CARBON.--SYMBOL C.--COMBINING WEIGHT 12.
By T.A. POOLEY, B.Sc., F.C.S.
This element, which next deserves our attention, is one of great importance and wide distribution; it occurs in nature in both the free and the combined states, and the number of compounds which it forms with other elements is very large. Unlike the previous elementary bodies we have studied, carbon is only known to us in the solid form when free, although many of its combinations are gaseous at the ordinary temperature and pressure. Carbon is known to exist in several different physical states, thus illustrating what chemists call allotropism, which means that substances of identical chemical composition sometimes possess altogether different outward and physical appearances. Thus the three states in which pure carbon exists, viz., diamond, graphite, or plumbago, and charcoal are as different as possible, and yet chemically they are all exactly the same substance. The diamond is the purest carbon, and occurs in the crystalline form known as a regular octahedron; the diamond is one of the hardest substances known, and is therefore, utilized for cutting glass; it has also a very high specific gravity, namely, 3.5, which means that it is three and a half times heavier than water, and it is far heavier than any of the other allotropic modifications of carbon. Graphite or plumbago, the second form in which carbon occurs, is widely distributed in nature, and the finer qualities are known as black lead, although no lead enters into their composition, as they are composed of carbon almost as pure as the diamond; the specific gravity of graphite is only 2.3. Charcoal, the third allotropic modification of carbon, is by far the most common, and is formed by the natural or artificial disintegration of organic matters by heat; we thus have formed wood charcoal, animal charcoal, lamp-black, and coke, all produced by artificial means, and we may also class with these coal, which is a natural product, and which contains from 85 to 95 per cent. of pure carbon.
Wood charcoal is made by heating wood in closed vessels or in large masses, when all the hydrogen, oxygen, and nitrogen are expelled in the gaseous state, and the carbon is left mixed with the mineral constituents of the wood; this form of carbon is very porous and light, and is used in a number of industrial processes.
Animal charcoal, as its name implies, is the carbonaceous residue left on heating any animal matters in a retort; and contains, in addition to the carbon, a large proportion of phosphates and other mineral salts, which, however, can be extracted by dilute acids. Animal charcoal possesses to a remarkable degree the property of removing color from solutions of animal and vegetable substances, and it is used for this purpose to a large extent by sugar refiners, who thus decolorize their dark brown sirups; in the manufacture of glucose and saccharums for brewers' use, the concentrated solutions have to be filtered through layers of animal charcoal in order that the resulting product may be freed from color. The decolorizing power of animal charcoal can be easily tested by any brewer, by causing a little dark colored wort to filter through a layer of this material; after passing through once or twice, the color will entirely disappear, or at all events be greatly reduced in intensity. Animal charcoal also absorbs gases with great avidity, and on this account it is utilized as a powerful disinfectant, for when once putrefactive gases are absorbed by it, they undergo a gradual oxidation, and are rendered innocuous, in the same way animal charcoal is a valuable agent for purifying water, for by filtering the most impure water through a bed of animal charcoal nearly the whole of the organic impurities will be completely removed.
Lamp-black is the name given to those varieties of carbon which are deposited when hydrocarbons are burned with an insufficient supply of oxygen; thus the smoke and soot emitted into our atmosphere from our furnaces and fireplaces are composed of comparatively pure carbon.
Coal is an impure form of carbon derived from the gradual oxidation and destruction of vegetable matters by natural causes; thus wood first changes into a peaty substance, and subsequently into a body called lignite, which again in its turn becomes converted into the different varieties of coal; these changes, which have resulted in the accumulation of vast beds of coal in the crust of the earth, have been going on for ages. There are very many different kinds of coal; some are rich in hydrogen, and are therefore well adapted for making illuminating gas, while others, such as anthracite, are very rich in carbon, and contain but little hydrogen; the last named variety of coal is smokeless, and is therefore largely used for drying malt.
Carbon occurs in nature also in a combined state; limestone, chalk, and marble contain 12 per cent. of this element. It is also present in the atmosphere in the form of carbonic acid, and the same compound of carbon is present in well and river waters, both in the free state and combined with lime and magnesia. All animal and vegetable organisms contain a large proportion of carbon as an essential constituent; albumen contains about 53 per cent., alcohol contains 52 per cent., starch 44 per cent., cane sugar 42 per cent., and so on. The presence of carbon in the large class of bodies known to chemists as carbohydrates, of which starch and sugar are prominent examples, can be easily demonstrated. If a little strong sulphuric acid be added to some powdered cane sugar in a glass, the mass will soon begin to darken in color and swell up, and in the course of a few minutes a mass of black porous carbon will separate, which can be purified from the acid by repeated washings; the sugar is composed of carbon, hydrogen, and oxygen, the two last-named elements being present in the exact proportion necessary to form water; the sulphuric acid having a strong affinity for water, removes the hydrogen and oxygen, and the carbon is then left in a free state.
Carbon forms two compounds with oxygen--carbon monoxide, commonly called carbonic oxide, and carbon dioxide, commonly called carbonic acid; and the last-named, being of most importance, will be studied first.
