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CHAPTER VIII

CARBON

Symbol, C. Atomic weight=12.00. Occurrence.—This element is capable of assuming three allotropic forms, and it occurs free in nature in each of these modifications, viz., diamond, graphite, and charcoal.

In combination with oxygen, carbon occurs in carbon dioxide, a gas which is present in the air, being a constant product of combustion and respiration. In combination with hydrogen it occurs as marsh gas. Carbon is a constituent of all the natural carbonates, such as limestone, dolomite, &c., which form an important fraction of the earth's crust, and it is also an essential constituent of all organic substances.

DIAMOND.

Occurrence.—This substance has been known and prized from the remotest antiquity. It is found in various parts of India, mostly in river gravels and superficial deposits, in Brazil, South Africa, Australia, and various parts of the United States. The diamond has also recently been obtained from extra-terrestrial sources. In a meteorite which fell in Russia on September 22, 1886, carbon was found, partly as amorphous and partly as adamantine carbon.

The diamond form of carbon is found of various colours ; sometimes it is dark grey, or even black, stones of these colours being known as carbonado and bort. The former of these is extremely hard, and is of great value for use in rock-boring and drilling instruments. Bort is used in the crushed condition by lapidaries for grinding and polishing.

Occasionally the diamond is found coloured blue, or red, or green by traces of foreign materials. Some of these coloured stones are of great value as gcms: the well-known “Hope" diamond, a stone weighing 44} carats, has a fine sapphire colour.

The origin of the diamond is unknown, although many theories have been put forward to explain its formation. Newton's famous suggestion, that diamond was “an unctuous substance coagulated," was based upon its remarkably high refractive index. The cellular structure which is sometimes to be seen in the ash that is left when the diamond is burnt seems to indicate that it is of vegetable origin.

Modes of Formation.-Innumerable attempts have been made to effect the crystallisation of carbon in the adamantine form ; but while it is readily possible to convert this variety of carbon into its allotropes graphite and charcoal, the transformation of these back again to the diamond is a problem that is beset with the greatest difficulties. Moissan has recently shown * that the carbon, which is capable of being dissolved in molten iron, and which is usually deposited in the graphitic form on cooling, can, under certain conditions, be caused to take up the adamantine form.

By raising the temperature of the iron to about 3000° by means of an electric furnace, and then suddenly cooling the molten mass by plunging the crucible into water or molten lead, until the cooled and solidified surface is at a dull red heat, an enormous pressure is brought to bear upon the interior and still liquid portion. Under these circumstances, a part of the carbon which is deposited by the slowly cooling mass was found by Moissan to be in the adamantine form. On dissolving the iron in hydrochloric acid, amongst the carbonaceous residue were found fragments having a specific gravity between 3.0 and 3.5, and sufficiently hard to scratch ruby. Some of the fragments were the black or carbonado variety, while others were transparent. On combustion in oxygen, Moissan proved that these were really carbon in the diamond form.

Properties.—The diamond in its purest condition is a colourless crystalline substance. Its crystalline forms belong to the cubic system, and appear to some extent to be characteristic of the locality in which the element occurs. It is extremely hard and moderately brittle. When struck with a hammer the diamond not only splits along its cleavage-planes, but also in other directions, with a conchoidal fracture. It does not conduct electricity. The

* Comptes Rendus de l'Académie des Sciences, vol. cxvi. p. 218.

specific gravity of diamond varies slightly in different specimens, the mean being about 3.5. Its resractive index is higher than that of any other substance, and it is this property which gives its peculiar beauty and brilliancy to the cut stone.

The value of diamond as a gem depends largely upon its colourlessness, except in the case of those rare instances where the colour is quite definite and also pleasing, such as distinct red, blue, or green.

When diamond is strongly heated it becomes black, and increases in bulk, being converted into a substance having the properties of coke. Lavoisier (1772) was the first to show that the diamond was a combustible body, and that it yielded carbon dioxide. Davy (1814) showed that carbon dioxide was the only product of its combustion, and proved that diamond was pure carbon.

ta The combustion of diamond in oxygen may readily be accomplished by means of the apparatus shown in Fig. 61. A fragment of diamond is supported upon a small gutter of platinum foil, which bridges across two stout copper wires, A. These wires pass through a cork in a perforated glass plate, and are lowered into a cylinder of oxygen. By the passage of an electric current the little platinum boat can be strongly heated, when the FIG. 61. diamond will become ignited, and continue to burn brilliantly in the oxygen, with the formation of carbon dioxide. The ash, which is always left after a diamond has been burnt, varies from 0.2 to 0.05 per cent of the stone. It is found usually to contain ferric oxide and silica.

