DIAMOND-WHERE FOUND-MODE OF CUTTING. 67 pounds contained is returned into the air in the gaseous state. The carbonic anhydride thus poured out by animals as a refuse and poisonous product, supplies food and sustenance to the vegetable world, which in its turn converts the carbon into a form suitable for the maintenance of life in animals. Each great division of animated nature is thus seen to be essential to the wellbeing and even to the support of the other. The fuel which has been burned and dissipated in vapour, is again reduced to the solid state, and by the agency of vegetable life, it is once more fitted for combustion. Plants are in fact the grand agents by which, under the influence of the chemical actions of the sun's rays, deoxidation is effected, while animals are the channels through which recombination with oxygen is unceasingly produced. (352) Diamond, Sp. Gr. from 3'33 to 3'55-Carbon is found in its purest state in the diamond, which occurs crystallized in forms belonging to the regular system. The crystals are generally derivatives from the octohedron, with a cleavage parallel to each of the planes of the octohedron; the faces are often convex, and the edges are generally rounded, or lenticular, as they are termed, in such crystals. Diamonds usually present themselves under the appearance of semi-transparent rounded pebbles, enclosed in a thin brownish opaque crust. The gem, when freed from this coating, is generally colourless; such specimens are the most prized; it is, however, met with of various tints, the more common of which are yellow and different shades of brown. The most famous diamond mines are those of Golconda and Bundelcund in India, of Borneo, and of the Brazils. The origin of the diamond is entirely unknown; it is not probable that it has been formed by crystallization after fusion, since intense heat reduces the diamond to the form of graphite. The circumstances under which diamonds are found in nature afford no clue to the process of their formation. In the year 1827, a diamond was found imbedded in a fine-grained quartzose rock (Itacolumite) in Brazil, but with few exceptions the gem is found scantily in an alluvial matrix, consisting chiefly of sandstone and rolled quartz pebbles, from which the diamonds are extracted by washing and careful sorting. Diamond is the hardest body known, crystallized boron approaching it most nearly in this respect: it is cut and polished by employing its own powder for the purpose. The fine diamond dust used for this object is mixed with a little olive oil, and spread over a revolving steel plate, and the diamond, cemented into a suitable support, has each of its faces in turn presented to the flat face of 68 PROPERTIES OF THE DIAMOND. the disk.* The most important use to which the diamond is applied is the cutting of sheets of glass: only the natural face of the crystal can be employed for this purpose, crystals with curved faces being the best; they are set in a convenient handle, and a line in the proper direction is traced with the diamond across the glass; slight pressure on each side of the cut then determines the fracture in the right direction. A true cut is effected by such a diamond if properly used, whilst a diamond with angles obtained by cleavage produces only a superficial scratch with ragged edges. The diamond has a very brilliant lustre and a high refracting power; it is a non-conductor of electricity. After exposure to sunshine, many specimens emit a feeble phosphorescent light, which may be seen in a darkened room. In vessels from which air is excluded, it may be heated intensely without change. If it be suspended in a cage of platinum wire, heated to bright redness, and plunged into oxygen gas, it will burn with a steady red light, and with the production of pure carbonic anhydride. The diamond, however, is not perfectly pure carbon: it always leaves a minute yellowish ash, which has been found to contain silica and oxide of iron. This ash has generally the form of a cellular network, and may perhaps, at some future time, assist in determining the origin of this valuable gem. No heat hitherto applied suffices for the fusion or volatilization of the diamond, or indeed of carbon in any of its forms, though in the intense heat of the voltaic arc, it appears to be mechanically transported from one electrode to the other (355). When the diamond is introduced into the flame of the voltaic arc, it undergoes a remarkable change; as soon as it becomes white hot it begins to swell up, loses its transparency, suddenly acquires the power of conducting electricity, becomes specifically lighter, and is converted into a black opaque mass, resembling coke. The density of a diamond thus altered was 2.6778, while in its crystalline condition it was 3'336 (Jacquelain, The Kohinoor diamond, which was cut in 1852, for the Queen, was imbedded in a copper vessel of about the size of a teacup, into which it was cemented with a mixture of equal parts of tin