II. HYDROGEN GAS.-The liquid substance water consists of oxygen gas, combined with another gas called hydrogen ; several processes are known for preparing hydrogen from water. This gas is, however, most readily obtained from another liquid containing it, called hydrochloric acid, by the action upon it of the metal zinc.

Exp. 8.-Fit a two-necked Woulffe's bottle* (A, fig. 3), with

FIG. 3.

air-tight perforated corks bearing a thistle funnel (B), whose end reaches nearly to the bottom of the bottle, and a tube bent as shown at c and terminating just below the cork: join to this, by means of a short piece of tightly fitting indiarubber tube, a bent delivery-tube D. Pour into the bottle sufficient granulated zinc to cover the bottom, replace the corks, and after ascertaining that the apparatus is air-tight by closing the end of the delivery-tube and blowing down the thistle-funnel, pour in through the funnel sufficient water to cover the zinc and the end of the funnel-tube; then add strong hydrochloric acid gradually until, after mixing the acid and water by shaking the bottle, the hydrogen is seen to rise from the zinc in numerous small bubbles :

Zn + 2HCl = 2H + ZnCl2.

Then dip the end of the delivery-tube under water contained

* A wide-necked bottle may be used instead, being fitted as shown in fig. 5 (p. 14).

in the pan or trough, and allow the gas to bubble out through the water for at least five minutes. This delay is necessary in order to give the hydrogen time to entirely remove the air which filled the bottle, and which when mixed with hydrogen produces a dangerously explosive mixture. Before collecting larger quantities of the gas for experiments, ascertain that the hydrogen is no longer mixed with air by inverting a test-tube filled with water over the end of the delivery-tube; as soon as the tube is full of gas close its mouth with the thumb, and hold it to a flame; if the gas burns with a slight explosion, the tube is again similarly filled with the gas and tried in the same way. As soon as the gas burns quietly with a pale flame, a jar (or a small thick glass cylinder or tube) may be filled with it in the same way as was directed for oxygen.

If during the preparation of gas for the following experiments the gas comes off too slowly, it is only necessary to pour in a little more strong acid through the funnel and mix it with the liquid in the bottle by gently shaking the latter.

EXP. 9.-Hold the jar filled with hydrogen with its mouth open and directed upwards for a short time, the gas will entirely escape; the absence of the gas may be shown by holding a lighted taper in the jar, when no flame will be seen at the mouth. If the jar be refilled with hydrogen and held for a short time mouth downwards, the hydrogen will remain in it, and its presence may be shown by the gas burning with a pale flame when a lighted taper is introduced : these results prove that hydrogen is much lighter than air, since its tendency to rise prevents it from passing out downwards through the open mouth, whilst it readily escapes upwards from the erect cylinder. Since hydrogen is so much lighter than air, it is possible to collect the gas without using water by a process called "displacement." The delivery-tube of the hydrogen apparatus is passed up to the top of the inverted jar containing air, the hydrogen rises to the upper part of the jar and gradually pushes out the heavier air downwards.

EXP. 10. Fill a jar with hydrogen by "displacement." For this purpose fit upon the india-rubber joint of the hy

drogen apparatus a delivery-tube bent as shown in fig. 4, and when a brisk effervescence of gas has been caused by pouring

is thus almost entirely prevented.

Remove this jar, keeping it mouth downwards, and push up inside it a burning wax taper five or six inches in length; the hydrogen will be lighted, and will burn with a pale flame at the mouth of the jar, but the flame of the taper will be seen to be extinguished by the gas: the taper may, however, be rekindled by holding it in the hydrogen flame burning at the mouth of the jar.

EXP. 11.-Cover the bottle and funnel-tube with a cloth, to prevent accident in case of an explosion, and light the hydrogen at the end of the delivery-tube used in the last experiment. Hold over the flame a perfectly clean, dry, and cool tumbler or beaker, the inside will become dimmed with moisture, showing that hydrogen gas burning in the air produces water::

H2+0 = H2O.

