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gas is often found in the lower part of caverns, wells, mines, and other subterranean places. In mines it proves frequently fatal to the miners, who call it the choke damp. Wells, or similar places, which have been shut up for a long time, should never be entered without first putting down a lighted candle : if this is extinguished, it is not safe to go down. There are some caverns in which this gas is produced in so great a quantity, that it runs out at the opening, like a stream of water: this is particularly the case with the celebrated Grotto del Cane. A dog is suffocated if it be held for a short time in the lower part of the cavern, but the upper part is free from

this gas.

cause.

Charcoal should never be burnt in rooms that have no chimney, because the red hot charcoal unites with the oxygen of the atmosphere, and forms carbonic acid, which cannot escape. Some melancholy accidents have happened from this

This
gas

has also been called mephitic air, from its suffocating quality.

Carbonic acid combines with all the alkalies, and with the alkaline earths, lime, magnesia, barytes, and strontia. With these it forms a class of salts, called carbonates. Carbon does not combine with any of the metals

This combination is the carburet of iron, called also plumbago and black lead; which, however, contains only five per cent. of iron. It is the substance used for making black lead pencils.

Carbonic Oxide.— This body is always gaseous. It contains only half as much oxygen as carbonic acid does. It is void of taste and smell, and fatal to animal life. It is inflammable, burning

except iron.

with a blue lambent flame, but does not explode when mixed with atmospheric air. 'It is procured by depriving carbonic acid of part of its oxygen. This is effected by exposing equal parts of chalk and filings of zinc to a gradual red heat, suffering the first product to escape, which is carbonic acid gas. The zinc deprives the carbonic acid in the chalk of part of its oxygen.

Carbon and Nitrogen.

This combination is called cyanogen. It is a gaseous body, having a penetrating and peculiar smell, and burning with a purple flame. It reddens vegetable blues.

When cyanogen combines with hydrogen, it forms a triple compound, called hydrocyanic acid : this is also called Prussic acid. Prussic acid is a liquid having a very pungent odour, like that of bitter almonds. It is extremely acrid, and highly poisonous. It is called Prussic acid, because it forms one of the constituents of the well-known pigment Prussian blue, which is a combination of hydrocyanate of iron with alumine.

PHOSPHORUS.

This highly inflammable substance is not met with in nature uncombined; but it exists combined with oxygen, forming phosphoric acid, in many animal and mineral substances.

Phosphorus is a yellowish semi-transparent matter of the consistence of wax. It is luminous in

the dark at the common temperature of the atmosphere.

To show this property in a striking manner, write with a stick of it upon black or purple paper, or any other smooth surface; the writing will be luminous as if on fire. The fiery appearance disappears and appears again by blowing upon it. It is necessary, in making this experiment, to cut the phosphorus under water, and to put it into a quill, in order to defend the hands, lest it should take fire; and great care ought to be taken lest any particle should be left under the nails, or in any other place, for if this were afterwards to take fire it might occasion very serious accidents, as a burn by it is extremely severe.

Very slight friction is sufficient to inflame phosphorus. Put a grain of it into brown paper and rub it with some hard body, and it will take fire and inflame the paper. It takes fire spontaneously, and burns rapidly in the open air at 1224 Fahr., with a brilliant flame. On this account, it is always kept under water ; and it should never be suffered to lie exposed to the air.

Phosphorus is obtained by decomposing the phosphoric acid by means of charcoal in a retort. The oxygen of the acid unites to the carbon, forming carbonic acid; and the phosphorus distils over into water. The phosphoric acid is obtained by decomposing calcined bones by sulphuric acid. Bones consist chiefly of phosphate of lime; and in this process the sulphuric acid joins to the lime, leaving the phosphoric acid free.

Phosphorus is soluble in oil in small quantity, which is thus rendered luminous. Sulphuric and nitric ether, and ardent spirit, dissolve it, though sparingly, in the cold.

Phosphorus and Oxygen.

Phosphorus unites with oxygen in two proportions, forming phosphorous acid, which contains the lowest proportion of oxygen ; and phosphoric acid, which contains the greatest proportion.

Phosphoric acid may be made by the rapid combustion of phosphorus in oxygen; but it is usually obtained from calcined bones, by decomposing them with sulphuric acid.

Phosphorus has the property of de-oxidizing several metallic solutions, as those of gold, silver, copper, mercury, lead, tin. If a stick of phosphorus be left in a concentrated solution of nitrate of copper, the copper will be precipitated upon the phosphorus in a metallic state.

It also combines with lime, forming phosphuret of lime. When pieces of phosphuret of lime are dropt into water, flashes of fire are seen to rise out of the water, which is occasioned by the phosphuret decomposing the water, and part of the phosphorus uniting to the hydrogen gas, forming phosphuretted hydrogen gas, described before.

The phosphurets of barytes and strontia have similar properties.

Phosphorus combines with chlorine and with iodine; and also with sulphur and the metals. Its union with hydrogen has been already noticed as phosphuretted hydrogen gas.

BORAX.

This elementary substance is known only in the boracic acid, which consists of it united to oxygen.

Boracic acid is rarely found native, but is generally procured from the salt called boraz.

Borax is boracic acid united to soda, or a borate of soda. This is the tincal brought from Asia purified. It is found at the bottom of certain lakes in Thibet, and in China. The boracic acid has been decomposed but lately, when it yielded to the application of galvanic electricity by Sir H. Davy. Borax is in the form of a powder of an olive colour. It is combustible.

Borax is much used as a flux in soldering metals, and also for such stones as cannot be brought into fusion by alkalies.

FLUORINE.

This name has been given, provisionally, to the supposed base of the fluoric acid, which is imagined to consist of fluorine and hydrogen. The

fluoric acid has hitherto resisted all the endeavours that have been made to decompose it completely; and its real nature, therefore, continues uncertain. According to the present nomenclature, it is now sometimes called hydrofluoric acid.

Fluoric acid is obtained by adding sulphuric acid to some pounded pure fluor spar,

and

applying heat.

is that mineral well known by the name of Derbyshire spar, because very abundant in that county. It is employed for making vases and other ornamental works. It consists of fluoric acid and lime, or, perhaps, calcium, the metal of lime, and is hence called also fluat of lime. The sulphuric acid, having a stronger attraction for lime

Fluor spar

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