unite with various vegetable substances, and in doing so remove hydrogen from these substances, by what is termed a principle of replacement or substitution. They are therefore intimately connected with H, the first member of the subsequent class. 4. They unite with a very large proportion of simple bodies, forming salts, with the exception of oxygen, which produces oxides.

OXYGEN-(symbol O).

History.- Spiritus nitro-aereus (Mayow, 1672).

Dephlogisticated air

(Priestley, 1774). Feuerluft (Scheele, 1775). Oxygen (ogus, acid, yrívoμas, Į form, Lavoisier, 1787). Vital air (Condorcet).

Sources. About one-fifth of the atmosphere consists of oxygen. It exists abundantly in the earth, in union with metals forming oxides, and in all animals and vegetables.

Physical Characters. A transparent, colourless gas, with the mechanical properties of common air, destitute of taste and smell. Specific gravity 1111 (Thomson), 1105·7 (Dumas), that of air being 1000. The weight of 100 cubic inches is 34-457 grains (Thomson), 34.289 (Dumas). Mean, 34-378 grains. Refracting power to light 924, air being 1. (Dulong), 861 (Biot and Arago). Oxone is supposed to be a variety of oxygen.

Preparation of Gases in general.- When gases are prepared in large quantities, it is usual to employ a gas-holder for their reception, and a retort for the generation of the gases.

The gas-holder has various forms. One of the most economical kinds consists of glazed earthenware, of the annexed shape (fig.11.).

b

11

It has two apertures at the top, one of which a is connected with a tube which passes down to the bottom of the vessel, and is surmounted by a funnel for the introduction of water; the other aperture b is at the summit, and is intended for the escape of the gas. There is a third: opening at the bottom c, by means of which the gas-holder is filled with gas. In order to fill it with water, which is the first step in preparing it for the reception of a gas, the two apertures at the top are opened, that at the bottom is closed. Water is then poured in by the funnel at the top, until the vessel is completely filled, the air escaping by the middle aperture. When filled, the two openings are closed, one by a cork, the other by a stop-cock; the lower aperture is now opened, and the beak of the retort containing the substance for generating the gas is inserted in it. The water cannot run out of the vessel, because the openings at the top are closed, and the

C

12

AND GASES IN GENERAL.

333

atmospheric pressure is therefore withheld from the surface of the water; neither can the air enter from below, because the lower aperture is placed obliquely like a bird's fountain glass, where the water exposed to the air is pressed upon by the atmosphere, upon the principle exhibited in Cooper's tube for collecting gases over mercury (fig. 12.). When we fill this tube with water or mercury, retaining the stopper in it, this tube may be held vertically without any escape of fluid; and the beak of a retort or tube connected with a vessel for generating gas, being inserted in the lower opening, the gas enters and displaces the fluid. The inferior aperture of the gas-holder is constructed in a similar manner. The beak of the retort being introduced into the gas-holder as described, as soon as the gas is disengaged water begins to flow from the vessel in proportion to the amount of gas which enters; and as soon as all the water has been displaced by gas, we close the aperture, and set aside the gas-holder for use. In order to remove gas from the vessel as it is required, a bent tube is fixed to one of the apertures at the top, connected with a pneumatic trough, which is simply in this case a cylindrical vessel of earthenware, filled two-thirds with water, and having an inverted cylinder standing in it with an opening on one side for the extremity of the tube to enter, and another on its upper surface to admit of the escape of the gas from the end of the tube, and convey it into an inverted glass jar filled with water. The glass jar is thus placed by filling it in its natural position with water, covering its mouth closely with a plate of glass, and while holding it firmly, inverting it, and placing it on the shelf. The apparatus being thus arranged, the stop-cock at the top of the gas-holder is opened, the funnel is adjusted to the other aperture and opened. Water being poured in by the funnel, the gas escapes by the bent tube and passes into the inverted glass jar in the pneumatic trough. Thus a succession of jars may be filled until the whole of the gas has been expended. A very convenient gasholder is one made of copper, or of japanned tin or zinc, usually called Pepys' gas-holder. (fig. 13.). This apparatus consists of a cylinder (b) for holding the gas, and of an upper vessel (a) for holding the water which is intended to displace the gas. They are connected by two tubes, one (c) dipping to the bottom of the cylinder, and the other (d) entering at the top. In order to fill the gas-holder with water, the stop-cocks on the two tubes (c d) are opened, and water is poured into the upper vessel,

13

a

a, Water-holder.
b, Gas-holder.
c, Water tube.
d, Gas tube.
e, Gauge.

