CARBON (symbol, C; atomic weight,12) is one of the most important of the chemical elements. It occurs pure in the diamond, and nearly pure as graphite or plurnbago ; it is a constituent of all animal and vegetable tissues and of coal ; and it also enters into the composition of many minerals, such as chalk and dolomite.
Carbon is a solid substance, destitute of taste and odour ; but it occurs in several modifications which exhibit very diverse physical properties. Thus, it is met with in the form of the diamond in transparent crystals belonging to the regular or cubical system, which conduct electricity but slowly ; and in the form of graphite in opaque crystals belonging to the hexagonal system, which conduct electricity nearly as well as the metals. The diamond is the hardest substance known, and has a relatively high specific gravity (3.33 to 3.55), but graphite is comparatively soft, producing a black shining streak when rubbed upon paper, and has a much lower specific gravity (2.15 to 2.35). In addition to these two crystalline modifications of carbon there are a number of varieties of non-crystalline or amorphous carbon, which, however, exhibit the greatest differences in physical properties.
By heating to the high temperature afforded by a powerful galvanic battery, both the diamond and amorphous carbon are converted into graphite. In the electric arc carbon appears to be converted into vapour ; but the temperature which is required to volatilize it is extremely high ; in fact, it has been calculated' that the boiling-point of carbon is not less than about 7000° on the centigrade scale.
Although the various allotropic modifications of carbon cannot always be satisfactorily distinguished by their physical properties, they may readily be distinguished, as Berthelot has shown, by their behaviour on treatment with certain oxidizing agents. The diamond is not affected thereby even after prolonged and reiterated treatment. The different varieties of amorphous carbon, however, are more or less readily entirely converted into humus-like substances, or" humic acids," soluble in water, whereas the different varieties of graphite furnish "graphitic oxides," which are insoluble in water, and especially characterized by the property of undergoing decomposition with deflagration when heated. The method of treatment adopted by Berthelot is as follows. The carbon in the form of an impalpable powder is mixed with the aid of a card with five times its weight of pulverized potassie chlorate, and this mixture is then formed little by little into a paste wi fuming nitric acid. In performing these operations great care is necessary in order to avoid explosions, and at most five grams of carbon should be taken. The mixture, contained in a small open flask, is allowed to stand several hours, and is then heated for three or four days without interruption to a temperature not higher than 50° or 60° C. ; the mass is then diluted with water and washed by decantation with tepid water. It is necessary as a rule to repeat this series of operations several times in order entirely to dissolve the amorphous carbons, or to convert the graphites into graphitic oxides.
Berthelot has examined a very large number of varieties of carbon in this manner with the following results. The carbon of wood charcoal, animal charcoal, coke, the so-called metallic carbon obtained by decomposing hydrocarbons by passing their vapours through a red-hot tube, gas-retort carbon, and various specimens of anthracite from different sources, all dissolved entirely with more or less readiness when treated in the above manner ; lamp black, however, furnished a small amount of graphitic oxide. The amorphous carbon of the meteorite of Cranbourne (Australia) furnished a graphitic oxide identical with that obtained by similarly treating graphite from cast-iron, -but the carbon of the Orgueil meteorite was entirely soluble. The carbon of the Greenland meteoric rock discovered by Nordenskiold also dissolved entirely with the exception of a very insignificant residue.
Berthelot also examined the action of various agents on carbon, and finds that heat alone is without influence ; that is to say, the graphites are not changed into amorphous carbon, or the amorphous carbons into graphite, when heated to whiteness in an atmosphere of hydrogen or of chlorine. When, however, a pencil of gas-retort carbon is inflamed in an atmosphere of oxygen, and then as soon as the point is fully incandescent plunged into water, the part which has been heated contains a small quantity of graphite. On examining the pencils of carbon employed in producing the electric light it was found that the spongy mass of carbon collected on the negative pole contained a large proportion of graphite, but that only traces were present in the pencil employed as positive pole, which appears to indicate that it is necessary for the carbon to undergo volatilization in order that it may be converted into graphite. The graphite thus produced is not identical with that contained in cast-iron, nor with natural plumbago,, the same variety of graphite is produced, however, when the diamond is heated in the electric arc. The carbon separated from the various hydrocarbons by heat alone consists entirely of amorphous carbon, but that obtained on decomposing marsh gas by the electric spark contains a small quantity of graphite, and the carbon resulting from the decomposition of perchloride of carbon and bisulphide of carbon at a red heat contains a considerable proportion of graphite ; that resulting from the decomposition of cyanogen by the electric spark contains only traces of graphite.
The specific heats of the several modifications of carbon also differ considerably ; that is to say, the amounts of heat required to raise equal weights through the same number of degrees of temperature are different. The diamond has the lowest, and amorphous carbon the highest specific heat ; or to raise the temperature of a given weight of the diamond from the temperature a to the temperature b will require less heat than to raise the temperature of the same weight of amorphous carbon from the temperature a to b.
