LEAD ORES. 283 



by stoves or from fermenting bark ; the re suit is that the lead 

 becomes carbonated from the acid fumes that rise from be 

 neath.* The carbonate is then removed by shaking the 

 plates smartly, and after washing and levigation, it is dried 

 for market. According to another good process, (Thenard's,) 

 carbonic acid, either from burning coke, brewers' vats, 01 

 some other source, is made to pass through a solution of sub- 

 acetate of lead, the solution of subacetate being formed by 

 digesting litharge and neutral acetate of lead. In place of 

 this solution, litharge moistened slightly with vinegar, has 

 been proposed. In the processes in the arts more litharge 

 is made than is demanded in trade, and this use of it is con- 

 sidered more economical than its reduction to lead. 



Carbonate of lead, mixed with sulphate of barytes, forms 

 what is called Venice white. 



Carbonate and sulphate of lead. There are two whitish or grayish 

 ores of this composition called dioxylite and leadhillite, or respectively 

 sulphato-carhonate and sulphato-tricarhonate of lead. The former 

 contains 71 per cent, of carbonate of lead ; the latter 47. Dioxylite has 

 a perfect basal cleavage. Gr=6 - 2 — 6 - 5. Leadhillite cleaves into lami- 

 nae that are flexible like gypsum. Gr=6'8 — 7. From Leadhills. 



Caledonite is a compound of the carbonates of lead and copper and 

 sulphate of lead, and is called the cupreous sulphato-carhonate of lead. 

 In crystals of a deep verdigris or bluish green color. Gr=64. From 

 Leadhills and Red Gill ; also from the Missouri mines. 



pyromorphite. — Phosphate of Lead. 



Primary form, a hexagonal prism. Cleavage lateral, in 

 traces. Usual in clustered hexagonal prisms, 

 forming crusts. Also in globules, or reniform, 

 with a radiated structure. 



Color bright green or brown ; sometimes fine 

 orange-yellow, owing to an intermixture with 

 chromate of lead. Streak white or nearly so. Luster more 

 or less resinous. Nearly transparent to subtranslucent. 

 Brittle. H=3«5— 4. Gr = 6-5— 7-1". 



Composition of a brown variety : oxyd of lead 78-58, mu- 

 riatic acid 1*65, phosphoric acid 19-73. Before the blow- 

 pipe on charcoal fuses, and on cooling, the globule becomes 



Describe pyromorphite. Of what does it consist? 



* A subacetate is supposed to form first, and then to be immediately 

 decomposed by the rising carbonic acid. 