Note. Since all gases which have been in contact with water contain more or less vapour of water, or are "moist," it is usual to employ for this Exp. a stream of hydrogen gas which has been freed from moisture or "dried." A gas is dried (or desiccated) by passing it through some substance which readily absorbs moisture. Strong sulphuric acid (oil of vitriol), calcium chloride, and quick-lime are the desiccating agents most commonly employed. The gas may be made to bubble through strong sulphuric acid contained in a bottle, B (fig. 10, p. 22), connected, as shown in the figure, with the generating apparatus. It may

be also dried by passing it through a tube containing fragments of calcium chloride, or of quick-lime (fig. 10a), or pieces of pumice-stone moistened with strong sulphuric acid: the moistened pumice may be contained in a U-tube (fig. 106), or in the bottle в (fig. 10, p. 22).

.

EXP. 12. Measure the height of a short thick glass cylinder, and divide it into three equal parts by small pieces of gum-paper stuck upon the outside. Fill the cylinder with water and, after inverting it in water, fill one-third with oxygen (Exp. 4), and the remainder with hydrogen (Exp. 8); let the jar stand with its mouth under water for five or six minutes to allow the gases to mix; then apply a lighted taper to the mouth of the jar directed downwards, taking care not to place the fingers beneath it: the gases combine to form water with a loud explosion.

Test. Hydrogen gas is recognised by burning with a pale flame in air or oxygen, the flame depositing water on any cold object held above it.

III. CARBON DIOXIDE GAS (CARBONIC ANHYDRIDE).*When carbon was burnt in oxygen (Exp. 5) a gas called carbon dioxide remained in the jar: the gas may be prepared in this way, but a much more easy method consists in pouring hydrochloric acid upon some pieces of marble:

CaCO3 + 2HC1 = CO2 + H2O + CaCl2.

Chalk or limestone may be substituted for marble, but it does not answer so well.

EXP. 13. Rinse out the apparatus used for preparing hydrogen, and place in it some small pieces of marble; fit into the india-rubber joint a delivery-tube bent at right angles, as shown in fig. 5; then pour through the funnel-tube sufficient water to cover the marble and the end of the funnel-tube, and then strong hydrochloric acid until gas comes off with brisk effervescence. Place the delivery-tube in a jar with its end nearly touching the bottom; cover the mouth of the jar with a small disk of cardboard which has had a slit cut in it for

* Formerly called carbonic acid, a name objectionable on chemical grounds.

the delivery-tube (or pass the delivery-tube through the brass cap of a deflagrating spoon), and allow the apparatus to stand for several minutes.

Carbon dioxide being much heavier than air, will soon fill

FIG. 5.

the jar by "displacement;" that is to say, it will collect in the lower part of the jar, and, by gradually rising in it, will lift out the air. Since this gas has the property of extinguishing a burning taper, it is easy to ascertain when the jar is full by holding a lighted taper just inside its mouth if the taper is extinguished, the carbon. dioxide has reached the top. Carbon dioxide gas, being very largely dissolved by water, is rarely collected over water, the process of "displacement" being very preferable.

EXP. 14.-Allow this jar of carbon dioxide to stand uncovered and with its mouth upwards for a few minutes; then place in the jar a burning taper: the carbon dioxide is shown to be still present in the vessel by the immediate extinction of the taper. Then hold the jar for several minutes with its mouth downwards; on testing with a lighted taper, only air will be found in the vessel. These experiments prove that carbon dioxide is heavier than air, since it remains in a vessel which is open only above, and falls out of one which is open below. EXP. 15. Since this gas is so much heavier than air, it can be poured from vessel to vessel like water. This may be shown by pouring carbon dioxide from a bottle filled with the gas into a jar full of air, the latter being somewhat the smaller. The bottle is gradually tilted a little beyond the horizontal position, with its mouth over that of the jar. After holding it in this position for a short time it may be proved by

FIG. 6.

a lighted taper that the gas has left the bottle and is present in the jar.