Graphite. - Graphite is found native near Travancore in Ceylon, and near Moreton Bay in Australia, in several parts of the United States, in South Siberia, and in Germany, principally at Griessbach near Passau, always in rocks belonging to the earliest formations. It occurs 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. Native graphite contains from 95 to nearly 100 per cent. of carbon, the impurity being usually small quantities of silicates. Graphite, also called plumbago or black lead, is used for making so-called lead pencils, for polishing iron work, for lubricating machinery, for making crucibles, and in the electrotype process for coating the surfaces of wood, plasterof-paris, gutta-percha, and other non-conducting materials, so as to render them conductive.
The behaviour of graphite on treatment with a mixture of potassic chlorate and nitric acid has been carefully studied by Brodie ; but our knowledge of its oxidation products is still very incomplete. He has shown that it is converted into a body to which he attributes the composition indicated by the formula C„11405; graphitic acid, as this compound is termed, forms yellow silky plates, insoluble in water and acids. It does not form salts, and Berthelot therefore prefers to call it graphitic oxide. When this substance is heated it decomposes almost with explosive violence, leaving a residue which still contains hydrogen and oxygen, but which is not distinguishable from finely divided graphite. When the graphite which crystallizes from cast-iron and that obtained by heating amorphous carbon in the electric arc are similarly treated, graphitic oxides are produced which differ from each other, and from the oxide formed from native graphite; it is therefore supposed that these graphites are distinct substances (Berthelot, Ann. Ch. Phys.  xix. 399).
Amorphous Carbon.. - Pure amorphous carbon is only obtained with great difficulty. That produced by heating pure organic substances, such as sugar and starch, still contains traces of hydrogen and oxygen, from which it can only be freed by long-continued ignition at a white-heat in an atmosphere of chlorine. The purest amorphous carbon ordinarily met with is lamp black, which is prepared by the imperfect combustion of highly carbonized bodies, such as resin. An amorphous carbon of considerable purity, known as gas-retort carbon, is obtained in the manufacture of coal-gas. The parts of the retort which are exposed to the highest temperature partially decompose the gas as it escapes, and by degrees a layer of very dense carbon is deposited in the upper parts of the retorts. It is a good conductor of heat and electricity, and burns with difficulty, and is therefore employed in producing the electric light, and to form the negative element in Bunsen's voltaic battery. Wood charcoal and coke are very impure forms of amorphous carbon, containing in addition to small quantities of hydrogen and oxygen a considerable proportion of mineral matters, which remain as ashes when the charcoal or coke is burnt.. Animal charcoal is a still more impure form of amorphous carbon.
Oxides of Carbon. - When carbon is burnt in oxgyen carbonic dioxide or carbonic anhydride, or, as it is commonly termed, carbonic acid, CO,, is formed ; if the supply of oxygen is deficient the lower oxide, carbonic oxide, CO, is also produced, and the latter may be obtained in a pure state by passing the dioxide over red-hot carbon. Both are colourless, odourless gases. The union of carbon with oxygen gives rise to the evolution of a very large amount of heat, but much less heat is produced by the union of the first half of the oxygen than by the union of the second half. Thus the combination of 12 grams of carbon with 16 grains of oxygen to form 2S grams of carbonic oxide gives rise to the evolution of about 25,000 units of heat, but no less than 69,000 units are produced by the addition of a second 16 grams of oxygen to form carbonic dioxide. It is supposed that in the first instance very much less heat is evolved because energy, which otherwise would appear in the form of heat, is expended in converting the solid carbon into the state in which it exists in gaseous carbonic oxide and dioxide, since it is observed that in those cases in which two oxides are formed, both of which are solid, sensibly equal quantities of heat are evolved in the fixation of each successive 16 grams of oxygen.
In the conversion, however, of the different varieties of carbon into carbonic dioxide, 002, by combustion in oxygen different amounts of heat are produced. The following table represents the number of units of heat (the unit of heat being the amount required to raise the temperature of 1 gram of water from 0° to 1° C) evolved in the conversion of 12 grams of each of the varieties of carbon into 44 grams of carbonic dioxide Carbonic oxide burns in the air or oxygen with a blue flame, forming carbonic dioxide. It is an extremely poisonous gas, being capable of displacing the oxygen in blood, owing to the formation of a compound with the hmmoglobin with which the oxygen is ordinarily combined. It is very sparingly soluble in water, which dissolves only about 410th of its bulk at 15° C. When a mixture of equal volumes of carbonic oxide and chlorine gas is exposed to sunlight, the two gases combine, forming chloro-carbouic oxide or phosgene gas, COC12.
