Obs. 1. When it is found ready formed in earths, or friable minerals, it is extracted by collecting the earth into proper vessels, and pouring on water, which passes through, dissolves the alum and holds it in solution. The water is then evaporated until the salt crystallizes.

2. When the mineral which furnishes this salt is an aluminous clay mixed with sulphur and iron, another method is taken. Here, although most of the ingredients which form alum are present, yet no salt is formed. To form the salt the clay containing the iron and sulphur is exposed to heat, or merely to the air until the sulphur becomes acidified or converted into sulphuric acid, this then unites to the alumine and "forms a sulphate. If no potash be present in the earth, a quantity is added, as this is necessary to form alum. The clay is then treated by lixiviation and evaporation as above.

3. The uses and common properties of alum are so well known as to need no description.

441. A combination of alumine, sulphur, and charcoal forms Homberg's pyrophorus.

Illus. To prepare this, powder an ounce or two of alum and mix with it an equal quantity of brown sugar. Put the mixture into a ladle, or earthern dish, and keep it stirring over a fire until all the water is driven off. Then pulverize it finely, and introduce it into a common vial coated with a mixture of clay and sand. Lute to the mouth of the vial a small glass tube, or stem of a tobacco pipe, to allow the moisture and gases to escape. The vial must then be set in a crucible surrounded by sand, and the whole set in a fire and gradually heated to redness. Water will soon issue from the end of the tube, and afterwards a gas, which on being lighted, burns with a blue flame.

After the flame goes out, keep up the heat for about fifteen minutes, and then remove the crucible from the fire, and immediately stop the orifice of the tube with a piece of clay. When the vial is cool enough to be handled, the pyrophorus must be hastily poured into warm and perfectly dry vials and corked from the air. It will be in the form of a black powder.

442. This powder is a curiosity, and sometimes a convenience.

Illus. If it is exposed to the air, it undergoes spontaneous ignition, giving out the smell of sulphur, and glowing with a red heat. If a small quantity be poured on a piece of paper it sets it on fire. If the vial is held a few feet from the ground and it is shaken out, a little at a time, it resembles a shower of fire.

In oxygen gas it burns or explodes with violence, giving out intense heat and light, affording a beautiful experiment.

443. Small vials filled with it may be kept for years, provided care be taken to prevent access of air. The most effectual method is to immerse their mouths under mercury.

Obs. 1. This powder affords a convenient method of obtaining fire for common purposes.

2. In regard to the chemical changes which the alum and sugar undergo to form this curious substance, Mr. Accum supposes that the red heat to which they are exposed, causes the sulphuric acid which the alum contains, and the charcoal to which the sugar is reduced, to react on each other, by which, part of the charcoal is converted into carbonic acid and is driven off in that form, at the same time the oxygen which the sulphuric acid imparts to the carbon, converts a part of this into sulphurous acid, which also makes it escape; the blue flame is probably caused by sulphurated hydrogen; the hydrogen being formed by the absorption of the oxygen of the water, by the carbon of the sugar. The remainder of the sulphur unites to the potash of the alum and forms sulphuret of potash.

Thus the powder consists of sulpuret of potash, alumine and carbon. When this is exposed to the air the sulphuret absorbs the oxygen, not only from the air itself, but also from the water which it contains with such avidity, as to occasion a degree of heat which sets fire to the charcoal and sulphur.

NITRATES.

444. The nitrates by the action of fire, furnish oxgen gas. Most of them scintillate when thrown on burning charcoal.

Obs. This last effect is in consequence of their increasing the combustion by imparting oxygen gas. Hence most of the ni. trates are capable of supporting combustion.

445. Nitrate of Potash. Nitre. Saltpetre. This well known substance is found native in all countries, where circumstances are favourable to its production. It however seldom occurs in large masses, but is generally found either in the form of a whitish efflorescence on the soil, or in small

masses, or incrustations in limestone caverns. Its particles are also diffused through particular kinds of soil, but never to any considerable depth.

Obs. 1. The mode of obtaining nitre is by lixiviation, that is by passing water through the earth which contains it. The water dissolves the nitre, which is then crystalized by evaporation.

2. The calcareous caverns which abound in Kentucky furnish large quantities of this salt. One of these, situated in Madison county, is upwards of 600 yards in length and about 40 feet wide. It extends through a hill, and affords a passage for horses and waggons to bring out the earth to be lixiviated. One bushel of this earth yields from one, to two pounds of nitre.

3. The earth taken from under old and decayed buildings, which have been inhabited, always contains a quantity of this salt, and is sometimes lixiviated for this purpose. During our revolutionary struggle, considerable quantities of nitre were manufactured in this way..

