place on metallic lead, 

 because there is insufficient moisture, or only mere vapor present ; 

 while lead burled in soft, wet earth, becomes covered with trans- 

 parent crystals of both carbonate and sulphate of lead. 



Where there is a continued moist, investing mass, chemical changes 

 taking place in the compound buried, will, as a necessary conse- 

 quence, induce polarity throughout that mass, and, the conditions of 

 transfer being present, compounds insoluble in water, or fluids 

 present, may form as the simple consequence of such conditions. 

 The crystals of red silver ore, in this case, are microscopic, not quite 

 regular, but they illustrate the dependence of their formation on the 

 previous decomposition of the compact ore. 