Carbonic dioxide will not burn, neither does it support combustion. Under the pressure of 36 atmospheres at 0° C. it is converted into a colourless mobile liquid. When the liquid is suddenly relieved from the pressure under which alone it can exist, part of it at once passes back into the state of gas, and heat is abstracted so rapidly that the remaining portion of the liquid solidifies. By allowing a jet of the liquid dioxide to pass into a cylindrical metal box, having within it an inclined metal tongue against which the jet of liquid impinges, a considerable quantity of the solid may be collected in the form of a white flocculent mass like snow. Like all flocculent substances, it conducts heat but slowly, and may be preserved for a considerable time. By mixing it with ether its heat-conducting power is greatly increased ; it therefore evaporates much more quickly, and a much lower temperature is obtained than with the solid alone, and by placing the mixture under tho receiver of an air-pump and exhausting, a still greater degree of cold is produced. According to Faraday, an alcohol thermometer plunged into a bath of the solid carbonic dioxide and ether in air indicates a temperature of 76° C., and in the same bath under a receiver exhausted to within 12 inches of the atmospheric pressure it fell to 110° C. ; at the latter temperature alcohol assumes the consistence of a thick oil.
Recent experiments of Sir B. Brodie (Royal Society Proceedings, xxi. p. 483, 1873) show that carbonic oxide and dioxide are not the only oxides of carbon which are capable of existing. When a current of pure and dry carbonic oxide is circulated through a Siemen's induction-tube, and there submitted to the action of electricity, a decomposition of the gas occurs. Carbonic dioxide is formed, and simultaneously with its formation a solid deposit may be observed in the induction-tube ; this deposit appears as a transparent film of a red-brown colour. It is entirely soluble in water, which is strongly coloured by it, and the solution has an intensely acid reaction. In the dry condition, before it has been in contact with water, it is an oxide of carbon. Samples, however, made in different experiments do not present precisely the same composition; but nevertheless they appear to belong to a certain limited number of forms, which repeatedly occur and may invariably be referred to the same general order or system. This system is, or appears to be, what may be termed an homologous series of " oxycarbons," of which the unit of carbon with the weight 12 may be regarded as the first term, and of which the adjacent terms differ by an increment of carbonic oxide, CO, weighing 28, precisely as homologous series of hydrocarbons differ by the increment CII2. Two at least of these substances have been identified by analysis, namely, the adjacent terms C403 and C504.
Carbonic Acid. - Carbonic dioxide dissolves in about its own volume of water at ordinary temperatures, forming carbonic acid, LE2CO3; the solution has a sharp and slightly acid taste, and turns the blue colour of litmus to wine-red. The volume of carbonic dioxide dissolved by water diminishes as the temperature rises, and at the boiling heat the whole is expelled from solution ; the volume dissolved by water at a given temperature is nearly the same, however, under all pressures, so that the weight of gas absorbed increases in nearly the same proportion as the pressure. On removing the pressure the gas is given off with effervescence. Ordinary soda-water consists merely of water impregnated with carbonic dioxide by mechanical pressure. When lime water is added to a solution of carbonic acid, or carbonic dioxide is passed into lime water, a white precipitate of calcic carbonate or carbonate of lime, the chief constituent of ordinary chalk, is produced : - On continuing to pass the gas the precipitate becomes dissolved owing to the formation of an acid carbonate or bicarbonate, which is fairly soluble in water, the carbonate being almost insoluble : - If the solution of the bicarbonate be heated, carbonic dioxide is given off and the caleic carbonate is precipitated, the bicarbonate being decomposed. The lime salt may also be removed from the solution, with the exception of the small amount of caleic carbonate which the water is capable of dissolving, by carefully adding lime water or a solution of ordinary washing soda as long as a precipitate is produced. The action of lime water in this case is to convert the soluble bicarbonate into the insoluble carbonate ; thus : - These facts serve to explain the "hardness," as it is termed, of many natural waters, and the methods employed to render such waters soft. A water which, like rain water, readily produces a lather with soap is said to be a soft water, whereas one which does not readily yield a lather, but forms a large amount of curd, is said to be a hard water. The hardness of most natural spring waters is chiefly due to dissolved calcic bicarbonate, which is formed by the action of the carbonic acid dissolved in rain upon the calcareous materials with which the water comes in contact during its passage through the earth. Ordinary soap consists of the sodie salts of certain fatty acids, and is soluble in water ; but the lime salts of these acids are insoluble, so that when the soap is used with the hard water a double decomposition takes place, the calcic bicarbonate being converted into sodic bicarbonate and the soda-soap into a lime soap or curd. Such waters may be rendered soft, that is to say, the calcic bicarbonate may be removed in a variety of ways, viz., by heating the water, which causes the decomposition of the bicarbonate and the precipi tation of the carbonate, and it is in this way that the fur is produced in our kettles and boilers ; by adding washing soda or sodic carbonate, a common practice in all households where hard water is used ; and lastly, by adding lime water. (n. E. A.)