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flask, and close the flask by a perforated cork and tube containing chloride of calcium. Then weigh accurately the whole apparatus. Lower down the suspended tube into the fluid, and gradually by agitation wash out the carbonate from the tube. After effervescence has ceased, heat gently the flask, release the cork slightly, apply a suction tube to the top of the chloride of calcium, and draw out the air from the flask. Then weigh the apparatus. The loss indicates the

carbonic acid in the carbonate used.

Carbonic Oxide. CO 175. Sp. gr. 9722. Weight of 100 cubic inches 30.149. Refracting power 1.157.

Process. First observed by Priestley in 1772, but accurately described by him in 1800. (1.) It may be procured by passing carbonic acid through a red hot tube filled with charcoal; CO2 and C becoming 2 CO. (2.) When oxide of zinc and charcoal are heated in an iron bottle similar to that employed in making oxygen, carbonic oxide is produced; ZnO and C becoming Zn and CO. (3.) The same gas is also obtained by heating the carbonates of lime, barytes, &c. with charcoal; CaO CO, and C becoming CaO 2 CO. (4.) Carbonic oxide may be prepared abundantly by pouring sulphuric acid on the yellow prussiate of potash in a retort (Dr. Thomson, 1818). (5.) The simplest process, however, for preparing carbonic oxide is to mix pounded oxalic acid (CO + CO2+3 HO) with sulphuric acid in a retort, and apply heat. The acid removes the water, and sets free the carbonic oxide and carbonic acid which pass over, and can be collected over water. By standing or agitation the CO2 is dissolved and the CO remains; but the combustible properties of the gas may be examined in the mixed state.

Characters. - Carbonic oxide is colourless, and without smell. Experiments.-(1.) When a taper is brought to the mouth of a jar containing it, the gas catches fire, and burns with a fine blue flame; (2.) but if the taper be plunged into it, the light is extinguished. Hence it is a combustible body, and not a supporter of combustion. (3.) When 1 measure of oxygen is mixed in a strong eudiometer (fig. 47.), with 2 of carbonic oxide, and an electric spark passed through the mixture, a violent explosion occurs, (CO and O being converted into CO2) and carbonic acid alone is generated. (4.) When a current of this gas is passed over potassium heated in a bulbed tube, the oxygen of the CO is taken up by the potassium, and carbon is dropped in the form of a black deposit (CO and K forming C and KO). (5.) It may be substituted for hydrogen in the same proportion in the oxyhydrogen blow-pipe, and is not liable to explode. Animals when immersed in it, speedily die, just as if they were plunged into water. This gas is formed in many cases of intense

ignition in presence of carbon, as in lime kilns, where it may be distinguished by its delicate blue flame. It is also formed in iron furnaces, and in charcoal stoves, and is very poisonous.

Oxalic Acid.-CO CO, 4.5, Acid of sorrel, Acid of sugar, is prepared by heating one part of sugar or potatoe starch with five parts of nitric acid (sp. grav. 1·42) and ten parts of water. By evaporation crystals of oxalic acid are obtained. Sugar consists of equal atoms of carbon, hydrogen, and oxygen; and when heated with nitric acid, the following action occurs.

It may also be obtained by decomposing the salt of sorrel of the shops (binoxalate of potash) by means of sulphuric acid.

Characters. The crystals of oxalic acid are four-sided oblique prisms. The taste is sour, and the acid reddens litmus paper. When heated to the boiling point of water the crystals lose 21 per cent. of their weight of water. Above this temperature the acid sublimes, or is decomposed. When swallowed, oxalic acid creates great pain in the stomach, and soon proves fatal. The best antidote is lime or chalk, the oxalate of lime being formed, which is innocuous. Oxalic acid is the best test for lime. When a solution of it is dropped into water containing lime a white precipitate is immediately produced, which, dissolved in hydrochloric acid, is again restored by the addition of caustic ammonia. Its characteristic property is its power of precipitating lime from its union with sulphuric acid or sulphate of lime. Oxalic acid is very useful in removing stains of ink from linen and cotton. It acts by dissolving the tannate of iron of which the black portion of ink consists. It is to be applied by dissolving a few grains of it in half an ounce of water, and then allowing it to stand in contact with the spot for some time. It answers also for removing spots of ink from tables.

Oxalate of Ammonia, NH3, C2O, 2 HO, 8-875, is prepared by adding carbonate of ammonia to a cold solution of oxalic acid until effervescence ceases. The salt crystallizes in right rhombic prisms, with a bitter taste. This is the form in which oxalic acid is usually employed as a test for lime. When distilled, oxalate of ammonia affords a white powder, oxamide, which differs from that salt in having lost the elements of water (HO), and hence its formula is NH, C2 O2. This substance is the type or model of the amides, as they are termed,-a series of organic bodies which contain amidogen NH, and which when boiled

CARBURETTED HYDROGEN.

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with caustic alkali give out the smell of ammonia (NH). Oxalic acid unites with various bases, forming finely crystalline salts with potash and soda.

