EXP. 16.-Pour a little lime-water from a test-tube or small beaker into a jar of carbon dioxide and shake the liquid round in the jar. The lime-water will at once become milky, owing to the lime which is dissolved in the water being converted by the carbon dioxide into common chalk (calcium carbonate), and this, being an insoluble substance, remains mixed as a white powder ("precipitate") with the water ::

CaH2O2+ CO2 = CaCO3 + H2O.

2

EXP. 17.-Carbon dioxide gas readily dissolves in cold water, forming a liquid which probably contains carbonic acid :

CO,+H,O=H,CO.

The solubility of the gas in water may be proved by displacing the air from a bottle, previously half filled with cold. water, by carbon dioxide: then tightly closing the mouth of the bottle with the wet hand, and shaking vigorously for a short time the bottle will adhere to the hand, owing to a partial vacuum being produced by the absorption of the gas by the water. A further proof is afforded by dipping the end of the delivery-tube employed in Exp. 13 to the bottom of a beaker containing water, so as to cause the gas to bubble up through the liquid. After the bubbles have passed for several minutes the water may be shown to contain carbonic acid by pouring some of it into a test-tube and adding a little lime-water, which will cause a milkiness ;* also, by adding to another part of the carbonic acid solution several drops of blue litmus solution, or dipping into it a piece of blue litmuspaper, which will become red, indicating the presence of an acid. If this liquid containing carbonic acid be tasted it will be found to possess a taste resembling that of soda-water, and in fact soda-water is merely water which contains a large quantity of carbonic acid, as may be proved by examining it with lime-water and litmus. If some of the water containing

.*

* Sometimes the milkiness disappears, for reasons explained in Exp. 18, unless much lime-water is added.

.

carbonic acid be boiled in a test-tube, the carbon dioxide gas is driven off again: the bubbles of gas are seen rising in the water long before the latter boils, and after the liquid has been boiled briskly for several minutes, it may be proved to be free from carbonic acid by giving no milkiness on addition of lime-water, and by not changing the colour of blue litmuspaper of solution.

EXP. 18.-Dilute some lime-water, contained in a small beaker, with an equal quantity of distilled water, and allow the carbon dioxide gas to bubble through it as in Exp. 17; a milkiness will be produced owing to the formation of chalk (Exp. 16); but if the gas is allowed to bubble for several minutes through the liquid, the milkiness will gradually disappear, since the chalk dissolves entirely in the carbonic acid which is formed by the carbon dioxide dissolving in the water. On boiling some of this clear liquid it again becomes milky, since the carbon dioxide is driven out of the water by heat, and therefore the chalk can no longer remain dissolved. It will be found, on pouring out the water, that part of the chalk remains adhering to the inside of the tube, whence it may be removed by pouring in a few drops of hydrochloric acid. The above experiment explains the origin of the coating or 66 incrustation" of chalk inside kettles and steam-boilers in which chalk-water is boiled. Such water contains chalk dissolved by carbon dioxide gas present in the water, this gas is driven off when the water is boiled, and the greater part of the chalk separates upon the sides of the vessels.

Tests for Carbon Dioxide.-It is evident that the presence of carbon dioxide gas is shown by its properties of extinguishing a burning taper, and turning lime-water milky: these are the ordinary "tests" for carbon dioxide. We may now proceed to employ them to prove that carbon dioxide is evolved from our lungs during the process of respiration, and also that it is produced by a burning candle.

EXP. 19.-Invert a bottle full of water in a pan of water, and fill it with air from the lungs by blowing the breath out through a glass tube, one end of which is dipped into the

water and held beneath the mouth of the bottle. In order to obtain air from the lungs a full breath should be drawn, and the nose then closed by pinching it with the finger and thumb: before allowing any air to pass up into the bottle, a portion should be breathed out through the tube so as to replace the air contained in the mouth and in the tube by air from the lungs, the remainder of the breath is then allowed to bubble up into the bottle. Close the bottle, remove it from the pan, and introduce into it a lighted taper, the flame will be immediately extinguished.

Now blow air from the lungs (obtained as just described) through a glass tube into lime-water contained in a small beaker, the lime-water will become milky.

FIG. 7.

EXP. 20.-Fasten a small piece of candle or wax taper upon the deflagrating spoon, and place it alight in a bottle of air the mouth of which is closed by the brass plate (fig. 7). After burning for a short time the flame will be extinguished, and if relighted will be again extinguished when placed in the jar. Now pour in some lime-water from a small beaker, and shake it round in the bottle, the liquor will be rendered milky.

