VARIETIES OF CHARCOAL.

77

charcoal progressively diminishing; but the per-centage of carbon in the residual charcoal was found to increase, as shown in the table.

A peculiar kind of charcoal, but imperfectly burned, and of a reddish-brown colour, termed by the French charbon roux, is occasionally prepared for the manufacture of the gunpowder used for sporting purposes. Powder made with this charcoal absorbs moisture more rapidly than ordinary gunpowder. Charbon roux is procured by forcing steam, under a pressure of about two atmospheres, through a coil of heated pipe, and directing this superheated steam, at about 540°, into the iron cylinder containing the wood in a few hours the charring of the wood is effected. The following is stated by Regnault to be its average composition in 100 parts:

Animal Charcoal, or ivory black, is prepared by heating bones in cylinders in a manner similar to that employed for wood charcoal. (355) General Properties of Carbon.-Carbon in all the forms above mentioned is chemically the same. At atmospheric temperatures it is one of those substances in which chemical attraction exhibits least activity; consequently a superficial charring is frequently resorted to with a view to protect wood from decay, as in the case of piles which are driven into mud or into the beds of rivers, to serve as foundations. For the same reason it is a common practice to char the interior of tubs and casks destined to hold liquids. Lampblack furnishes the most indestructible of black pigments, and has long been employed on this account as the basis of printing ink. The diamond is a non-conductor of electricity; in its other forms carbon is an excellent conductor, ranking next to the metals in this respect. In a state of fine subdivision it is a bad conductor of heat, but its conducting power increases with its density. Finely divided charcoal is usually stated to have strong antiseptic powers. It certainly has a remarkable action upon putrescible substances; but Stenhouse has shown that this action consists in a rapid process of oxidation dependent upon the power which charcoal, when in a finely divided state, possesses of condensing oxygen. The offensive effluvia from animal matter in an advanced state of putrefaction disappear when the putrefying substance is covered with a layer of charcoal;

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POWER OF CHARCOAL TO PROMOTE OXIDATION.

it continues to decay, but without emitting any odour, till at length all the carbon is dissipated as carbonic anhydride, the hydrogen as water, and the nitrogen remains as nitric acid. The remarkable power possessed by charcoal of absorbing various bodies, particularly colouring matters and many bitter principles, when in a finely divided state (54), as well as its property of condensing a large proportion of gaseous matters within its pores (65), has been already mentioned. So rapid is this action, that Stenhouse has proposed to use a respirator filled with charcoal, to protect the mouth and nostrils in an infected atmosphere; and the employment of trays of powdered wood charcoal in dissectingrooms, in the wards of hospitals, and in situations where putrescent animal matter is present, is found to exert a most beneficial influence in sweetening the atmosphere by absorbing and decomposing the offensive gases. These properties render charcoal a valuable material in the construction of filters, not only for decolorizing purposes, but likewise for assisting in purifying water for domestic use. It is now also employed most successfully to prevent the escape of noxious vapours at the ventilating openings of the sewers, as it allows the free passage of air, but condenses the offensive effluvia in its pores, where they are destroyed by a process of oxidation. It will continue active for years if kept dry.

Carbon is usually regarded as neither fusible nor volatile; but in the course of some experiments with a voltaic battery of intense energy, consisting of 600 cells of Bunsen's construction, connected so as to form a battery of 100 pairs of 6 cells each, Despretz found on operating upon carbon points in an exhausted receiver, that the vessel became filled with a dark cloud, which was deposited upon the sides of the glass as a black crystalline powder; and by exposing charcoal obtained from pure sugar, or from essence of turpentine, to the action of the battery in a small crucible of pure charcoal connected with the positive electrode, the whole of the charcoal powder became cemented into a coherent mass which appeared to have been fused, and which exhibited the properties of graphite.

At high temperatures carbon combines rapidly with oxygen, and will remove it from a great number of its compounds, especially from the oxides of the metals; hence the various forms of carbon are very extensively employed in the reduction of these substances to the metallic condition. The deoxidizing power of carbon is sometimes exerted at the ordinary temperature of the air. Schönbein found that ferric salts may be reduced to the condition of ferrous salts, by simply agitating their solutions with

SYNTHESIS OF CARBONIC ANHYDRIDE.

79

charcoal powder, and the mercuric are, in like manner, converted into mercurous salts. Charcoal decomposes steam at a red heat; hydrogen is liberated, and a mixture of carbonic oxide and carbonic anhydride is formed.

It was long supposed that sulphur is the only non-metallic element besides oxygen with which carbon can be made to unite directly, and a high temperature is required in this case also to effect the combination. The experiments of Berthelot have proved that by igniting charcoal intensely by means of the voltaic arc in a current of pure hydrogen, a particular hydrocarbon, acetylene, ЄH, is formed.

The compounds of carbon with the metals are termed carburets or carbides.

