CARBONIC ACID AND CARBONIC OXIDE.

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at the very commencement of the process, when it | with air or in oxygen gas: it is also produced by rewould otherwise be most abundant. As the spiration and fermentation. It may be prepared concoking of the coal advances most slowly and regularly veniently by the action of muriatic acid on fragments from the top of the heap to the bottom, only one of marble carbonic acid escapes with effervescence, layer is affected at a time, and in succession down- and chloride of calcium is left in solution. wards, while the surface is always covered with a stratum of red-hot cinders, ready to consume every particle of carburetted or sulphuretted hydrogen gases which may escape from below. . . . The coke being perfectly freed from all fuliginous and volatile matters, by a calcination of upwards of 40 hours, is cooled down to moderate ignition by sliding in the dampers and sliding up the doors, which had been partially closed during the latter part of the process. It is now observed to form prismatic concretions, somewhat like a columnar mass of basalt. These are loosened by iron bars, lifted out upon shovels furnished with long iron shanks, which are poised upon swing chains with hooked ends, and the lumps are thrown upon the pavement, to be extinguished by sprinkling water upon them from the rose of a watering-can; or they might be transferred into a large chest of sheet-iron, set on wheels, and then covered up. Good coals thus treated yield 80 per cent. of an excellent, compact, glistening coke, weigh ing about 14 cwt. per chaldron. The loss of weight in coking, in the ordinary ovens, is usually reckoned at 25 per cent.; and coal, which thus loses one-fourth in weight, gains one-fourth in bulk."

Fig. 471.

CARBONIC ACID and CARBONIC OXIDE. Carbon and oxygen possess a very powerful affinity for each other when the temperature is sufficiently raised: under ordinary circumstances, and at the usual atmospheric temperatures, they do not combine. The only binary compounds of carbon and oxygen which have been isolated are carbonic oxide, which is composed of 6 parts, by weight, of carbon, and 8 of oxygen, and carbonic acid, which contains 6 carbon and 16 oxygen. We will first notice carbonic acid, which is always produced when charcoal burns in contact

The gas is colourless, of a pungent odour, and acidulous taste. Its sp. gr. is 1.524, and 100 cubic inches at 60°, and at a pressure of 30 inches as indicated by the barometer, weigh 47.262 grains. This gas extinguishes all burning bodies even when largely diluted with air, for a candle will not burn in a mixture of 4 volumes atmospheric air and 1 volume of carbonic acid. Nor will this gas support animal respiration, for a small portion of it in the air soon proves fatal. An animal cannot live in a mixture which extinguishes a lighted candle, and hence the old practical rule of letting a light down into a well before any one ventures to descend. If the candle burn, it is judged safe, but Dr. Christison relates cases in which a light has not been extinguished, but the men who descended instantly became insensible. When an attempt has been made to inspire pure carbonic acid, there was a violent spasm of the glottis, which prevented the gas from entering the lungs. If sufficiently diluted with air to admit of its passing the glottis, it acts as a narcotic poison. Many persons have lost their lives either intentionally or by accident by sleeping in a confined room with a pan of burning charcoal. Carbonic acid is quite incombustible, and this gas consists of carbon in its highest degree of oxidation.

When lime-water comes in contact with this gas it becomes turbid. The lime unites with the gas, forming carbonate of lime, which being insoluble in water renders the solution of lime-water milky.

Recently boiled water absorbs its own volume of carbonic acid at 60° and 30 inches pressure; but it will take up much more if the pressure be increased. The quantity absorbed is in exact ratio with the compressing force, the water dissolving twice its volume when the pressure is doubled, and three times its volume when the pressure is trebled. On removing the pressure the greater part of the gas escapes, and produces that effervescence which we see when a bottle of ginger-beer, soda-water, cider, or brisk champagne is opened. Water saturated with carbonic acid sparkles when it is poured from one vessel to another. The solution has a pleasant acid taste, reddens litmus paper, but the blue colour returns as the gas escapes. Lime-water is at first rendered turbid by the solution, but it soon becomes clear, as carbonate of lime is soluble in excess of carbonic acid. The agreeable pungency of malt liquors is in great measure due to the presence of carbonic acid, the loss of which by exposure to the air causes them to become stale. Spring and well-water contain carbonic acid absorbed from the air, and to which they partly owe their pleasant flavour. The insipid taste of water that has been boiled is from an absence of carbonic acid. This gas is always present in the atmosphere, even at the height of several thousand feet. Its presence may be proved by exposing lime-water

