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than fluoric acid has, expels the latter and unites itself to the lime.

It appears, that before the researches of Gay Lusac and Thenard, the pure fluoric acid had never been procured; what had been considered as fluoric acid, being, in fact, a different acid, the siliceo-fluoric acid. In their experiments, the leaden receiver was cooled by ice, and the fluoric acid condensed into a liquid form. In this state it is. the most caustic substance known, corroding the skin instantly, and causing dangerous sores.

Fluoric acid combines with silica, and becomes with it a permanent acid gas, called the siliceo-fluoric acid. This was formerly called fluoric acid gas. It has a pungent irritating odour, will not support combustion, and forms white

vapours

when it comes in contact with the air. It corrodes glass, and combines rapidly with water, forming the liquid siliceo-fluoric acid. This acid, formerly called the fluoric, also acts on glass, and is very acid and corrosive.

In the process for making it from fluor

spar and sulphuric acid, a little silicious matter generally existed in the spar, or glass vessels, that had been used; and thus the siliceo-fluoric, and not the fluoric acid, had been obtained.

Fluoric acid forms fluates with the alkalies and salifiable earths.

It also unites to borax, forming an acid called the fluo-boric acid. This does not act on glass, and is not so corrosive as fluoric acid. It gives rise to fluo-borates.

ALKALIES.

Alkalies are an important class of bodies. They have received this name because one of them, soda, was procured in great abundance from a plant called kali, by the Arabians.

Alkalies have peculiar properties. They change blue vegetable colours to green, and yellow to reddish brown. They unite to oil and fat, forming soap; thus rendering them miscible with water. They have an acrid and peculiar taste. They are caustic, or act powerfully úpon animal substances. They combine with acids, forming with them a peculiar class of salts in which the properties of the acid and alkali disappear.

Until lately, only three alkalies were known, potash, soda, and ammonia. The two former were called the fixed, the latter the volatile alkali. Their number is now increased by the addition of lithia.

POTASH.

Potash had long been known by the name of the vegetable alkali.

It is procured from the ashes of burnt vegetables in the following manner. Dried vegetables are burned in heaps ; the ashes are collected and lixiviated with water. Thus the potash in the ashes is dissolved, while the rest of the ashes is insoluble. The solution is poured off from the sediment, and evaporated : what remains is the potash of commerce, which is of a grayish colour, and contains some impurities: these are separated by being heated in a furnace. It is then white, and is called pearlash.

All wood ashes have in them more or less of this alkali, and hence they are applied to the same purposes as potash, or pearlash.

Potash and pearlash procured in this manner are combined with a certain proportion of carbonic acid, but not so much as to destroy completely its alkaline properties : hence it is a sub-carbonate of potash. To free the alkali from the carbonic acid, another process is necessary. Twice its weight of quicklime is added to the pearlash, and the whole mixed with water. The carbonic acid having a stronger affinity to the lime than to the alkali, quits the latter, and forms carbonate of lime, which, being insoluble, falls down, while the purer alkali is kept in solution by the water, and is afterwards separated by evaporation. Sometimes it is still farther purified, if necessary, by mixing the whole with alkohol, which dissolves the pure alkali alone. The alkoholic solution ascends to the top of the fluid, and is drawn off by decantation.

Potash, when thus prepared, is a solid white substance, and is called caustic potash, from its property of corroding the skin and flesh when it is applied to it: on this account it is frequently employed by surgeons.

Caustic potash, when prepared by alkohol, is united to a portion of water, and is therefore a hydrate of potash. It may be obtained free from water by another process.

Potash may be made to combine with a greater proportion of carbonic acid than in the state of sub-carbonate, by causing a stream of carbonic acid gas to pass through a solution of the latter salt : when this solution is then evaporated, it affords crystals of bi-carbonate of potash. This salt is milder than the subcarbonate, and its crystals are not deliquescent.

The fixed alkalies were, until lately, regarded as simple bodies, and one of the most brilliant disco. veries of modern chemistry has been that which showed them to be the oxides of peculiar metals.

The decomposition of the alkalies was effected by means of voltaic electricity. By acting upon a very small piece of caustic potash, the metallic base was liberated, and proved to be solid, malleable, and having a high metallic lustre resembling mercury. This new metal is called potassium. It differs considerably in its properties from all the metals previously known. It is lighter than water, and has so strong an attraction for oxygen, that it almost instantly attracts it from the atmosphere and returns to the state of oxide. If thrown into water, it produces a very singular phenomenon; it decomposes the water so rapidly that an explosion takes place, accompanied by a flame. The same effect is seen if a globule of the metal is placed upon a piece of ice. This metal can only be preserved by keeping it under naphtha, a liquid that does not contain oxygen as one of its constituents.

Potash combines with all the acids forming neutral salts.

Nitrate of potash, called also nitre, or saltpetre, is produced in considerable quantities naturally, particularly in Egypt. It has also been produced artificially by making beds of animal and vegetable substances, mixed with calcareous and other earths. In

process of time, an efflorescence of nitrate of potash appears, and is separated by lixiviation. By the decomposition of these substances, nitrogen is disengaged, which, uniting to the oxygen of the atmosphere, forms nitric acid ; and this uniting to

l the alkali furnished by the vegetables and soil, produces the nitre. Nitrate of potash has the property

VOL. II.

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of detonating when inflamed with charcoal or other easily inflammable bodies. It is

upon this property that gunpowder is formed, which consists of five parts of nitrate of potash, one of charcoal, and one of sulphur. Chlorate of potash is formed by passing chlorine gas through a solution of caustic potash. It is also called ory-muriate of potash. This salt detonates violently when three parts of it mixed with one of sulphur are triturated in a mortar, or struck on an anvil. With phosphorus the effect is still greater. It makes a powerful gunpowder when employed as an ingredient.

If a small quantity of it be mixed with some sugar, and sulphuric acid be added, a sudden and vehement inflammation will be produced. These experiments require great caution.

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SODA.

This has also been called the fossil or mineral alkali, because supposed peculiar to the mineral kingdom. It is obtained chiefly from the ashes of marine plants; all the fuci yield it in abundance ; when burnt, their ashes are called kelp, which contains a considerable proportion of this alkali. Ba. rilla is the same, procured by burning a plant of that name in Spain. Soda is also found in large quantities in different parts of the earth, particularly Egypt; and common sea-salt consists of it united to muriatic acid.

In all these cases, however, the soda is combined with carbonic acid. Of this there are two varieties, the carbonate, obtained by dissolving the soda of commerce and crystallizing it, and the bi

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