and lead. When it was necessary to change the position of the gem, the solder was softened by immersing the cup, with the diamond imbedded, in a charcoal fire, and heating the metal till it assumed a consistence resembling that of wet sand; in order to cool the diamond more quickly, it was plunged first into warm water and then into cold water; the cutting was effected by means of a cast-iron wheel revolving on a vertical axis about 2400 times per minute; the diamond rested upon the upper surface of the wheel, being held in its position by a kind of vice, and the pressure against the revolving disk was increased or diminished by adding or removing weights. From time to time the face of the diamond was touched with a hair pencil dipped in a cream of diamond dust and oil. GRAPHITE, OR PLUMBAGO. 69 Ann. de Chimie, III. xx. 467). The heat of the oxyhydrogen jet was found to be insufficient to produce this change. (353) Graphite, or Plumbago (Sp. Gr. from 2.35 to 2′15), is a second form in which carbon occurs native. Its once celebrated mine at Borrowdale is now nearly exhausted. Ceylon, and in several parts of the United States, always in rocks belonging to the earliest formation. It has also been found abundantly in the Batougal mountains, near the frontier of China, in South Siberia. The Borrowdale graphite occurs in clay-slate; in other localities it is imbedded in gneiss, mica-slate, or granular limestone. Graphite occurs either massive or in six-sided crystalline plates belonging to the rhombohedral system. Carbon, in the two forms of diamond and plumbago, offers an excellent instance of dimorphism; the properties which it displays in these two states are as widely different as those of any two dissimilar elements. Graphite has a metallic, leaden-grey lustre, whence its familiar name of black-lead. It is very friable, and consequently feels unctuous to the touch, and leaves traces on paper upon which it is rubbed. The particles of which it is composed are, however, extremely hard, and they rapidly wear out the saws employed to cut it. It appears to exist in two distinct modifications, one of which, like the Borrowdale graphite, is fine-grained and amorphous; the other, like the Ceylon variety, is composed of small flat plates, united by a cementing material; this form of graphite generally occurs in a matrix of quartz (Brodie). Graphite is an excellent conductor of electricity. It is never met with in a state free from foreign admixture: when burned in oxygen it leaves from 2 to 5 per cent. of ash, which generally contains quartz, and oxides of manganese and iron; these bodies, however, are merely accidental impurities. The fine-grained amorphous graphite is highly prized for the manufacture of 'lead pencils': where pieces of sufficient size can be obtained they are sawn into thin slices, and these again into small rectangular prisms, which are placed in cedar wood for use. It has been found that the smallest fragments, if of good quality, and the fine powder (which was formerly consolidated by melting it with a minute quantity of sulphur, and was used for the coarser kinds of pencils) may be again reduced into coherent plates by subjecting it to enormous pressure, and may thus be fitted for the manufacture of the best pencils. Black-lead is extensively used for lubricating machinery, and as it is quite unaltered by exposure to the weather, it forms a serviceable coating to protect coarse iron work from rust. An application of graphite which is of some importance to the chemist, is its use in the 70 OXIDIZED PRODUCTS FROM GRAPHITE. manufacture of what are termed black-lead crucibles, or blue-pots: the clay employed in making them is mixed with a coarse kind of graphite; the pots made from this mixture are much less likely to crack when heated than if made of fire-clay only. Brodie (Ann. de Chimie, III. xlv. 351) has described a method of obtaining graphite in a state of purity, and in a very finely divided form. It consists in mixing coarsely-powdered graphite with a fourteenth of its weight of chlorate of potassium; the mixture is introduced into an iron pot, and diffused through a quantity of concentrated sulphuric acid, equal to twice the weight of the graphite employed. The mixture is heated over a steam bath so long as any peroxide of chlorine is disengaged: it is then allowed to cool, thrown into water, and washed thoroughly. If graphite which has been subjected to this treatment and dried, be heated to redness, it gives off gas, increases greatly in bulk, and becomes reduced to an exceedingly fine powder. In cases in which the graphite was originally mixed with silica, this impurity may be got rid of by adding a small quantity of fluoride of sodium to the mixture of graphite with chlorate of potassium and sulphuric acid; the silica is then expelled in the form of fluoride of silicon. It appears that during this treatment the graphite becomes oxidized; and that a new compound of carbon, hydrogen, and oxygen