SODA-WATER-SPARKLING DRINKS.

71

sphere that is, if twice or thrice the quantity of carbonic acid be compressed into the same space, the water will still dissolve one pint of the gas, but the weight of this pint will now be twice or thrice that of the pint of uncompressed gas, so that the water will have dissolved 32 or 48 grains of the gas, accordingly as the pressure had been doubled or trebled. As soon, however, as the pressure is removed, the compressed carbonic acid will resume its former state, with the exception of that portion which the water is capable of retaining in solution under the ordinary pressure of the atmosphere. Thus, if the water had been charged with carbonic acid under a pressure equal to thrice that of the atmosphere, and had therefore absorbed 48 grains of the gas, it would only retain 16 grains when the pressure was taken off, allowing 32 grains to escape in minute bubbles, producing the appearance known as effervescence. This affords an explanation of the properties of soda-water, which is prepared by charging water with carbonic acid gas under considerable pressure, and rapidly confining it in strong bottles. As soon as the resistance offered by the cork to the expansion of the gas is removed, the excess of the carbonic acid, above that which it can hold in solution at the ordinary pressure of the air, escapes with effervescence. In a similar manner, the waters of certain springs become charged with carbonic acid, under high pressure, beneath the surface of the earth, and when, upon their rising to the surface, this pressure is removed, the excess of carbonic acid escapes with effervescence, giving rise to the sparkling appearance and sharp flavour which renders spring water so agreeable. On the other hand, the waters of lakes and rivers are usually flat and insipid, because they hold in solution so small a quantity of uncombined carbonic acid.

Fig. 71.

The sparkling character of champagne, bottled beer, &c., is due to the presence in these liquids of a quantity of carbonic acid which has been generated by fermentation, subsequent to bottling, and has therefore been retained in the liquid under pressure. In the case of Seidlitz powders and soda-water powders, the effervescence caused by dissolving them in water is due to the disengagement of carbonic acid, caused by the action of the tartaric acid, which composes one of the powders, upon the bicarbonate of soda, producing tartrate of soda and carbonic acid gas. In the dry state these powders may be mixed without any chemical change, but the addition of water immediately causes the effer

vescence.

The solubility of carbonic acid in water is of great importance in the chemistry of nature; for this acid, brought down from the atmosphere dissolved in rain, is able to act chemically upon rocks, such as granite, which contain alkalies-the carbonic acid combining with these, and thus slowly disintegrating or crumbling down the rock, an effect much assisted by the mechanical action of the expansion of freezing water in the interstices of the rock. It appears that soils are thus formed by the slow degradation of rocks, and when these soils are capable of supporting plants, the solution of carbonic acid is again of service, not only as a direct food, by providing the plant with carbon through its roots, but as a solvent for certain portions of the mineral food of the plant (such as

72

LIQUEFACTION OF CARBONIC ACID.

phosphate of lime), which pure water could not dissolve, and which the plant cannot take up except in the dissolved state.

57. Although carbonic acid retains its state of gas under all temperatures and pressures to which it is commonly exposed, it is capable of assuming the liquid and even the solid state.

When exposed to a pressure of 38.5 atmospheres (577.5 lbs. upon the square inch) at 32° F., carbonic acid condenses to a colourless liquid of sp. gr. 0.83 (water = 1), and at a temperature of -70° F. (70° below the zero, or 102° below the freezing point, F.), becomes a transparent mass of solid carbonic acid resembling ice.

A small specimen of liquid carbonic acid is easily prepared. green glass (A, fig. 72) is selected, about 12 inches long,

A strong tube of inch diameter in the bore, and inch thick in the walls. With the aid of the blowpipe flame this tube is softened and drawn off at about an inch from one end, as at B, which is thus closed (C). This operation should be performed slowly, in order that the closed end may not be much thinner than the walls of the tube. When the tube has cooled, between 30 and 40 grs. of powdered bicarbonate of ammonia (ordi. nary sesquicarbonate which has crumbled down) are tightly rammed into it with a glass rod. This part of the tube is then surrounded with a few folds of wet blottingpaper to keep it cool, and the tube is bent, just beyond the carbonate of ammonia, to a somewhat obtuse angle (D). The tube is then softened at about an inch from the open

end, and drawn out to a narrow neck (E), through which a measured drachm of oil of vitriol is poured down a funnel-tube, so as not to soil the neck, which is then carefully drawn out and sealed by the blowpipe flame, as at F. The empty space in the tube should not exceed cubic inch.

