48 LEAD 



and then into metallic lead. It consists, according to Klaproth, of 71 oxide of lead, 

 25 sulphuric acid, 2 -water, and 1 iron. The prevailing form of crystallisation is the 

 rectangular octahedron, -whose angles and edges are variously modified. This mineral 

 was first recognised in Anglesea, hence its name. It was found in the Channel Islands 

 at Sark Mine, and is occasionally met with in the Leadhills and at Wanlockhead in 

 Scotland, at Glemalure in Wicklow, and at Ballycorus Mine, Co. Dublin. 



5. Phosphate of lead. Pyromorphite.Th\8, like all the combinations of lead with 

 an acid, exhibits no metallic lustre, but a variety of colours. Before the blowpipe, 

 upon charcoal, it melts into a globule externally crystalline, which by a continuance 

 of the heat, with the addition of iron and boracic acid, affords metallic lead. Its con- 

 stituents are 80 oxide of lead, 18 phosphoric acid, and 1-6 hydrochloric acid, according 

 to Klaproth's analysis of the mineral from Wanlockhead. The crystalline forms are 

 derived from an obtuse rhombohedron. Phosphate of lead is a little harder than white 

 lead ; it is easily scratched, and its powder is always grey. Its specific gravity is 6'9. 

 It has a vitreous lustre, somewhat adamantine. Its lamellar texture is not very 

 distinct ; its fracture is wavy, and it is easily frangible. The phosphoric and arsenic 

 acids being, according to M. Mitscherlich, isomorphous bodies, may replace each other 

 in chemical combinations in every proportion, so that the phosphate of lead may include 

 any proportion, from the smallest fraction of arsenic acid to the smallest fraction of 

 phosphoric acid, thus graduating indefinitely into arsenate of lead. The yellowish 

 variety indicates, for the most part, the presence of arsenic acid. It is found in 

 Cornwall, Devonshire, Yorkshire, Derbyshire, and Cumberland, very fine specimens 

 being found in the Alston Moor mines. 



6. Arsenate of lead. Mimetesite. The name is derived from fu/jirjrfis, imitator, the 

 species so nearly resembling pyromorphite. The colour of this ore varies from straw- 

 yellow and wax-yellow to brown, reddish-brown, orange, yellow, and red. Before the 

 blowpipe, on charcoal, it emits arsenical fumes, and yields a bead of lead. The analysis 

 by Dufrenoy gives the following as its composition : 



Arsenate of lead 84'55 



Phosphate of lead 4'50 



Chloride of lead 9'05 



At Drygill, in Cumberland, this ore has been met with in sufficient abundance to be 

 worked to some extent as an ore of lead. The mimetesite from this mine was at one 

 time used in the manufacture of flint-glass, to which it gave great brilliancy. The 

 form of the arsenate of lead, when it is crystallised, is a prism with six faces, of 

 nearly the same dimensions as that of phosphate of lead. When pure, it is reducible 

 upn charcoal, before the blowpipe, into metallic lead, with the copious exhalation of 

 arsenical fumes ; but only in part, and leaving a crystalline globule, when it contains 

 any phosphate of lead. The arsenate of lead is tender, friable, sometimes even 

 pulverulent, and of specific gravity 5'04. That from the Saxon mines of Johann- 

 Georgenstadt, consists, according to Rose, of oxide of lead, 77"5 ; arsenic acid, 12*5; 

 phosphoric acid, 7'5 ; and chlorine, 1*5. 



7. Sulphide of lead. Galena (Bleiglanz, Ger.). This is the most abundant ore of 

 lead ; it may be indeed regarded as the only commercial ore of value, if we except 

 the carbonates, which are probably formed by the decomposition of galena. Its pre- 

 vailing forms are the cube and a combination of the cube and octahedron ; lustre 

 metallic, opaque, colour and streak lead grey. Fracture conchoidal, but difficult to 

 obtain, owing to the readiness with which it cleaves. The localities of galena need 

 not be named here, as the lead-producing districts, of which a list will be presently 

 given, will include them, galena occurring in them all. Thomson's analysis of 



Lead 86-13 



Sulphur 13-02 



Iron . 0-50 



' It is a remarkable fact that silver should invariably be present in galena, some- 

 times, indeed, in very minute proportion ; and the same generalisation may now be 

 received as established with respect to the presence of gold. The silver, it is certain, 

 usually exists in galena in the state of sulphide ; and so, probably, does the gold. 

 The mode of existence of sulphide of silver in galena is not always the same, as may 

 be inferred from the fact that by washing, nearly the whole of the silver is carried 

 away from some kinds of galena ; while by the same treatment of other kinds of galena 

 the loss of silver is inconsiderable. It is an error to suppose that largely-crystalline 

 galena is generally poor in silver.' Percy. 



8. Jamesonite is a combination of lead, antimony, and sulphur. It occurs in acicular 

 crystals, or in parallel or diverging groups, and more frequently in fibrous masses. It 

 is found in many places in Cornwall and Devon. Hose's analysis gives the following 

 as its composition : 