D

14

until no more air escapes by the tube which enters at the top of the cylinder. There is also a stop-cock at the side, which may be opened to facilitate the escape of the air. When full, the stop-cocks are closed. The gasholder is filled with gas in the same way as the earthenware gas-holder. To procure the gas for use after it has been collected, an inverted jar, filled with water, is placed in the upper vessel over the opening of the short tube, and the two stop-cocks being opened, the gas rises and displaces the water in the inverted jar. Connected with the cylinder there is a glass tube or gauge, which exhibits the amount of gas in the vessel in proportion to the rise or fall of the water contained in it. Another form of gas-holder resembles that which is used in gas works (fig. 14. ). It consists of a cylinder (a) containing water, and an inverted cap (b), which rises by the assistance of weights and pulleys as the gas enters.

a

15

Gases, however, may be collected in smaller quantities, and then a gasholder is unnecessary. The substances to supply the gas are placed in a retort (fig. 15.), the beak of which communicates with an inverted jar, placed in a pneumatic trough. Heat being generally required to facilitate the evolution of the gas, is applied cautiously by means of a spiritlamp, or a gas-flame, covered with a

wire gauze cylinder; or, when the amount of gas required is

16

to fusion from its containing lead.

minute, a wide test tube of Bohemian glass, supplied with a perforated cork and exit tube, supported on a grasping stand, may be used (fig. 16.).

English glass does not answer for such experiments, in consequence of its liability

Preparation of Oxygen. -Oxygen is obtained from various bodies, which contain it in a state of union. The substance which supplies the gas is placed in a retort or tube, and exposed to heat. The gas is evolved, and must be collected over water.

1. From Chlorate of Potash. —The most convenient method of

17

obtaining oxygen is to mix equal
parts of chlorate of potash, a
white salt crystallizing in plates,
and black oxide of manganese to-
gether in a porcelain mortar (fig.
17.); introduce the mixture finely
pounded into a retort by means

of a dry funnel (fig.18.), which is

18

Y

placed in the tubulture of the retort, and insert the beak of the retort either in the lower aperture of a gas-holder, if the amount of gas to be obtained is considerable, or supported on a grasper

19

stand, and under an inverted glass jar filled with water in a pneumatic trough, if the quantity sought is smaller; or in a glass tube (fig. 16.), if the amount is to be still more minute; the mixture may also be introduced into a flask with a perforated cork and bent tube (fig. 19.). Heat being applied to the flask, the gas is disengaged. On the application of a heat of about 660°, the salt begins to decompose, and in a short time the gas is evolved in great abundance. The first bubbles of gas, being common air, should be allowed to escape, and the gas should not be collected until a glowing piece of wood inserted in the beak of the retort is relighted, the characteristic evidence of the presence of oxygen. The manganese suffers no change, but prevents the salt from fusing, and by communicating its heat to the finely divided particles of the salt renders its decomposition, which is accompanied with frequent scintillations, gradual and complete.

The mode in which the chlorate of potash is decomposed, is represented in the following scheme. It is a compound of potash or oxide of potassium (KO), and chloric acid (Cl 0); when it is heated the whole of the oxygen passes away, while chloride of potassium remains in the retort.

Hence we observe, that every 15 grains of chlorate of potash yield 6 grains of oxygen gas, or 1 grain will yield 387 grains= 1.128 cubic inches.

2. From Black Oxide of Manganese. This substance is employed by itself for the preparation of large quantities of oxygen. The pounded oxide is introduced into an iron bottle, to the mouth of which is adapted an iron tube, luted with common pipe-clay and water (fig. 20.). To the extremity of this tube is

attached another tube, in which there is a joint, the outer end of which is introduced into a gas-holder. The iron bottle is placed in a furnace, and surrounded with red-hot coals. At a red heat the manganese parts with a third of its oxygen, which passes into the gas-holder, while the red oxide of manganese remains in the retort, represented as follows (Mn O, becoming Mn 01):

From this diagram, we learn that 16 grains of perfectly pure black oxide of manganese yield 2 grains of oxygen; but as the manganese of commerce is very impure, the product is much inferior to this. The oxide often contains carbonates, the carbonic acid of which is evolved by heat, but is easily removed by allowing the gas to stand over water, when the carbonic acid is dissolved in that fluid.

3. From Manganese and Sulphuric Acid. If we introduce 2 or 3 ounces of sulphuric acid into a retort (as in fig. 15.), and add to it through a funnel as much black oxide of manganese as will