Plumbago, and molybdenite and copper possess similar powers, and can be used as detectors in radio-telegraphy.
He sometimes held the carbon powder against the diaphragm in a small tr ans' shallow cell (from a quarter to half an inch in diameter and about an eighth of an inch deep), and sometimes he used what he describes as a fluff, that is, a little brush of silk fibre with plumbago rubbed into it.
In another form the plumbago powder was worked into a button cemented together with syrup and other substances.
In the specification of the patent applied for on the list of July 1877 he showed a sketch of an instrument which consisted of a diaphragm, with a small platinum patch in the centre for an electrode, against which a hard point, made of plumbago powder cemented together with india-rubber and vulcanized, was pressed by a long spring, the pressure of the carbon against the platinum disk being adjusted by a straining screw near the base of the spring.
In small works the cupellation is finished in one furnace, and the resulting low-grade silver fined in a plumbago crucible, either by overheating in the presence of air, or by the addition of silver sulphate to the melted silver, when air or sulphur trioxide and oxygen oxidize the impurities.
Before it can be cupelled it has to be freed from most of the zinc, which is accomplished by distilling in a retort made of a mixture similar to that of the plumbago crucible.
In the best-known form a plumbago crucible was used with a hole cut in the bottom to receive a carbon rod, which was ground in so as to make a tight joint.
Formerly bullion was melted in crucibles made of refractory clay, but they are liable to crack and require careful handling These were succeeded by iron crucibles, especially for melting silver, and these have now been generally replaced by graphite (plumbago) crucibles made of a mixture of clay and graphite.
Earlier names, still in common use, are plumbago and black-lead, but since the mineral contains no lead these names are singularly inappropriate.
In 1778 he proposed a new method of making calomel and powder of algaroth, and he got molybdic acid from mineral molybdaena nitens which he carefully distinguished from ordinary molybdena (plumbago or black lead of commerce).
In the following year he showed that plumbago consists essentially of carbon, and he published a record of estimations of the proportions of oxygen in the atmosphere, which he had carried on daily during the whole of 1778 - three years before Cavendish.
The solidified chloride is then broken up, the shots and fused masses of magnesium are picked out, run together in a plumbago crucible without flux, and poured into a suitable mould.
In the mountains near Nanking, coal, plumbago, iron ore and marble are found.
Iron is abundant and plumbago is worked.
Copper ore is found in many tracts throughout India, plumbago in Madras, and corundum in southern India.
Minerals produced in small quantities include gypsum, millstones, salt and sandstone, and among those found but not produced (in 1902) in commercial quantities may be mentioned allanite, alum, arsenic, bismuth, carbonite, felspar, kaolin, marble, plumbago, quartz, serpentine and tin.