Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/99

Rh 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, plaster- of-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 composi tion indicated by the formula C 11 H 4 O 5 ; 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 crystal lizes 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. [4] 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 2, 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 grams of oxygen to form 28 grams of carbonic oxide gives rise to the evolution of about 25,000 units of heat, but no less than G9,000 units are produced by the addition of a second 1C 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 grains of oxygen. In the conversion, however, of the different varieties of carbon into carbonic dioxide, C0 2, 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 to 1 C) evolved in the conversion of 12 grams of each of the varieties of carbon into 44 grams of carbonic dioxide:—

Units. Diamond 93,240 Iron graphite 93,144 Natural graphite 93,564 Gas-retort carbon 96,564 &quot;Wood charcoal 96,960 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 haemoglobin with which the oxygen is ordinarily combined. It is very sparingly soluble in water, which dissolves only about ^njth 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-carbonic oxide or phosgene gas, COC1 2. Carbonic dioxide will not burn, neither does it support combustion. Under the pressure of 36 atmospheres at 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 nmch more quickly, and a much lower temperature is obtained than with the solid alone, and by placing the mixture under the 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 tho consistence of a thick oil. Recent experiments of Sir B. Brodie (Royal Society Pro ceedings, 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 decomposi tion 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 con dition, 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 j 