Carbon Dioxide, or Carbonic Acid, Symbol CO2.--Carbonic acid occurs, as we have already stated, in large quantities in combination with lime and magnesia, forming immense rock formations of limestone, chalk, marble, dolomite, etc.; it also issues in a gaseous state from volcanoes, and it is always present in small quantities in the atmosphere; it is found dissolved in well and river waters, and it is a product of the respiration of animals. Brewers also are well aware of the existence of this body, for it is evolved in enormous quantities during the alcoholic fermentation of saccharine fluids. When carbonaceous substances are burnt the bulk of the carbon is converted into carbonic acid, and thus our furnaces and fireplaces are continually emitting enormous quantities of carbonic acid into the atmosphere. With these different sources of supply it might reasonably be thought that carbonic acid would be gradually accumulating in our atmosphere; the breathing of animals, the eruption of volcanoes, the combustion of fuel, and the fermentation of sugar, are ever going on, and to a fast-increasing extent with the progress of civilization, and yet the proportion of carbonic acid in our atmosphere is no greater now than it was at the earliest time when exact chemical research determined its presence and quantity. A counteracting influence is always at work; nature has beautifully provided for this by causing plants to absorb carbonic acid, holding some of the carbon, and allowing the oxygen to escape again into the atmosphere to restore the equilibrium of purity. This mutual evolution and absorption of carbonic acid is continually going on; occasionally there may be either an excess or a deficiency in a particular place, but fortunately any irregularity in this respect is soon overcome, and the air retains its original composition, otherwise animal life on the face of the globe would be doomed to gradual but sure extinction.
Carbonic acid can be prepared for experimental purposes by causing dilute hydrochloric acid to act upon fragments of marble placed in a bottle with two necks, into one neck of which a funnel passing through a cork is fixed, and into the other a bent tube for conveying the gas into any suitable receiver. The evolution of carbonic acid by this method is rapid, but easily regulated, and the gas may be purified by causing it to pass through some water contained in another two-necked bottle, similar to the generator. The chemical change involved in this decomposition is expressed by the following equation:
CaCO_3 + 2HCl = CO_2 + H_2O + CaCl_2
Calcium Hydrochloric Carbonic Water. Calcium
Carbonate. Acid. Acid. Chloride.
By referring to the table of combining weights given in a previous paper, it will be seen that 100 parts of calcium carbonate will yield 44 parts of carbonic acid. Instead of hydrochloric acid any other acid may be used, and in the practical manufacture of carbonic acid for aerated waters sulphuric acid is the one usually employed. Carbonic acid is colorless and inodorous, but has a peculiar sharp taste; it is half as heavy again as air, its exact specific gravity being 1529; one hundred cubic inches weigh 47.26 grains. It is uninflammable, and does not support combustion or animal respiration. Under a pressure of about 38 atmospheres, at a temperature of 32° F., carbonic acid condenses into a colorless liquid, which may also be frozen into a compact mass resembling ice, or into a white powder like snow. Carbonic acid is soluble in water, and at the ordinary pressure and temperature one volume of water will hold in solution one volume of the gas; under increased pressures, far larger quantities of the gas can be held in solution, but this is rapidly evolved as soon as the excess of pressure is removed. Upon this property the manufacture of aerated waters depends. The presence of free carbonic acid can be easily detected by causing the gas to pass over the surface of some clear lime-water. If any be present a white film of carbonate of lime will at once be formed. In testing carbonic acid in a state of combination, the gas must first be liberated by acting upon the substance with a stronger acid, and then applying the lime-water test. The presence of large quantities of carbonic acid in a gaseous mixture can be readily detected by plunging into the vessel a lighted taper, which will be immediately extinguished. This ought always to be adopted in a brewery, where many fatal accidents have happened through workmen going down into empty fermenting vats and wells without first taking this precaution.
The presence of carbon in this colorless gas can be demonstrated by causing some of it to pass over a piece of the metal potassium placed in a hard glass tube, and heated to dull redness; the potassium then eagerly combines with the oxygen, forming oxide of potassium, and the carbon is liberated and can be separated in the form of a black powder by washing the tube out with water.
Carbon Monoxide, or Carbonic Oxide. Symbol CO.--This is formed when carbon is burnt with an insufficient supply of oxygen, or when carbonic acid gas is passed over some carbon heated to redness. This gas is continually being formed in our furnaces and fire-places; at the lower part of the furnace, where the air enters, the carbon is converted into carbonic acid, which in its turn has to pass through some red-hot coals, so that before reaching the surface it is again converted into carbonic oxide; over the surface of the fire this carbonic oxide meets with a fresh supply of oxygen, and is then again converted into carbonic acid. The peculiar blue lambent flame often observed on the surface of our open fire-places is due to the combustion of carbonic oxide, which has been formed in the way we have just described. Carbonic oxide is a colorless, tasteless gas, which differs from carbonic acid by being combustible, and by not having any action on lime water.--Brewers' Guardian.