first produced in a generator, from which it is distilled into a receiver for the purpose of separating it from the sulphate of soda, &c. The products of several successive operations are collected in this receiver. The generator, a Fig. 473, is a cylindrical chamber of

to the air. A pellicle of carbonate of lime soon forms | Dr. Faraday's glass tube. The liquid carbonic acid is on its surface. The sources of carbonic acid in the air are numerous. It is an abundant product of combustion, of respiration of animals, which is in fact only a kind of slow combustion, and it is generated by fermentation, and in all those changes which take place in dead animal and vegetable matter. When these processes. take place in low and confined situations, the poisonous carbonic acid, or choke-damp as it is sometimes called, may accumulate and prove fatal to animal life. But in general this gas is equally diffused throughout the air, and never accumulates unless there is some local source of supply; for although so much heavier than the atmospheric air, the strong diffusive tendency of gases prevents it from separating from the lighter gases of the air. By its equal diffusion it ministers food to growing plants, which decompose the carbonic acid, retain the carbon, and give out an equal volume of pure oxygen in return. Many mineral springs also contain carbonic acid, and in combination with lime it forms extensive masses of rock in all countries. Carbonic acid unites with alkaline substances, forming salts which are termed carbonates.

Carbonic acid has been obtained in a liquid, and also in a solid form, the first example of a solidified gas. The liquefaction of carbonic acid was effected by Faraday in the following manner :-A strong tube,

Fig. 472.

Fig. 472, about

inch diameter and 8 inches long, being bent at about 2 inches from its end to an obtuse angle, and sealed at the shorter end, sulphuric acid was poured in through a small funnel, so as not to soil the larger leg, which was then loosely filled with fragments of carbonate of ammonia, and also hermetically sealed. The acid was then made to run upon the salt, and the evolved carbonic acid gradually appeared condensed by the mutual pressure of its particles into a liquid form. Many precautions are necessary to guard against explosion; the face and eyes must be protected by a strong mask with goggles for the eyes, and the hands must be shielded by thick gloves. Great care is required in handling the tubes, for in some cases, after having held the fluid safely for weeks together, they have exploded from some slight increase of temperature. It appears from some experiments by Mr. Addams, that the elastic force of liquid carbonic acid at different temperatures is as follows:

lead enclosed within a copper cylinder, and strengthened by wrought-iron rings. The top and bottom are further strengthened by thick iron plates. The generator is supported on an axis at p p. The construction of the receiver B is similar to that of the generator. The opening at o in the generator is closed by a screw-plug s, pierced in the direction of its axis, and furnished with a stop-cock c. The screw. plug is moved by a double handle h h. A ring of lead compressed into the double throat between the neck of the generator and the screw-plug, renders the vessel perfectly tight. The receiver в has an opening which is furnished with a copper tube a, passing nearly to the bottom of the vessel, and is furnished with a stop-cock e on the outside. Communication is established between the two vessels A and B, by means of a copper tube x tx.

For the production of the liquid carbonic acid, the following is the charge for a generator 2 feet long, and 4 inches internal diameter:-6.25 lbs. of water at 100°: 2.75 lbs. of bicarbonate of soda in fine powder, and 1.47 lbs. of sulphuric acid of full commercial strength. These proportions leave an excess of alkaline salt, which is desirable in order completely to neutralize the acid. The generator is charged by taking out the valve-plug and introducing some of the tepid water by means of a funnel: the carbonate is then added, then the remaining water, and the whole is stirred up with a rod. The acid is introduced separately in a copper tube, Fig. 474, in order to prevent it from acting upon the materials before the screw-plug is replaced. This being done, the generator is inclined, when the acid flows out of the tube and mingles with the alkaline solution, and in order to mix them more completely, the vessel is turned over and over several times upon its axis pp. The generator is then Fig. 474. allowed to stand a few minutes erect with its

CARBONIC OXIDE may be obtained from carbonic acid, by subjecting that gas to the action of substances which abstract a portion of its oxygen; as for example, by passing carbonic acid over red-hot charcoal or metallic iron, one half of its oxygen is removed. There are various other methods of obtaining this gas known to the chemist. Carbonic oxide differs essentially in its properties from carbonic acid. It is combustible, burning with a beautiful pale blue flame, producing carbonic acid as the result of combustion. It is colourless, has very little odour, but is very poisonous. It explodes when mixed with oxygen and fired by the electric spark. Its sp. gr. is .973, and 100 cubic inches weigh 30.21 grains.