When the tube is thoroughly cold, it is suspended by strings in such a position that the operator, having retired behind a screen at some distance, may reverse the tube, allowing the acid to flow into the limb containing the carbonate of ammonia; or the tube may be fixed in a box which is shut up, and reversed so as to bring the tube into the required position.

If the tube be strong enough to resist the pressure, it will be found, after a few hours, that a layer of liquid carbonic acid has been formed upon the surface of the solution of sulphate of ammonia. By cooling the empty limb in a mixture of pounded ice and salt, or of hydrochloric acid and sulphate of soda, the liquid acid can be made to distil itself over into this limb, leaving the sulphate of ammonia in the other.

On a larger scale the gas is liquefied in iron vessels. The liquid carbonic acid is employed for illustrating the laws of heat. When a jet of the liquid is allowed to escape into the air, the evaporation of one portion absorbs enough heat to solidify the remainder, which becomes a snow-like mass, evaporating rapidly when exposed to air, with production of intense cold. A mixture of the solid carbonic acid with ether forms one of the most powerful frigorific mixtures, and has rendered great service in the liquefaction and solidification of gases.

58. Carbonic acid may be separated from most other gases by the

ANALYSIS OF ORGANIC SUBSTANCES.

7333

action of hydrate of potash, which absorbs it, forming carbonate of potash.. The proportion of carbonic acid is inferred either from the diminution in volume suffered by the gas when treated with potash, or from the increase of weight of the latter.

In the former case the gas is carefully measured over mercury (fig. 73), with due attention to temperature and barometric pressure, and a little concentrated solution of potash is thrown up through a curved

pipette or syringe, introduced into the orifice of the tube beneath the surface of the mercury. The tube is gently shaken for a few seconds to promote the absorption of the gas, and, after a few minutes' rest, the diminution of volume is read off. Instead of solution of potash, damp hydrate of potash in the solid state is sometimes introduced, in the form of small sticks or balls attached to a wire. To determine the weight of carbonic acid in a gaseous mixture, the latter is passed through a bulb-apparatus (C, fig. 74), containing a strong solution of potash, and weighed before and after the passage of the gas. When the proportion of carbonic acid in the gas is small, it is usual to attach to the bulb-apparatus a little tube, containing solid hydrate of potash, or chloride of calcium, or pumice-stone moistened with sulphuric acid, for the purpose of retaining any vapour of water which the large volume of unabsorbed gas might carry away in passing through the solution of potash.

Fig. 73.

59. Ultimate organic analysis.-It is necessary to determine in this manner the weight of carbonic acid, in order to ascertain the proportion of carbon present in organic substances. For this purpose, an accurately weighed quantity (usually from seven to ten grains) of the organic substance is very carefully mixed with some compound from which it can obtain oxygen at a high temperature, such as oxide of copper (CuO) or chromate of lead (PbO.CrO), care being taken to employ a large excess of the oxidising agent. The mixture is introduced into a combustion-tube of German glass (which is free from lead and noted for its infusibility) of the form shown in A, fig. 74. This tube is provided with a small tube B,

D

Fig. 74.-Apparatus for organic analysis.

containing chloride of calcium, which is connected by a tube of caoutchouc with the potash-bulbs C. On gradually heating the tube in a charcoal

74

CALCULATION OF FORMULA.

furnace, or over a properly constructed gas-burner, the hydrogen and carbon contained in the organic substance are converted, respectively, into water and carbonic acid, by the oxygen derived from the chromate of lead or oxide of copper. The water is absorbed by the chloride of calcium in B, and the increase of weight in this tube will indicate the quantity of water formed in the combustion, whilst that of the potash bulbs will show the weight of the carbonic acid. When the whole length of the tube is red hot, and no more gas passes through the bulbs, the sealed point D of the tube is broken off, and air drawn through by applying suction at E, in order to sweep out the last traces of water and carbonic acid into the chloride of calcium and potash. Sometimes the organic substance is heated in a little platinum tray, placed within a glass tube, through which a stream of pure oxygen is passed, the products of combustion being afterwards made to pass over red-hot oxide of copper, to convert any carbonic oxide into carbonic acid, and collected for weighing as before.

