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tongue. They have the power of changing the blue colours of vegetables to green.-They are soluble in water, and give out heat during the mixture. They corrode, and, if sufficiently powerful, reduce woollen cloth to the form of a jelly. They render oils miscible with water, by uniting with them, and forming thereby soaps.-Combined with sulphur they form alkaline hepars or livers, now denominated sulphurets; and with acids they form neutral salts, distinguished by different names, according to the acid and alkali that enter into their union; hence we have the sulphate of potash, compounded of sulphuric acid and potash; the nitrate of soda, formed of nitric acid and soda, &c.

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Potash is a dry, solid, white, crystallized substance, fusible at a high temperature, very deliquescent; and, when combined with silex in fusion, the compound is glass. It is procured from the burnt ashes of vegetables by combustion in iron or other pots, whence the compound word pot-ash. It was known under the name of vegetable alkali, in consequence of its being chiefly obtained from plants.

Soda is usually procured from the ashes of sea-weeds, or marine plants, but chiefly from the salt water of the sea, soda being one of the constituents of sea-salt. It is found in large quantities combined with carbonic-acid in different parts of the earth, especially in Egypt, hence its name mineral alkali; and common culinary salt is a compound of soda and muriaticacid. But the soda of commerce is obtained from ashes of different species of salsola, a genus of plants that grow upon the sea-shore. Almost all the algæ, or sea-weeds, contain a considerable quantity of soda, and the ashes of those plants are known in this country by the name of kelp. The fixed alkalies are used in medicine, as caustics in surgery; for if a piece of flesh be put into a strong solution of potash, or soda, it will be soon dissolved: they are used in the manufacture of glass, in soap-making, in dying, and in the formation of various colours for painting; also in the manufacture of alum, and in bleaching. The potash used in this country, is chiefly

brought from America and Russia; but the kelp of our own coasts, and the barilla of Spain and Sicily, furnish us with the greater part of our mineral alkali.

Ammonia, or volatile alkali, exists in its most simple state in gas: it has a great affinity for water, with which it readily combines, and forms liquid ammonia. It was, till within these six years, regarded as a compound, formed of hydrogen and nitrogen, in the proportion of one to four; but, as we shall soon see, Sir Humphry Davy has, by the most decisive experiments, proved that it has a metallic base. Ammonia is given out by all animal and vegetable substances when in a state of putrefaction; but it is chiefly procured by a dry distillation of bones and horns. It is used in medicine, in dying, and the manufacture of sal-ammoniac, or, as it is called in chemical language, the muriate of ammonia; for the muriatic-acid and ammonia, in a state of gas, unite and form a solid.

ACIDS have the property of changing the blue vegetables to red: they, for the most part, combine readily with water, and, when united with the earths, alkalies and metallic oxydes, produce salts. In general, acids are in the state of liquids; but some of them are solid, as the Benzoic acid; and some are met with in a state of gas, as the carbonic and muriatic acids. Some are mild, others are corrosive; some are pungent and volatile, others are fixed and inodorous. Acids are formed by the combination of certain substances with oxygen, which is, in most cases, unquestionably the acidifying principle; on that account the name oxygen was assumed from the Greek words ožus and yivoμai, signifying to produce acidity. Acids, as we have seen, may exist in three states, which are caused by the different degrees in which the base is combined with oxygen. In the first they contain the least possible quantity of oxygen to render them acid; in this state they are designated, in the chemical nomenclature, by the termination. ous: thus we have the sulphurous, the phosphorous, the nitrous and acetous acids. In the second state they contain a

larger portion of oxygen, and are in general saturated with it: this state is expressed by the termination ic, as the sulphuric, nitric, phosphoric and acetic acids. In the third state, which belongs to but few, they contain an excess of oxygen; and are distinguished by the prefix oxy, as in the case of the охуmuriatic-acid.

When metallic or other substances are combined with a less proportion of oxygen than is sufficient to render them acid, they are said to be oxydated, and the substances are called oxydes. Formerly acids were divided into the mineral, vegetable, and animal, according as they were supposed to derive their origin; but it is now usual to class them according to the number of principles, or simple substances, of which they are composed: the acids of the first class contain only two principles, viz. oxygen, and some substance called the radical, as the sulphuric and sulphurous acids are formed of the radical sulphur, and a greater or less quantity of oxygen; those of the second class are composed chiefly of oxygen, hydrogen, and carbon, in different proportions. Some of them contain a portion of nitrogen.

Many of the acids are found in great abundance in nature, but in combination with other substances: thus the vast masses of limestone, chalk and marble, found in every part of the world, are combinations of lime and carbonic acid; Gypsum, of which there is so much in different parts of the globe, is composed of lime and sulphuric acid; the fluor spar, so abundant in Derbyshire and other places, is a compound of fluoric acid and lime; and the constituents of common culinary salt, which is found in such immense quantities, both in the old and new world, are muriatic-acid and soda.

CHAP. VII.

CHEMISTRY,

Continued.

Chemical attraction or affinity illustrated: Chemical apparatus-Experiments-Decomposition of the fixed alkalies.

CHEMISTRY is a science depending on experiments which are chiefly founded on very simple laws, that ought to be well understood in the outset of the study. The different actions which result from the proper application of the bodies already enumerated, whether taken in their simple state, or combined, are founded on certain agencies which apparently exist in all matter. Of these the unknown causes are denominated attraction and repulsion. We have already explained what philosophers mean by these terms in Electricity, Voltaism and Magnetism, without pretending to guess even at the causes which produce them. In chemistry, the term attraction is frequently or generally exchanged for that of affinity. There are, however, some writers who make a distinction between the terms. Chemical affinity with them is to be regarded as the principle of chemical action; and chemical attraction as the same principle in a state of operation, by which alone the degree of affinity can be measured; for the affinity still exists, whether the attraction be in action or not.

VOL. II.

M

The terms are, however, frequently taken without this distinction, and chemical or elective affinity, are often employed to express the very same ideas. The term attraction, when used generally, and without limitation, as has been already observed, denotes the principle which brings, or has a tendency to bring bodies remotely situated, to one another. Such is the action subsisting between the earth, and bodies that tend to fall to its surface: such is the action between the planets and their satellites, and between the sun and planets; and probably the same principle extends to all the fixed stars, and the indefinite number of planets depending upon them, as the earth, and other planets known to us, depend upon the sun. This principle, then, it may be assumed, acts, at indefinitely great distances, in comparison of which the distance between the Georgium Sidus and the sun, though 1,800,000,000 miles, can be regarded only as a point. But chemical, or elective affinity or attraction, acts only upon particles at imperceptibly small distances. The following experiment will illustrate the idea of what is intended by chemical affinity.

Ex. 1. A spoonful of salt, thrown into a vessel of water, very soon diffuses itself through the whole of the fluid, and the salt is said to be combined with the water; the water and the salt have a certain affinity for each other, and they cannot be separated by any mechanical means: but if another substance be introduced, to which water has a greater affinity than it has to the salt, it will quit the salt, to unite to this third substance. If, therefore, alcohol be the third body, the water will leave the salt to join the spirit; and the salt, by its superior gravity, will fall to the bottom of the vessel.

Ex. 2. Alcohol will dissolve camphor, and the fluid be perfectly clear, which is another iustance of chemical combination: the two substances have a strong affinity for each other but the spirit has a still stronger affinity for water than for the camphor; and if a little of that fluid be added to the solution, the camphor will fall down in flakes, that is, in a solid form. The following instance of the nature of simple

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