4

Chlorides of Carbon.- Carbonic oxide unites with chlorine, and gives rise to a gas, chlorocarbonic acid (CO Cl). The most interesting chloride of carbon (C, H, Cl2) is that which is formed by mixing chlorine and olefiant gas together, covering up the vessel, and allowing the gases to act upon each other. The gases gradually diminish in bulk, and an oil swims on the surface of the water which has the preceding composition. It is by this action that we are enabled to determine the amount of olefiant gas present in coal or other gases. Carburetted Hydrogen CH2. Fire damp. Heavy inflammable air. Marsh Gas. Atm. Wt. 1. Spec. grav. 555. Wt. of 100 c.i. =17.211 grs. 1. This gas may be procured by inverting a bottle filled with water in a stagnant pool, supplied with an inverted funnel, stirring the bottom of the pool with a stick, and catching the bubbles which rise in the funnel. 2. It may be also obtained by passing the vapour of alcohol through a red hot porcelain tube; CH, and carbonic acid pass over. 3. When coal is heated to

71

redness the principal products of the ignition are tarry matter, and gases containing 70 per cent of carburetted hydrogen, varying according to the heat applied and to the quality of the coal. The experiment may be performed by filling with pounded coal a blacking bottle supplied with a tobacco pipe stem luted on with clay to carry off the gas, or in a

tube of Bohemian glass, into which a smaller tube is fitted by means of a cork; the whole being supported by a grasping stand (fig. 71.). On applying the heat of a lamp or chauffer to the coals, the gas comes over, and may be set on fire at the point of the fine tube. The same gas is produced in the ignition of most substances of organic origin. It occurs frequently accumulated in deep coal mines, derived from the decomposition of the coal, and often occasions fatal accidents from the property which it possesses of exploding when mixed with common air, and fired by a light. To prevent such explosions the Davy lamp was invented by Sir H. Davy. Its nature may be understood by taking a piece of fine

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12

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wire gauze, and holding it across the flame of a candle or lamp (fig. 72.). It will be observed that the flame does not pass through the gauze, and that if the gauze be held over a gas lamp before ignition, if the gas be allowed to pass out and be lighted above the gauze, when the latter is sufficiently fine it will be found that the flame never descends below the gauze. The experiment with the gauze at the same time enables us to study the structure of flame, which we find to be a hollow cone of light, free from flame in the centre. A Davy lamp simply consists (fig. 73.), of a flame surrounded by wire gauze, which thus prevents the flame from passing to the exterior atmosphere, and igniting the fire damp or carburetted hydrogen. The gauze, which is composed of iron wire is supposed to cool the flame below the point of ignition, and thus to prevent its transference through the meshes. When 1 part of carburetted hydrogen is mixed with from 7 or 8 to 14 parts of common air it is capable of exploding most powerfully. With from three or four times its bulk it does not explode. Hence the danger of omitting to close gas lamps, and of having leaks in the gas pipes. Fortunately coal gas possesses a powerful naphtha odour, which is soon perceptible when it happens to escape. When 2 volumes of oxygen are mixed with 1 volume of this gas they are resolved into 1 of carbonic acid and 2 of vapour of water; CH, O, becoming CO, 2 HO; a volume of oxygen being equivalent to 2 atoms.

Olefiant Gas (oleum, oil; fieri, to be made).— Heavy carburetted hydrogen, CH, 3.5, spec. grav. 9722, may be obtained by the dry distillation of many organic substances, as fats, &c., or in the dry distillation of coal peat, &c. in close vessels. It is usually pre

pared by mixing 1 part of alcohol with from 4 to 7 parts of sulphuric acid in a retort or gas bottle, and applying heat. The fluid becomes first brown and then black; the gas comes over in great abundance, but is latterly contaminated with much sulphurous acid (SO) and carbonic acid (CO2). — Churacters. Colourless, odour ethereal, and burns with a fine white flame, affording much light. Lime water, poured into the jar after combustion, is precipitated, showing the presence of carbonic acid; the products of the burning being CO2 and HO. It is the most important constituent of gas as a lighting agent. When 3 volumes of oxygen

2

and 1 of olefiant gas are mixed in a strong glass bottle enveloped in a towel, and the mouth of the bottle is brought to a lighted candle, a violent explosion results, carbonic acid and water being formed (C, H, and O becoming 2 CO, and 2 HO). When 2 measures of chlorine are mixed with 1 of olefiant gas in a large bottle, no action occurs - but if a lighted taper or gold leaf be brought in contact with the gases, combustion takes place with a red light, and lamp-black is copiously deposited, proving the presence of carbon in the gas (C2); this is a very striking experiment. When exposed to strong pressure and cold, this gas condenses into a clear liquid. This gas does not support combustion, since it is a combustible body. It destroys an animal when plunged into it precisely like water. It is slightly soluble in water. 100 cubic inches weigh 30.15 grains.

Carburets of Hydrogen. Several carburets of hydrogen have been obtained by the condensation of oil gas, which differ in volatility, and may therefore be separated by different degrees of heat. Coal Gas. - Which is prepared by heating coals in an iron cylinder and collecting the gas which comes over; consists, when well prepared, of 20 per cent. of olefiant gas, 60 per cent. of carburetted, and 20 of carbonic acid and hydrogen. The spec. grav. of the best coal gas is 622, but generally as prepared in this country is not above 500. The best coal for the preparation of gas is cannel coal, termed in Scotland parrot, from the crackling sound which it emits when burned. It yields a beautiful light when used as a combustible agent, and may be frequently seen in Glasgow employed for this purpose, in consequence of its comparative abundance in the neighbourhood.

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Cyanogen (Kvavos and yevvaw, I produce blue), C2 N.3.25, spec. grav. 1·8055.—May be easily prepared by introducing the cyanide of mercury, a salt which crystallizes in square prisms (Hg Cy), into a small retort with a turned up beak, or into a Bohemian test tube fitted with a cork, and bent quill tube (fig. 74.) applying heat and receiving the gas over mercury. Characters: it is a transparent colourless gas, possessing a peculiar, dis. agreeable odour. By a force of 4 atmospheres and cold, it condenses into a fluid, and is solid at -30°. If a light be brought to the beak of the retort while the gas is passing over, it catches fire and burns with a beautiful violet blue flame. It neither supports combustion nor respiration. Water takes up 4 measures of it, and acquires its smell and peculiar taste. It is very soluble in alcohol; it reddens solution of litmus. Potassium, when heated in it, burns with a brilliant

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