acid and ether in the air, as measured by a spirit | tained; we must therefore examine and reflect upon thermometer, was found by Faraday to be -106° the various circumstances which may produce such Fahr., but when this mixture was placed beneath the loss. The first and most obvious loss arises from the receiver of an air-pump, and exhaustion rapidly made, escape of the heated air from the chimney before it the temperature sank to -166° Fahr. In 1845 Dr. has surrendered to the boiler the full amount of heat Faraday took advantage of this low temperature to which it is capable of relinquishing. It is manifest, resume his experiments on the liquefaction of the that the best method of obviating this consists in so permanent gases, which had been undertaken by him arranging the chimney and passages for the products with such happy results 22 years before. By using of combustion, that they shall circulate thoroughly narrow green glass tubes of great strength, powerful around the boiler, and that sufficient time may be condensing syringes, and the low temperature obtained allowed for them to part with their high temperature as just described, several of the compound gases before escaping into the external air. Doubtless this were liquefied and even frozen into colourless trans- is an extremely difficult thing to effect to the best parent solids alcohol did not freeze, but it became advantage, and is the part where some loss is inthick and oily; while oxygen, hydrogen, nitrogen, evitable; but it is so striking a point that I need not nitric oxide, carbonic oxide, and coal gas, could not here dwell longer upon it. Another important source be liquefied at 166° Fahr. while subjected to pres- of loss is the cooling down of the pipes and boiler sures varying from 27 to 58 atmospheres. after the steam has been generated, but before it has been used. And here science has rendered most important service; in the Cornish engines especially this point has received attention,-and four times the quantity of work may now be obtained with the same expenditure of fuel from an engine which Mr. Watt considered was doing the full amount of work that could be expected from it. There is, however, another very important but more unsuspected mode in which loss is sustained, and one which is intimately connected with the chemistry of combustion. It depends upon an insufficient supply of air. It is a fact, not less singular than important, that charcoal, or coke, may be dissipated in vapour, and may apparently be wholly consumed, by one half of the amount of air that is usually required in an open fire, under circumstances where the full quantity of heat is given out; and it is to be observed, that in this case one pound of charcoal instead of emitting heat enough to convert thirteen pounds of water into steam, will only give out one-fifth of the heat, and will therefore raise but little more than two pounds and a half of water into steam. This important fact depends upon the property which charcoal has of forming two compounds with oxygen; in the first case where the most heat is emitted, twice the amount of oxygen is taken up, and carbonic acid gas, or fixed air, is produced; in the second case, a gas is obtained also, called carbonic oxide; it is colourless, and therefore escapes notice; but it is combustible, which carbonic acid is not, and in burning, it gives out a large amount of heat; in short, the other four-fifths of the heat which are deficient when charcoal is burned into this gas.

Carbonic oxide is sometimes produced from the carbonic acid which results from the combustion of coal in our furnaces, and the cause of this is clearly stated in the following extract from an admirable lecture by Professor Miller of King's College. He is tracing the progress of the combustion of a pound of pure carbon.

"When this pound of pure charcoal is burned, when it combines with oxygen from the air, it gives out a certain amount of heat, which, from careful experiment, appears to be sufficient to convert thirteen pounds of water at 60°, the ordinary temperature of the air, into steam at 212°, the boiling-point of water. In other words, it will boil away thirteen pounds of water; and more than the heat sufficient to effect this, no ingenuity can make it furnish. It is evident, however, if it is capable of furnishing this quantity of steam, that unless we actually generate this full amount in our steam boilers,-unless the whole thirteen pounds of water is turned into steam for every pound of charcoal which we consume, that we are incurring a loss greater or less in proportion to the quantity of steam produced. Some waste is almost inevitable; but our exertions cannot be considered to be crowned with the success which is attainable so long as any considerable loss is sus

(1) An Introductory Address to the Medical Classes of King's King's College, given October 6, 1845. By W. A. Miller, M. D.

College, delivered October 1, 1845; with an Inaugural Lecture at

F.R.S. Professor of Chemistry, King's College, London. 1845.

"Now, the way in which this gas is produced is worthy of notice. It is not formed, in the first instance, by the direct union of the coke or charcoal with the oxygen of the air; for carbonic acid is the compound which is invariably obtained; but when this carbonic acid is made to pass over red-hot cou's, it dissolves a portion of the coal, becomes dilated to twice the bulk it occupied, and actually, instead of increasing the heat of the furnace by the quantity of coal with which it thus unites, it most materially diminishes it, and carries it off in sheer waste.