GRAPHITE.

Occurrence.—This second allotrope of carbon is much more plentiful in nature than the first. It is found in large quantities in Siberia, Ceylon, and various parts of India. In England the chief source of graphite has been the mines at Borrowdale in Cumberland ; this supply is now practically exhausted. Enormous quantities of very pure graphite are now obtained from the Eureka Black-Lead Mines in California. Graphite also occurs in many specimens of meteoric iron.

Mode of Formation.—Molten iron, especially when it contains silicon, is capable of dissolving a considerable amount of carbon, which, on cooling, is deposited in the form of black shining crystals of graphite. Occasionally considerable quantities of graphite are found deposited in this way in iron-smelting furnaces, to which the name “kish” has been applied.

Graphite is now manufactured by heating a mixture of 97 parts of amorphous carbon (charcoal or coke) and 3 parts of iron in an electric furnace. It was formerly believed that at the high temperature of the electric arc amorphous carbon was converted directly into the graphitic modification ; but it has recently been shown (Acheson) that pure charcoal does not by itself undergo this transformation ; that the change, in reality, takes place through the intermediate formation of a metallic carbide. The product obtained is practically free from iron, as the metal is volatilised at the high temperature.

Properties. --Graphite is a soft, shiny, greyish-black substance, which is smooth and soapy to the touch. It is usually found in compact laminated masses, but sometimes crystallised in six-sided plates. Its specific gravity varies in different specimens, averaging about 2.5. Graphite is a good conductor of both heat and electricity.

When strongly heated in oxygen, graphite takes fire and bums, forming carbon dioxide, and leaving an ash consisting of silica, alumina, and oxide of iron. Graphite has been found by Regnault to contain, usually, traces of hydrogen. Graphite is employed for the manufacture of ordinary lead pencils ; for, on account of its softness, it leaves a black mark upon paper when drawn across it. For the purposes of the pencil manufacture the natural graphite is ground to powder and carefully washed free from gritty matter. It is then mixed with the finest washed clay, and the pasty mass is forced by hydraulic pressure through perforated plates. The name “graphite," from the Greek to write, is given to this substance on account of its use for this purpose. It was formerly supposed that this material contained lead, hence the names black-lead and plumbago.

When powdered graphite is subjected to prolonged treatment with boiling nitric acid and potassium chlorate it undergoes partial oxidation, and is converted into a greyish crystalline substance which was termed by its disquiere (Brodie) graphitic acid. It contains carbon, hydrogen, and oxygen, and is believed to have a composition represented by the formula H_C0g. When heated, this compound undergoes a very curious transformation. If a fragment about the size of a pea is heated in the bottom of a test-tube, feeble signs of visible combustion are seen, and a light, porous black mass is produced which fills and overflows the tube. Tnis porous mass appears to be pure graphite. At the same time a little moisture condenses upon the tube.

Graphite is largely employed, on account of its refractoriness, for the manufacture of the so-called plumbago crucibles, which consist of fireclay mixed with finely-ground graphite.

Other uses to which graphite is put are for glazing or polishing gunpowder, especially the larger grained varieties ; as a lubricant for machinery, where oil is inadmissible on account of high temperature ; for electrotyping processes, and also as a coating for ironwork, to prevent rusting.

AMORPHOUS CARBON. This non-crystalline form of carbon may be obtained by the decomposition of a great variety of carbon compounds, by the process known as destructive distillation. The carbon so obtained differs very much as regards its purity, according to the particular organic compound used for its preparation. The commonest forms of amorphous carbon to be met with are lampblack or soot, gas carbon, coke, charcoal, animal charcoal or bone - black. None of these substances is pure carbon ; animal charcoal, for example, usually containing only about 10 per cent. of carbon.

Lampblack.—This substance is manufactured by burning substances rich in carbon, and which burn with a smoky flame (as turpentine, petroleum, or tar), with a limited supply of air. The smoke is passed into chambers in which are suspended coarse blankets, upon which the soot collects. The lampblack always contains hydrogen in the form of hydrocarbons. If the soot be heated to redness in a stream of chlorine, this hydrogen can be removed, and pure amorphous carbon will be left.

Lampblack is used for printers' ink and for black paint.

Gas Carbon.-- This form of carbon is obtained by the destructive distillation of coal in the manufacture of illuminating gas. It remains in the retorts as an extremely hard deposit, lining the roof and sides. It is a very pure carbon, coming second to purified lampblack. Its specific gravity is about 2.35. Gas carbon is a good conductor of electricity, and is extensively used for the manufacture of carbon rods for the arc light.

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