284 Mr Haidinger on the Parasitic formation of Minerals, 



comes 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 en- 

 tirely of sulphate of lead (Pb S 2 ,) in which the two ingre- 

 dients, 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 of the oxygen in the sulphate. 

 The form of the hexahedral lead-glance, however, is not al- 

 ways recognizable in the products of its decomposition, though 

 there can be no doubt, that, in many cases, the numerous 

 crystalline species of the genus lead-baryte are formed in this 

 way in the veins. Those who might be still inclined to doubt 

 should visit the repositories of these species at Leadhills, a 

 place conspicuous in the annals of the mineral collector for the 

 beauty of the specimens with which his cabinet is adorned. 

 They occur there in a vein in greywacke, filled with a clayey 

 mass, in which nodules of the minerals containing the lead are 

 imbedded. On their outside they are almost uniformly co- 

 vered with crystals of the carbonate, more rarely of the phos- 

 phate, of lead. In the drusy cavities which they include, are 

 deposited the rarer species of the sulphato-carbonate, the sul- 

 phato-tri-carbonate, the cupreous sulphate, and the cupreous 

 sulphato-carbonate, and likewise the phosphates and sulphates 

 of lead. These cavities also are frequently lined with fine 

 crystals of the carbonate itself. A piece of the sulphuret, 

 with bright cleavage planes, is often discovered, engaged 

 among all these species, whose formation so much depends 

 upon its previous existence. In such cases, we find the sul- 

 phuret corroded and rounded, presenting a surface nearly si- 

 milar to that of hexahedral rock-salt, or gypsum that have 

 been exposed to the dripping of water. The space between 

 it and the external coating is often filled with water, when the 

 nodules are found in the mine. Mr Baird gave a pretty com- 