(356) Synthesis of Carbonic Anhydride.-Since a knowledge of the composition of carbonic anhydride is a fundamental datum for the analysis of organic compounds, the proportion in which oxygen combines with carbon to produce carbonic anhydride has been determined with the greatest care, by the combustion of weighed quantities of diamond, of graphite, and of charcoal, in a stream of dry oxygen. The apparatus employed for this purpose is indicated in fig. 288. A represents a gas-holder filled with oxygen; в a

tube containing fragments of caustic potash, or pumice-stone moistened with sulphuric acid, for removing all traces of moisture from the oxygen; c d is a tube of hard glass traversing the sheetiron furnace, E. At c is a platinum tray containing the weighed portion of diamond or graphite; the front of the tube d is occupied by a column of oxide of copper, the object of which is to oxidize completely any trace of carbonic oxide which might be formed. The apparatus is filled with dry oxygen by opening the stopcock of the gas-holder, A, to a regulated distance, and the

80

SYNTHESIS OF CARBONIC ANHYDRIDE.

fore-part of the tube, d, is brought to a red heat by means of heated charcoal; the heat is then applied to the spot, c, where the carbon lies. The carbon burns and becomes converted into carbonic anhydride, which traverses the column of heated oxide of copper; r is a weighed tube, filled with chloride of calcium, which, if water were present, would be found to increase in weight, but in which no deposit of moisture is formed if the experiment be properly conducted. The carbonic anhydride passes or, and is absorbed by a strong solution of potash which is contained in the bulbs of the Liebig's apparatus, shown at G. The excess of oxygen absorbs moisture as it passes through this liquid, but before it is allowed to escape into the air it is rendered perfectly dry by causing it to pass through an additional tube, H, filled with fragments of caustic potash. The increase in weight acquired by the tubes & and H furnishes the weight of the carbonic anhydride corresponding to the quantity of carbon consumed, and the quantity of carbon burned is ascertained by weighing the platinum tray and its contents after the experiment has terminated. By experiments conducted upon this principle it has been determined that 12 parts of carbon require for conversion into carbonic anhydride exactly 32 parts of oxygen (Dumas and Stas; Ann. de Chimie, III. i. 5).

Diamond, graphite, and charcoal are thus shown to be chemically the same substance, though they differ entirely in properties; these three conditions being allotropic modifications of carbon (87), the differences in properties arising not from differences in their chemical nature, but in their molecular arrangement.

If a piece of pure carbon be burned in a jar of oxygen over mercury, it will be found after the combustion is over, and the gas has cooled to the initial temperature of the oxygen, that its volume has undergone no permanent change: the bulk of the oxygen, therefore, is not altered by this combination; the carbonic anhydride which is formed occupies precisely the same space as the oxygen which produced it.

(357) CARBONIC OXIDE (CO = 28); Sp. Gr. o'967; Atomic and Mol. Vol.-It has been stated that carbonic anhydride is wholly deprived of its oxygen when heated with potassium; but if some metal, such as zinc or iron, which has a less powerful attraction for oxygen, be substituted for the potassium, the carbonic anhydride will only be partially deoxidized; the metal will deprive it of exactly half the oxygen which it contains, and a new gaseous body, termed carbonic oxide, will be produced. The bulk of this new gas is exactly equal to that of the carbonic

CARBONIC OXIDE-ITS PREPARATION.

anhydride that furnished it.

81

Carbonic oxide, mixed with free hydrogen, is also obtained abundantly when steam is transmitted over charcoal heated to bright redness.

Preparation.-1. Carbonic oxide may be conveniently prepared by mixing powdered chalk with an equal weight of iron or zinc filings, and exposing the mixture to a red heat in a gun-barrel. The chalk when ignited gives off carbonic anhydride, which in contact with the heated metal is decomposed; oxide of iron or of zinc is formed, quick-lime remains in the retort mixed with the metallic oxide, and the carbonic oxide gas after being washed with water containing slaked lime in suspension, with a view to absorb any undecomposed carbonic anhydride, may be collected over water, in which it is but slightly soluble. These chemical changes may be represented in the following manner; ¤a¤Ð ̧ + Zn = Ca✪ + Zn✪ + €0.

2.-Carbonic oxide is often produced abundantly in the ordinary process of combustion in stoves and furnaces: this mode of its formation is important, for it exercises a material influence upon the economy of combustion, inasmuch as all the carbonic oxide thus carried off unburnt represents so much fuel wasted; while in many metallurgic operations the carbonic oxide so produced plays a conspicuous part in the reduction of the ore to the metallic state, the oxides of iron, lead, copper and many other metals, being reduced when heated with it, whilst carbonic anhydride is formed. It is owing to the production of carbonic oxide that anthracite can be employed in roasting the copper ores at Swansea, flame being essential to the due performance of the process (869). The formation of carbonic oxide in an open fire which is burning steadily without emitting smoke is often evidenced by the flickering blue flame seen playing over the glowing embers. In this case carbonic anhydride is first formed at the bottom of the grate, from the free access of air to this part of the burning fuel; but the carbonic anhydride as it traverses the redhot coke enters into combination with an additional quantity of carbon, and the anhydride, by losing half its oxygen, is converted into its own bulk of carbonic oxide: at the same time the carbon of the heated fuel which has entered into combination with this removed oxygen furnishes another equal quantity of the same gas: the heated carbonic oxide takes fire as soon as it mixes with the air which passes over the upper surface of the fire. The reaction between the hot carbon and carbonic anhydride may be thus represented: €0, + € = 2 €0.