depending on their Internal Changes. 283 



V. Changes in Minerals containing Lead. 

 The mineral called Native Minium is probably, in every in- 

 stance in which it has yet been observed, the product of de- 

 composition of some other substance containing lead. Such is 

 the variety which M. Bergemann of Berlin found in the lead 

 mines of Kail, in the Eiffel in Germany, where the ore, chiefly 

 the sulphuret and carbonate of lead, is dug out in irregular 

 masses, from the loose earth, to the inconsiderable depth of a 

 few fathoms. To him I have been indebted for several dis- 

 tinct crystals, possessing the regular forms of the di-prismatic 

 lead-baryte, not only in regard to the simple prisms and pyra- 

 mids of which the combinations consist, and the striae on the 

 surface of some of them, but also in regard to the identical 

 mode of being joined in twin-crystals. The beautiful red 

 colour, which in these compact masses much more nearly ap- 

 proaches the colour of vermilion, than in the best varieties of 

 the usual minium in the state of powder, and the apparent ho- 

 mogeneity of the mass in the conchoidal fracture, together with 

 the external crystalline appearance of it, at first rendered it 

 extremely probable that this was actually a species of original 

 formation ; a supposition which proved to be erroneous, on 

 the substance being more accurately examined. In the pre- 

 sent case, it is carbonate of lead, or Pb C 2 , according to Ber- 

 zelius^ formula, corresponding to 83.52 oxide of lead, and 

 16.48 carbonic acid, which is changed into the red oxide of 

 lead, or Pb, containing 10.38 per cent, of oxygen. In order 

 to explain this change, we must suppose, that of the two atoms 

 of carbon contained in the original compound, one goes away 

 in the state of carbonic acid, and the other in that of oxide of 

 carbon, one of the atoms of oxygen being employed to convert 

 the yellow oxide contained in the carbonate of lead into red 

 oxide. The best artificial minium is obtained by a change ex- 

 actly analogous to what we find in nature. Carbonate of lead, 

 in the state of an impalpable powder, is exposed to heat, care 

 being taken to stir it continually, in order to renew the sur- 

 face exposed to the air. If crystals of the di-prismatic lead- 

 baryte be heated in a glass tube, the first application of heat 

 changes them into a red mass, which, however, at a higher 

 temperature, loses an additional portion of oxygen, and be- 