A similar experiment may be performed, substituting the flame of coal-gas which is burnt from a bent tube for that of a candle, and closing the mouth of the

bottle with a piece of cardboard. The extinction of the flame, and the milkiness then produced by shaking lime-water in the bottle, will prove the production of carbon dioxide by the combustion.

Hence carbon dioxide gas is constantly being introduced into the air by respiration and combustion, and we should therefore expect to be able to detect its presence in air by the above tests. It is manifestly not present in sufficient quantity to extinguish a burning taper, but the presence of carbon dioxide in air may be shown by lime-water in the following way:—

B

EXP. 21. Pour some clear lime-water into a watch-glass (or better a clock-glass), and allow it to stand for a few minutes in the air; a film of chalk will gradually form on the surface, and will be seen as white flakes when the water is stirred.

III. a. Nitric oxide gas may be prepared in the apparatus used for making carbon dioxide (fig. 5, p. 14).

EXP. 22. The pieces of marble are removed and the apparatus washed out. Some scraps of copper (copper clippings or turnings) are then placed in the flask, and nitric acid diluted with an equal measure of water poured in :

3Cu+8HNO3=2NO+3Cu(NO3)2+4H2O.

A reddish-brown gas soon fills the inside of the vessel, and should be allowed to bubble off for a time through water; it may then be collected over water as directed for hydrogen gas. Nitric oxide is colourless, but it forms a reddish-brown gas (chiefly nitrogen tetroxide) when mixed with free oxygen. This is shown by filling a jar with the gas over water, and then allowing it to stand with its mouth open in the air, the entrance of the oxygen of the air at once produces red fumes. The appearance of red fumes in the preparation vessel is thus explained, since the vessel is at first filled with air with which the first portions of the gas mingle.

IV. AMMONIA GAS.-The familiar smell of common "smelling salts" is due to ammonia gas which is constantly being

FIG. 8.

given off from the solid "carbonate of ammonia" contained in the bottle. The gas is thus evolved only very slowly, it may be made to come off much more rapidly if the "carbonate of ammonia" is mixed with lime and the mixture is then gently heated: sal ammoniac is usually employed instead of the carbonate of ammonia :

22

2NH4Cl + CaH2O2 = 2NH3+ CaCl2 + H2O.

EXP. 23.-Powder some ammo

nium chloride (sal ammoniac) in a mortar, and mix with

it thoroughly on a sheet of paper about an equal quantity of slaked lime in fine powder. Pour some of this mixture into a small flask (fig. 8) until it is about one-third filled, and close the neck of the flask with a tightly fitting perforated cork, into which is inserted a straight piece of glass tube 8 or 9 inches long. Heat the mixture gently by placing the flask upon a piece of wire-gauze on a tripod stand and putting underneath it a lighted rose-burner. Ammonia gas will soon be smelt issuing from the end of the tube, and, since it is much lighter than air, may be collected by "displacement," as described in Exp. 10. To ascertain when the vessel is filled with the gas it is only necessary to hold at the mouth of the jar a piece of moist red litmus or yellow turmeric paper; since ammonia gas changes the colour of the former to blue and of the latter to reddish-brown, it is easy to see whether it has reached the mouth of the jar by observing whether any change is produced in the colour of the paper.

Ammonia gas does not burn continuously in air at the ordinary temperature, but it burns readily either in strongly heated air or when lighted in oxygen gas. Show this by holding the end of the delivery-tube, from which a stream of the gas is issuing, in the top of a Bunsen-flame; a pale yellowish-green flame of burning ammonia will be seen; and if the end of the tube is dipped into a jar of oxygen, the NH, may be inflamed as it issues into the oxygen gas.

Ammonia should always be collected by displacement since it is extremely soluble in water, and therefore its collection over that liquid would lead to great waste of the gas. For this reason, also, the vessels in which ammonia is to be collected should always be perfectly dry inside.

EXP. 24. Place a jar full of ammonia mouth downwards in a vessel of water, and gently shake the jar so as to agitate the water at its mouth; the water rapidly absorbs the gas and rises in the jar to fill the space formerly occupied by the gas.

If a little water be rapidly poured into a jar of ammonia by momentarily partly opening it and at once covering it again