is formed, which enters into combination with the sulphuric acid,* and this compound is decomposed by ignition. *This oxidized substance may be obtained in a state of purity by the following process (Q. J. Chem. Soc. xii. 261):-Mix intimately I part of finely powdered Ceylon graphite with three parts of chlorate of potassium, and add sufficient of the strongest nitric acid to render the mixture fluid; after which expose it for three or four days to the heat of 140° on a water bath. Exposure of the mixture to the direct rays of the sun abridges the time required. The residue must be washed with water freely, dried, and subjected four or five times to the same treatment. Graphic Acid (H), as this compound is termed by Brodie, forms yellow silky plates, which are insoluble in water and in acids. It is slowly attacked by ammonia and by potash, the ammonia gradually combining with it, forming a gelatinous body susceptible of decomposition by acids, which occa sion the separation of a white gelatinous mass. When graphic acid is exposed to a temperature of between 500° and 600°, it undergoes decomposition with almost explosive violence, with evolution of heat and light, giving off gas, and producing an exceedingly bulky, flocculent, sooty-looking substance which still retains both carbon and hydrogen. If, in order to regulate the heat applied, the graphic acid be placed in paraffin oil, and the temperature be gradually raised to 520°, the hydrocarbon becomes of a deep red colour, and the acid gives off water and carbonic acid, leaving a substance of graphitoid appearance, consisting of €22H24; if this new body be further heated in an atmosphere of nitrogen, water and carbonic oxide escape, leaving a residue containing H. Even if heated to redness in 66 PIT COAL CONSUMPTION OF SMOKE. 71 The graphitic modification of carbon may be obtained artificially by several processes. When cast iron is melted in contact with an excess of charcoal, it takes up a considerable quantity of it, and if the iron be allowed to cool slowly, the carbon crystallizes out in the six-sided plates peculiar to graphite. In the manufacture of coal-gas, those parts of the retort which are exposed to the highest temperature, partially decompose the gas as it escapes; a part of the carbon which it held in combination is deposited, and by degrees a layer of very pure dense carbon is formed, possessed of a lustre resembling that of a metal. The density and appearance of this mass vary according to the temperature, and the gaseous pressure to which it has been subjected. Pit coal is a substance originally of vegetable origin, which has become altered in appearance and composition by the combined action of heat and moisture under great pressure. The composition of coal varies considerably according to the extent to which these decomposing actions have advanced: the different varieties of coal will be noticed hereafter, but in all cases it consists, like vegetable matter in general, of carbon, hydrogen, and oxygen, with a small proportion of nitrogen; and in addition, it contains a variable quantity of saline and earthy substances, which always exist in the juices of plants, besides a variable amount of iron pyrites or bisulphide of iron. These saline matters are left, when the coal is burnt in an open fire-place, and constitute the ashes; whilst the carbon and hydrogen are entirely converted into carbonic anhydride and water, if an adequate supply of oxygen from the air be furnished; but the burning of coal, even in an open fire, is never complete, so that it gives off a quantity of gaseous and tarry matters, holding finely divided carbon or soot in suspension.* When nitrogen, it retains a portion of oxygen and hydrogen, giving off water, carbonic anhydride, and carbonic oxide. 65 22 Brodie considers that in these compounds the graphite retains its allotropic state, which he terms Graphon, and that it possesses in this form a combining number of 33, with the symbol Gr. If this be so, graphic acid 1,H,, might be represented as Gr.HO, the first residue €,,H,, as Gr. He, and the second He, as Gr2H; graphic acid being regarded by Brodie as analogous to the hydrated oxide of silicon Si,H,0, discovered by Wöhler and Buff (471a.) *Consumption of Smoke.-When a quantity of fresh coal is thrown upon a hot fire, the coal immediately begins to undergo decomposition; various compounds of carbon with hydrogen being abundantly extricated in the form of gas or vapour: a portion of these immediately takes fire and burns with a bright, luminous flame, but a large proportion of these hydrocarbons, on coming into contact with the glowing embers, is further more or less completely decomposed, the carbon and hydrogen experiencing a separation from each other; the hydrogen, which is the more combustible element, becomes burned, or, if the supply of oxygen be inadequate, it passes off in the gaseous form, whilst the carbon, owing to the minute state of subdivision of its particles, |