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2. The reproduction of oxygen appears in the process of vegetation; healthy plants exposed in the sunshine to air, containing small quantities of carbonic acid gas, destroy that elastic fluid and evolve oxygen gas ; so that the two classes of organized beings are thus dependant.

3. It has lately been found, that azote generates uric acid, and gravel in the stones and kidneys, and hence animal food has properly been forbidden to persons subject to those af. flicting disorders.

523. Chlorine, or oxymuriatic Acid Gas, is an elementary substance, of a yellowish green colour, and it is this circumstance which suggested its name. Its odour is extremely disagreeable. It is not capable of being respired. And even when mixed in

very

small quantities with common air, renders the air extremely pernicious to the lungs. When an inflamed taper is introduced into a phial filled with it, the light continues, but of a dull red colour.

Obs.-Chlorine has never been found pure in nature, but exists in many compounds, particularly in common salt, and it may even be produced from that substance.

524. SULPHUR is an inflammable substance found in the neighborhood of volcanoes, combined with earths and metals, from which it is separated by sublimation in a furnace.

Obs.-When combined with oxygen, it forms a sulphuric and sulphurous acid. This is usually done by burning the sulphur in leaden chambers, the floors of which are covered with water, with a quantity of nitre (that is, one part to nine parts of sulphur) which affords oxygen for the formation of the acid which is condensed by the water. The water is then drawn off, evaporated by boiling, and afterwards distilled in retorts: the result is sulphuric acid, or oil of vitriol. Sul. phur combines with hydrogen, the alkalies, the earths, and metals. These combinations are called sulphurets.

525. Phosphorus is a simple substance, found in a state of combination with the bones of animals, from which it is extracted.

Its tendency to unite with oxygen is so great, that it always burns in the open air; and bursts into flame, at a degree of heat a little above that of the human body.

Its combinations with earths and metals are called phosphurets.

Obs.- Many amusing experiments may be performed with it; but great care should be taken, and a basin of water kept at hand, for it will kindle into an unextinguishable flame by slight friction.

226. Chemists, by separating earths from foreign matters, and from each other, have discovered nine primitive earths, which are not soluble in water or by heat: they are, Alumine,

Silex,
Barytes,

Strontia,
Glucine,

Yitria,
Lime,

Zircon, Magnesia, Obs.-These earths combine with acids, alkalies, oxygen, sulphur, phosphorus, the metals, and with each other, in various proportions; and four of them merit special notice. 1. Alumine, argil

, or pure clay, is infusible by the greatest heat; but heat makes it so hard as to scratch glass. It readily absorbs water, and also grease; and hence its use, as fuller's earth, in scouring cloths, &c.

2. Lime, or calcareous earth, is obtained by applying heat to chalk, marble, lime-stone; &c. by which carbonic acid gas and water are expelled, and the lime left as a product.

When used as a cement in building, water is used to make it plastic; and, with the addition of ashes, or sand, to supply carbonic acid, acquires its original hardness.

Lime is also used in tanning: in making sugar and soap; and it forms 80 parts, combined with 20 of phosphoric acid, in 100 parts of animal bones.

Lime is antiseptic; a few drachms, stirred in a cask of impure water, will render the whole perfectly sweet.

3. Magnesia, is a soft, white earth, generally found in combination with sulphur. United with sulphuric acid it forms that sulphate called Epsom-salts.

4. Silex, or flint, is the principal ingredient of stones, crystals, sand, &c., and cannot be melted by itself in any heat; but in contact with alkalies, as soda or pot-ash, it forms that useful production called glass.

527. The immense stony masses of which the globe is composed, are found in the earth, lying in strata one above another; a rock of one kind covering another species of rock; this a third, and so on. The

arrangement is not arbitrary ; but each species occupies its regular place, from the deepest part yet explored to the surface.

Obs.-Rocks are divided into five classes or formations ; called primitive, transition, stratified, alluvial, and volcanic.

1. The primitive formations of rocks are the lowest ; and are supposed to have been chemical precipitations, formed in the chaotic state of the earth; because they have no petrifactions or trace of organized beings. They are chiefly composed of silicious and argillaceous earth, as granite, slate, &c.

2. Transition-rocks are supposed to have been formed during the transition of the earth into a habitable state ; and differ from primitive, in the variety of their colours, and in containing the remains of marine animals.

3 Stratified rocks are disposed in horizontal strata ; they contain the remains of animals and vegetables, and consequently were formed after the creation of animals and vegetables

4. Alluvial formations consist of the constituent parts of previous rocks, separated by water, air, &c. and deposited in beds. These are compounded of sand, gravel, loam, clay, turf, &c.; and contain plants, roots, moss, bones, &c.; likewise petrified wood, and skeletons of quadrupeds ;-the remains of destroyed worlds.

