92 Mr HAIDINGER on the Parasitic Formation 



conchoidal fracture, together with the external crystalline ap- 

 pearance 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 present case, it is carbonate of lead, or Pb C 2 , 

 according to BEBZELIUS'S 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 exactly ana- 

 logous 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 surface 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 addition- 

 al portion of oxygen, and becomes yellow on cooling. It then 

 contains lead 92.83, and oxygen 7.17, and is Pb, or protoxide of 

 lead. 



The hexahedral lead-glance, consisting of one atom of lead 

 and two of sulphur, Pb S 2 , in the proportions of 86.55 and 

 13.45, is very liable to decomposition by means of the natural 

 agents. There are examples of compact varieties of prismatic 

 lead-baryte formed by its decomposition, and still presenting the 

 traces of fissures parallel to the hexahedral cleavage planes of the 

 original species. The prismatic lead-baryte consists entirely of 

 sulphate of lead (Pb S 2 ), in Avhich the two ingredients, lead and 

 sulphur, are in the same ratio as in the lead-glance : the two species 

 are chemically distinguished from each other only by the presence 



