anhydride be plunged into a bath of the solid anhydride mixed with ether, and placed in the vacuum of the air-pump, the liquid in the tube will speedily be frozen into a clear, transparent, icelike mass, which melts at -70°. The solidified anhydride is heavier than the liquid portion in which it is being formed.

Gaseous carbonic anhydride is not inflammable, neither will it support the flame of burning bodies: the extinction of a taper is one of the means frequently resorted to for detecting its presence. Many other gases, however, have the same property; some additional test, therefore, becomes necessary. Such a test is afforded by its action upon lime-water, which, when agitated with the gas, is immediately rendered milky from the formation of chalk; a few drops of any strong acid dissolve the chalk and restore transparency to the liquid; an excess of even carbonic acid has the same effect.

Carbonic anhydride in its concentrated form is irrespirable, for by producing spasm of the glottis it is prevented from entering the lungs; when diluted with air, however, it may be breathed without even a suspicion of its presence. If the proportion exceed 3 or 4 per cent. of the air it acts as a narcotic poison; and even in much smaller quantities its depressing effects are very injurious. The ill effects experienced in crowded and ill-ventilated rooms are partly due to the presence of this gas in undue quantity,* but partially also to the accumulation of volatile putrescible organic particles given off from the surface of the lungs and skin. It is the combination of these circumstances which renders attention to ventilation a matter of such high importance.

Gaseous carbonic anhydride is more than half as heavy again as atmospheric air; 100 cubic inches of it at 60° F. and 30 inches Bar. weigh 47'303 grains; from its density it may easily be collected in dry vessels by displacement, in the manner represented in fig. 284, and may be poured from one vessel into another like water.

FIG. 284.

No definite hydrated carbonic acid is known; the anhydride,

*The maximum observed by Roscoe in his experiments on the atmosphere of dwelling-houses, was o'33 per cent., and this occurred in a crowded schoolroom.-(Q. J. Chem. Soc. x. 265.)

58

PROPERTIES OF CARBONIC ACID.

2

both in the form of gas and in its denser conditions of liquid and solid, being, as its name indicates, free from water; but it appears convertible into a true acid by solution in water €0, + H2O yielding H ̧ ̧. At the ordinary temperature, the gas is soluble in about its own bulk of water; and its solubility increases if the pressure be increased;* but when the compression is suddenly removed, the gas escapes with brisk effervescence. Advantage is taken of this circumstance in the preparation of soda-water, as it is called. For this purpose the water, which may or may not contain soda or other substances in solution, is mechanically charged with a large quantity of carbonic acid, by the use of a condensing syringe, attached to a reservoir filled with the gas. The excess of the gas thus forced into the liquid occasions the agreeable briskness and pungency so much prized in this beverage.

A solution of carbonic acid in water reddens tincture of litmus; but the red colour disappears if the liquor be boiled for a few minutes, owing to the expulsion of the gas. The aqueous solution of the acid possesses solvent powers which, though in many instances extremely feeble, are yet far more extensive than those of pure water. By the continuous action of water charged with carbonic acid, even granite and the hardest rocks are disintegrated, few finely divided minerals being able to resist its gradual and long continued action. The proportion of gas dissolved is in many instances very minute, but as few natural sources of water exist which are not to a greater or less extent impregnated with carbonic acid, either by absorption from the atmosphere or from the soil, the solution, insignificant as it may at first sight appear, is continually proceeding, and in the lapse of time it effects changes of great importance and extent.

The briskness of spring water, and the preference given to it as a beverage, is partly occasioned by the carbonic acid which it contains; though its usual coolness and the abundance of atmospheric air dissolved in it are still more important. It is the absence of these qualities which renders boiled or distilled water flat and insipid.

Carbonic acid was originally termed fixed air, from the circumstance of its having been discovered by Dr. Black, in 1757, as a solid or fixed constituent in limestone, and from its becoming fixed or absorbed by solutions of the pure alkalies.

(349) Natural Sources of Carbonic Acid.-Besides the processes

* If the gas be simply transmitted through the water, the liquid seldom takes up more than two-thirds of its bulk.

NATURAL SOURCES OF CARBONIC ACID.

59

for procuring the gas already described, there are a variety of cases in which it is produced on a very large scale in nature.

1.-Respiration in man and animals is always attended with the formation of a large proportion of the gas. This fact may be easily proved by forcing air from the lungs by means of a tube through lime-water, which will speedily become milky from the deposition of carbonate of calcium. The proportion of carbonic anhydride. in respired air varies from 3 to 4 per cent., being usually about 3 per cent.

2.-Carbonic acid is also abundantly formed in the process of fermentation, and is the cause of the briskness in bottled beer, champagne, and other fermenting liquors. Many accidents have occurred from persons incautiously descending into an empty fermenting-vat before the gas has had time to escape and mix with the air: it is usual to facilitate the escape of the dense gas by leaving the plug at the bottom of the vessel open for some hours.

3. In the operation of burning lime in the lime-kiln, the heat expels from the limestone the carbonic anhydride, which escapes in large volumes. Many a poor houseless wanderer, tempted by the warmth of the kiln, has lain down in the stream of air proceeding from it, and has slept to wake no more. By the operation of subterranean heat in volcanic districts upon limestone beneath the surface, large volumes of carbonic anhydride are continually finding their way into the atmosphere; immense quantities are discharged from open craters or from fissures and cavities in the soil; the springs in such districts are also frequently highly charged with it, and the gas escapes with effervescence when they reach the surface. The springs of Seltzer, Pyrmont, and Marienbad, on the Continent, and of Tunbridge, in our own country, exhibit this phenomenon.