When the organic substance contains carbon, hydrogen, and oxygen, the weight of this last is inferred by subtracting the weights of the carbon and hydrogen from that of the substance. As an example of the ultimate analysis of an organic substance, the results of an analysis of oxalic acid are here given

10 grs. of oxalic acid, dried at 212° F., gave 9.78 grs. of carbonic acid and 200 grs of water.

9 : 1 :: 2.00 : y

y=0.22 gr. of hydrogen in 10 grs. of oxalic acid.

It having been ascertained by preliminary experiments that oxalic acid contains only carbon, hydrogen, and oxygen, 10 (oxalic acid) minus 2.89 (carbon and hydrogen) = 7·11 grs. of oxygen in 10 grs. of oxalic acid. It appears, therefore, that

Empirical and rational formulæ.-In order to deduce from these numbers the chemical formula for oxalic acid, that is, the formula expressing the number of combining weights of each element, it will be necessary, of course, to divide the weight of each element by the number representing its combining weight in the table at p. 2.

88

And the formula of oxalic acid might be written C4H.220.89. But as fractions are not admissible in such a formula, it would be written CHO This, however, is only an empirical formula for oxalic acid, that is, a formula which represents its composition only, without reference to its constitution, i.e., to the absolute number of combining weights present, and to the mode in which they are grouped or arranged within the compound. A formula professing to give such information would be termed a rational formula, and can only be arrived at by the careful study of the relation of the substance under examination to others of which the combining weights are certainly known. Thus, it is found that one combining weight (47 parts) of potash requires 45 parts of dry oxalic acid to neutralise it and form the oxalate of potash. Hence it is reasonable to regard 45 as the combining weight of dry oxalic acid. Since the above analysis has proved this quantity of oxalic acid to contain 12 (two combining weights) of carbon, 1 (one combining weight) of hydrogen, and 32 (four combining weights) of oxygen, the formula would be written CHO. In determining whether this formula represents only one grouping of the elements, or whether it contains two or more groups in combination, the chemist is guided by the results of a more minute study of the decompositions which the compound undergoes under varied conditions.

60. Salts formed by carbonic acid.-Although so ready to combine with the alkalies and alkaline earths (as shown in its absorption by solution of potash and by lime-water), carbonic acid must be classed among the weaker acids. It does not neutralise the alkalies completely, and it may be displaced from its combinations with bases by most other acids. Its action upon the colouring matter of litmus is feeble and transient. If a solution of carbonic acid in water be added to blue infusion of litmus, a wine-red liquid is produced, which becomes blue again when boiled, losing its carbonic acid; whilst litmus reddened by sulphuric, hydrochloric, or nitric acid, acquires a brighter red colour, which is permanent on boiling.

With each of the alkalies carbonic acid forms two well-defined salts, the carbonate and bicarbonate. Thus, the carbonates of potash and soda are represented by the formulæ, KO. CO, and NaO, CO, whilst the bicarbonates are KO. HO. 2CO, and NaO. HO. 2CO2. The existence of the latter salts would favour the belief in the existence of a hydrate of carbonic acid (HO. CO2), when they would become

KO.CO,, HO. CO2 and NaO. CO2, HO. CO2,

although no such combination of water with carbonic acid has yet been obtained in the separate state. Perfectly dry carbonic acid gas is not absorbed by pure quicklime (CaO), but when a little water is added combination at once takes place. This supports the view entertained by some chemists, that CO, is not an acid until it is associated with water, and they therefore speak of it as carbonic anhydride, reserving the name carbonic acid for the as yet undiscovered compound HO.CO, (or HCO3).

The following are some of the principal carbonates which are found in nature or employed in the arts :