PLUMBAGO.

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to admit distinct varieties of this substance: the principal are thediamond, plumbago, and charcoal (animal and vegetable). Of these thetwo latter only require consideration in this work.

1. Plumba'go. — Graph'ite or Black Lead .

History. —This substance was probably known to the ancients; butit was first accurately distinguished from other bodies with which it hadbeen previously confounded, especially with molybdena ( bisulphide ofmolybdenum), by Scheele ( Essays, p. 246), in 1779. The terms plumbago,plumbum nigrum, and molybdena, met with in Pliny {Hist. Nat.), do notapply to graphite.

Natural History. —It is found in various parts of the world;chiefly in primitive rocks and the coal formations. The finest occurs atBorrowdale, in Cumberland. It is brought to London , and sold byauction, at a public-house in Essex-street, Strand, on the first Mondayin every month. The best quality usually sells for two guineas or moreper pound, and is employed for making pencils. The ordinary kindsused in this country are imported from Ceylon and Hamburgh.

Properties. —It occurs either crystallized in regular six-sided prisms,or in kidney-shaped masses, or disseminated in rocks. Its colour is ironor steel-grey, with a metallic lustre. It has a greasy feel, and writeseasily on paper. Its specific gravity is 2'08 to 2.45.

Characteristics. —It is known to be a variety of carbon by its yield-ing carbonic acid when burned in oxygen gas. Its physical propertiesdistinguish it from most other varieties of this element. Some kinds ofcoal-gas charcoal closely resemble it. Of non-carbonaceous substances,molybdena (bisulphide of molybdenum) is the only substance that canbe confounded with it in external appearance.

Composition. —It consists essentially of carbon, but is mixed withvariable properties of iron and other substances. The following areanalyses of three varieties by Vanuxen (Silliman’s Journal, vol. x.p. 105):

Borrowdale.

Borrowdale.

Bustletown,

(pure.)

(impure.)

(pure.)

Carbon.

. . . 95-4

. . 1*23 . . .

... 0*6

Silica.. . . . .

Alumina.

Oxides of Iron , Manganese, &c. .

. . 3-60 . . .

... 1-4

99-30 . . .

. . . 99-90 . . .

I suspect, however, that the finest varieties of the Borrowdale graphitecontain a less quantity of foreign matter than is here stated. On thesupposition that the carbon was chemically combined with iron, graphitewas formerly called carburet or percarburet of iron. From some ob-servations of Schrader’s, however, it would appear that the iron is incombination with titanic acid.

Physiological Effects. —Various properties have been assigned toit; but farther evidence is wanting to establish its action on the body.Richter (ausfuhrliche Arzneimittellehre, 3". Bd. p.486) says it alters insome way the lymphatic secretion and the condition of the skin, andafter some days’ use, causes increased secretion of urine, with difficultyin passing it.