4.-The carbonic acid met with in spring water is in many instances derived from the gradual oxidation of the vegetable and other organic matter which it holds in solution, by the action of the oxygen of the air which all waters naturally contain. The lake waters from the primitive districts, such as those in the northern parts of Scotland, leave scarcely any residue on evaporation except a little organic matter; they are very free from carbonic acid, and the bulk of oxygen which they hold in solution is somewhat more than one-half that of the nitrogen. If such waters be kept in closed vessels for a few weeks in a warm room, the oxygen gradually decreases, and in its place a corresponding volume of carbonic acid is found. The pure water of Loch Katrine, for example,

60

ANALYSIS OF AIR CONTAINING CARBONIC ANHYDRIDE.

when first collected did not yield more than o'06 cubic inches of carbonic anhydride per gallon; but the quantity of this gas which the same sample yielded after it had been kept in a closed vessel for some weeks, in a warm room, rose to 0.38 cubic inches in the gallon, whilst the oxygen had diminished to similar extent. Spring waters which rise in a sandy district, the surface of which is sparingly clothed with vegetation, and from which consequently they can take up but little organic matter, contain but small quantities, often mere traces, of carbonic acid; whilst the springs of highly cultivated districts, such as those which rest more or less directly upon the chalk, become charged with organic matter, which gradually undergoes oxidation in the soil, and the quantity of carbonic acid contained in such waters is always considerable, whilst the quantity of oxygen which they hold in solution is proportionately reduced.* The extent to which this change

takes place in river water is very remarkable. It is well exhibited in the case of the Thames. On one occasion, some samples were taken from the river at low water at different points on the same day, in August, 1859; those collected above the metropolis being nearly free from contamination with sewerage products, whilst those obtained lower down were extensively impregnated with them. The

*The analysis of these gases or of any mixture of air with carbonic anhy dride, such, for example, as respired air, may be effected with sufficient accuracy for most purposes in the following manner :-Supposing that the gas had been collected over either water or mercury, it becomes necessary to transfer a portion of it from the jar in which it has been collected to the one in which it is to be analysed. A method of effecting this is shown in fig. 285. Upon the board, a d, is fastened a pipette, designed for effecting this transfer; a is a cylindrical funnel of a capacity of about two cubic inches; at c is a small steel stopcock, or a piece of vulcanized caoutchouc tubing compressed by a screw, which is simpler and less expensive; by either of these contrivances the contents of the funnel can be admitted to a wide thermometer-tube, which is furnished at d with a second steel stopcock or caoutchouc tube and screw clamp; b is a bulb of the capacity of one cubic inch, or rather less; from the upper part of b proceeds another piece of thermometer-tube, bent as shown in the figure, to allow of its introduction into the gas-jar. To use the instrument, the funnel, a, is filled with mercury, the stopcocks are both opened, and as soon as the air has been displaced from the vertical portion of the fine tube, and mercury escapes through d, the stopcock, d, is closed; the mercury quickly displaces the air from the rest of the tube, and from the bulb 6, and as soon as it begins to flow out at the open extremity of the recurved portion, the stopcock, c, is closed. The instrument being now full of mercury is introduced into the jar, e, of the gas to be transferred, and its open extremity is raised above the level of the water in the jar, e; the stopcock, d, is then opened, and whilst the mercury runs out into a vessel placed for its reception, the gas enters from e, and occupies the place of the mercury in the bulb, b. When a sufficient quantity has been admitted, the tube is depressed below the level of the water in the jar, e; the stopcock, d, is closed, and the pipette, which is sealed by the admission of a little water into the capillary tube, is withdrawn from the jar, f. The gas can now be transferred to the graduated tube, h, standing in the jar of mercury, g; the bent limb of the pipette is

FORMATION OF CARBONIC ACID IN WATER.

61

gases were expelled from each sample by boiling, within twentyfour hours from the time of its collection, and the results obtained are given in the following table, in which they are contrasted with the proportions of each gas furnished by a similar experiment upon

introduced into the tube, h, which has been previously filled with mercury. Fresh mercury is poured into the funnel, a, of the pipette, and on opening the

gas

stopcock, C, the is expelled into the tube, h; the gas should not occupy more than two-thirds of the capacity of this tube.

The proportions of carbonic anhydride, of nitrogen, and of oxygen are now easily ascertained in the following manner :-The bulk of the gas in tube, h, is to be carefully read off, care being taken to bring the mercury to the same level within and without the tube; the temperature and the pressure being observed as usual. Supposing that it has thus been ascertained that a bulk of gas of about two-thirds of a cubic inch is to be subjected to the analysis, the operator, by means of a glass syringe, throws up ten or twelve drops of a solution of caustic potash (sp. gr. 1'4) into the tube. The glass syringe may be extemporaneously prepared from a strong tube which is softened in the flame of a lamp, drawn off and recurved at one end, as shown in the figure at i; this constitutes the body of the syringe, whilst the piston is easily formed of a piece of glass rod, provided with a plug of caoutchouc.

The operator then agitates the contents of the tube by rapidly thrusting down the tube into the mercury, and withdrawing it, taking care to keep the mouth below the surface of the mercury: this manoeuvre is several times repeated in quick succession; the tube is again left at rest for a minute or two, and the absorption is noted by a second time reading off the volume of the gas at the proper level. The difference indicates the amount of carbonic anhydride.

In order to ascertain the proportion of oxygen in the remainder, the plan