5. Volcanic formations are minerals thrown out of the crater of a volcano, consisting of pumice-stones, lava, and basalts.

528. All acids consist of certain bases combined with oxygen, which is considered as the general cause of acidity.

Obs. 1.-Acids excite a sour taste on the tongue ; change vegetable blue colours into red ; and are either solid, liquid, or gaseous. Acids unite with water in every proportion, and some of them have such an affinity for it, as never to quit their fluid state. They also combine with the alkalies, earths, and metallic oxydes ; forming salts.

2. Salts may be formed by art; but many exist in nature, ready formed. Salts possess double names; for example, the Nitric Acid combined with Potass forms what is called Nitrate of Potass.

529. There is also another property in nature, called the alkaline, which is distinguished from the acid by its burning and urinous taste; and it has the

distinct properties of converting vegetable blues into greens.

The two alkalies are potass and soda; and there is also a volatile alkali called ammonia.

Obs. 1.-Alkalies and acids have the property, when combined, of neutralizing eacb other; and hence acids, when combined with alkalies, form what are called Neutral Salts.

2. Alkalies, mixed with fats, make soap; and, when melted, with silex, glass.

3. Potass is chiefly obtained by burning vegetables, as kali, &c., and hence called pot-ashes ; but Soda, the other alkali, is obtained from sea-salt, from natron beds, and even from mines. Ammonia is obtained from animal substances, viz. bones and urine; and also in the distillation of coals for gas.

530. The several Acids hitherto discovered are 44 in number. 11 Mineral. 8 Metallic. 13 Vegetable Acids. 12 Animal Acids. Sulphurous, Arsenious, Acetic, Phosphorous, Sulphuric, Arsenic, Malic, Phosphoric, Nitrous, Tungstic, Oxalic, Bombic, Nitric, Molybdic, Citric, Sebacic, Muriatic, Chromic, Tartaric, Laccic, Oxy-Muriatic, Columbic, Tartareous, Lactic, Carbonic, Molybdenous, Benzoic, Saccho Lactic, Fluoric,

Camphoric, Prussic, Boracic,

Gallic, Amniotic, Mellithic,

Lithic, Hyper-Oxy-Muriatic. Suberic, Rosacic.

Mucous,

Moroxylic. 531. The principal Acids obtained by the mixture of oxygen with other substances, are

Oxygen and sulphur, called sulphuric acid, or oil of vitriol.

Oxygen and nitrogen, called nitric acid, which dissolves silver, and thence the art of plating with silver.

Oxygen and Chlorine called muriatic acid, or acid of sea-salt ; this acid, united to soda, is called muriat of soda, which is the common salt of the table,

Succinic,

Obs. 1.-Oxygen, and Fluorine, obtained from Derbyshirespar, is called 'fuoric acid, and is employed for etching on glass. Nitro-muriatic acid, or aqua regia, dissolves gold and platina.

2. Acids may combine with two or more substances, and the product is called a triple compound. Thus sulphuric acid, combined with alumine and potass, forms alum, or sulphat of alumine and potass ; and Tartar emetic is the tartrate of the metal antimony, and potass.

532. Salts formed from acids combined with alkalies, earths, or oxydes, amount to many thousands.

Those of which sulphuric acid, or oil of vitriol,
forms a part, are-
Sulphate of Soda,

Sulphate of Magnesia,
Potass,

Alumine,
Barytes,

Iron,
Lime,

Copper, &c. Those of which Nitric Acid are formed, are Nitrate of Potash,

Nitrate of Mercury,
- Silver,

Copper, &c.
Muriatic Acid is a component part of
Muriate of Soda,

Muriate of Mercury,
Ammonia,

Copper,
Barytes,

Iron,
Lime,

Lead,
Silver,

Cobalt, &c.
Oxymuriatic Acid forms a part of
Oxymuriate of Potass,

Oxyinuriate of Mercury,
Soda,

&c. Nitric and Muriatic Acid combined, form NitroMuriates. These are Nitro-Muriate of Gold,

Nitro-muriate of Cobalt.

Tin, 1 533. Metals are' seldom found in the earth in a pure state, but generally in combination with oxygen, earths, sulphur, and acids. In these states of combination, they are called ores; and are generally found in mines. (See article Metallurgy, p. 19.)

Obs. 1.-Metals are fusible by beat, and when suffered to cool gradually, they crystallize. If they are continued in filsion, they absorb a certain portion of oxygen, lose their brilliancy, and become metallic oxydes. Water also oxydates

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