446. The formation of nitre depends on circumstances which are easily imitated by art. Hence a considerable proportion of what is used in the arts, is produced by what are called artificial nitre beds. These consist of certain materials thrown together in heaps, with attention to the circumstances which are known to favour its production.

Illus. These beds are made by digging dry ditches, which are covered from the rain by sheds open at both ends, for admitting the air, which appears to be an important part of the pro

cess.

These ditches are filled with the earth from old and decayed buildings, as old mortar, dirt from cellars, &c. The refuse of slaughter houses, as bits of skin, horns, &c. are added, as also the soil from barn yards, &c. These substances being mixed together in the heap, are now and then turned up with a spade to expose all parts to the air. After a succession of several months, or less, according to the management of the operation, the nitre forms in the mass and is obtained by lixiviation.

Obs. In the above process the nitric acid appears to be formed by a combination of the nitrogen, which forms a part of all animal matters, and the oxygen of the atmosphore. Hence the necessity of a free circulation of air on the materials. The acid

thus formed, unites to the potash of the vegetable materials which the heaps contain, and thus nitrate of potash is formed. 447. This salt is composed of nearly equal proportions of the alkali and the acid.

448. When submitted to a red heat, it is decomposed in consequence of the destruction of its acid.

Illus. By distilling nitre in an iron retort or gun barrel, oxygen gas is obtained in abundance. It is not perfectly pure, but sufficiently so for ordinary experiments.

419. This salt is rapidly decomposed by charcoal at a high temperature. It is also decomposed by sulphur.

Illus. Mix powdered nitre and sulphur, and throw the mixture, a little at a time into a red hot crucible. The sulphur will unite with the oxygen of the nitric acid, and form sulphuric acid. The potash being thus left free, there is formed a union between this and the new acid, and sulphate of potash remains in the crucible.

450. Nitre is the base of gun powder, fulminating powder, &c.

Illus. 1. A mixture of five parts powdered nitre, one part sulphur, and one of powdered charcoal composes gunpowder. The materials are first finely powdered separately, then mixed together, and beaten with a wooden pestle, a quantity of water being added to prevent an explosion. The mixture is afterwards granulated by passing it through sieves, and cautiously dried.

2. Fulminating powder is made by mixing three parts of powdered nitre, two of carbonate of potash, or common salts of tartar and one of sulphur. These ingredients must be carefully mixed by grinding them together in a mortar.

When a small quantity of this powder is gently heated nearly to redness on a shovel, or by other means, it explodes with violence, giving a loud and stunning report. Only 15, or 20 grains ought to be exploded at a time, within doors.

451. Nitrate of Ammonia. This salt is prepared by saturating dilute nitric acid with carbonate of

ammonia.

Exp. Dilute some aqua-fortis with three, or four parts of water. Put this into a porcelain or earthern dish, and set it in a

sand bath, or in hot ashes; then throw in pieces of carbonate of ammonia until it ceases to effervesce. Continue the evaporation until about two thirds of the solution is exhausted, or until a drop readily shoots into crystals on being placed on a piece of glass. Then set the dish aside until the crystals are formed.

Remark, If the solution is evaporated slowly, and with a gentle heat, and the vessel in which it crystalizes has a broad flat bottom, the crystals are very beautiful, long, shining, triated, prisms. If the solution is exhausted nearly to the point of crystalization while it remains hot, and this is done with a higher heat, it either shoots into small fibrous crystals, or concretes into a shapeless mass.

Obs. The most important property of this salt is its yielding, when decomposed by heat, the nitrous oxide.

MURIATES.

452. The muriates have a saltish taste, more or less pure. They emit white fumes when mixed with sulphuric acid. With nitric acid they emit oxymuriatic acid gas. They are all soluble in water, and with difficulty decomposed by heat.

453. Muriate of Soda. Common salt. Sea salt. Of all the saline substances this is the most com mon and abundant in nature. It is frequently found in extensive solid masses in the earth, or dissolved in springs and lakes far inland. The ocean, is however, the great depository of this salt, about a thirtieth of its weight being muriate of soda.

Illus. In Cheshire, England, there is a mine of this salt, whose beds are alternate with those of clay. The first bed of salt commences about 90 feet below the surface, and varies from 60 to 90 feet in thickness. Below this there is another bed whose thickness is not known, though it has already been penetrated to a great depth. The salt from this mine is carried to Liverpool where it is purified by solution in sea water, and by subsequent crystallization. Many thousand tons are annually shipped from that place, and hence it has acquired the name of Liverpool salt.

454. In Spain are many salt springs; and in Catalonia there is a mountain of rock salt, whose