their constitution in a strong heat. Notwithstanding the varied appearance of the earth under our feet, and of the mountainous parts of the world, whose diversified strata present to our view substances of every texture and of every shade, the whole is composed of only nine primitive earths; and as three of these occur but seldom, the variety which is produced by the other six becomes the more remarkable. One of the most valuable earths with which we are acquainted is silex or pure flint. It is the most durable article in the state of gravel for the formation of roads; it is a necessary ingredient in earthenware, porcelain, and cements; it is the basis of glass, and of all vitreous substances. It is white, inodorous and insipid in its pure state, and the various colours, which it assumes in different substances, proceed from the different ingredients with which it is mixed. Alumine obtained its name from its being the base of the salt called alum. It is distributed over the earth in the form of clay, and on account of its aptitude for moulding into dif ferent forms, and its property of hardening in the fire, is employed for various useful purposes. In making earthenware, a due proportion both of silex and alumine are necessary; for if alumine alone were used, the ware could not be sufficiently burnt without shrinking too much, and even cracking; and a great excess of silex would lessen the tenacity and render the ware brittle. Lime is never found pure in nature; it is obtained by decomposing calcareous matters by the action of fire, which deprives them of their acid. In its pure state it is used in many of the arts. It is employed by the farmers as a manure; and by bleachers, tanners, iron-masters and others, in their several manufactories, and in medicine. The use of lime in agriculture may be attributed to its property of hastening the dissolution of all animal and vegetable matters, and of imparting to the soil a power of retaining a quantity of moisture necessary for the nourishment and vigorous growth of the plants. Magnesia, besides being the basis of several salts, is of great use in medicine; and is employed by the manufacturers of enamels and porcelain.

The alkalies are distinguished by an acrid and peculiar taste; they change the blue juices of vegetables to a green, and the yellow to a brown, and have the property of rendering oils miscible with water. They form various salts by

combination with acids, act as powerful caustics when applied to the flesh of animals, and are soluble in water. Potash and soda have been called fixed alkalies, because they will endure a great heat without being volatilized: and yet in a very high temperature they are dissipated in vapour. They were formerly considered to be simple substances, but they are now found to be compounds of metallic substances, called potassium and sodium, with oxygen. They have various uses in surgery and medicine, and are employed in large quantities by the glass-maker, the dyer, the soapmaker, the colour-maker, and by many other manufacturers. Ammonia is so extremely volatile as to exhale at all known temperatures. When combined with carbonic acid, it takes a concrete form, and a beautiful white colour, and is known in commerce by the name of volatile salts. With muriatic acid it forms what is termed sal ammoniac, which is employed in many of our manufactories, particularly by dyers to give a brightness to certain colours. In tinning metals it is of use to cleanse the surfaces, and to prevent them from oxydizing by the heat which is given to them in the operation. Ammonia is furnished from all animal substances by decomposition. The horns of cattle, especially those of deer, yield it in abundance, and it is from this circumstance that a solution of ammonia in water has been called hartshorn.

QUESTIONS.-1. What are earths? 2. What the names of the nine earths? 3. What is said of silex? 4. Of alumine ? 5. Of lime? 6. Of magnesia? 7. How are alkalies distinguished? 8. Why are potash and soda called fixed alkalies? 9. Of what are they compounds? 10. What is said of their uses? 11. From what is ammonia furnished? 12. What is said of its combinations and uses? [NOTE. Besides the nine earths, above enumerated, we have now thorina, which is a rare earthy substance lately discovered. A new alkali, called lithia, has recently been discovered, which, like potash and soda, is found to be a metallic oxyd: its base is called lithium. Three new vegetable alkalies have also been discovered, called morphia, picrotoxine, and vauqueline. Clay, as it exists in soils, is commonly called argillaceous earth; and lime in soils is called calcare. ous earth.]

Acids which contain different quantities of oxygen are distin-
guished by their termination. The name of that which con-
tains most oxygen ends in ic, the other in ous.
Thus we say
sulphuric acid, and sulphurous acid. All salts that are com-
posed of acids ending in ic, take an ending in ate; as sulphate
of lime, a compound of lime with sulphuric acid. All salts
composed of acids ending in ous, take an ending in ite, in-
stead of ate; as sulphite of lime. When there is an excess
of acid, the preposition super is added; and when an excess of
the base, then sub is prefixed, as super-sulphate of potash, or
sub-borate of soda, (borax.)

THE name acid, in the language of chemists, has been given to all substances, whether liquids or solids, which produce that sensation on the tongue which we call sour. Most of the acids owe their origin to the combination of certain substances with oxygen; and they have the property of changing the blue, green, and purple juices of vegetables to red, and of combining with alkalies, earths, or metallic oxyds, so as to compose those compounds termed salts. The acids were formerly divided into three classes, mineral, vegetable, and animal; but the more useful and scientific way of dividing them is into two classes only. The undecomposable acids, and those which are formed with two principles, are comprised in the first class; while those acids which are formed with more than two principles compose the second class.

Sulphuric acid is procured by burning sulphur, in contact with some substance containing oxygen; by which process the sulphur combines with the oxygen, and becomes acidified. In commerce it is commonly called the oil of vitriol. That peculiar acid which is called muriatic is usually obtained from muriate of soda, which is the chemical name for common salt. Carbonic acid is a combination of carbon and oxygen. It was formerly called fixed air, on account of its being so intimately combined in chalk, lime-stone, and other substances. If you pour some diluted sulphuric acid over pulverized chalk or marble contained in a glass vessel, which has a tube connected with it, an effervescence will take place, and carbonic acid gas will escape through the tube. This gas is more destructive of life than any

taste.

other, and it extinguishes flame instantaneously. Water may be made by pressure to absorb three times its bulk of this gas; by which it acquires an acidulous and not unpleasant Soda water, cider, and other fermented liquors owe their briskness and sparkling to the presence of this gas. Fatal accidents often happen from the burning of charcoal in chambers, for wherever charcoal is burned this gas is always formed. It so often occupies the bottoms of wells, that workmen ought not to venture into such places without previously letting down a lighted candle. If the candle burns they may enter it with safety; if not, a quantity of quick-lime should be let down in buckets, and gradually sprinkled with water. As the lime slakes, it will absorb the gas, and the workmen may afterwards descend in safety.

The number of acids that are well known amounts to more than forty, and their uses are so many and important that it is impossible to enumerate them. They are indispensable to various arts and manufactures; they are employed for culinary purposes, and for medicine; they act an important part in the great elaboratory of nature, and form a great proportion of many of the mountainous districts of the globe in their various combinations.

The precise number of the salts is not known, but they probably amount to more than two thousand. The different salts are known from each other by the peculiar figure of their crystals, by their taste, and other distinctive or specific characters. The separation of salts from the water in which they may be dissolved, is generally effected by evaporation and cooling. When a certain portion of the water of solution is evaporated, and the remainder left in a proper temperature at rest, the salts will shoot into crystals, and will be found dispersed through the water at the bottom and at the sides of the vessel, and sometimes also on the surface of the solution. Their crystallization is owing to the abstraction of the heat or water by which they were dissolved. Crystallized salts are liable to changes in their appearance by exposure to atmospheric air. Some have so great an af finity for water that they absorb it with avidity from the atmosphere, and thus becoming moist or liquid, they are said to deliquesce. Others, having less affinity for water than atmospheric air has, lose their water of crystallization by exposure, and readily fall into powder. Such salts are said

to effloresce. Salts have not only the property of dissolving in water, but by exposure to great heat they will melt, and they require different degrees of heat to put them in a state of fusion, as well as different quantities of water for their solution.

Many of the salts are found native, and the carbonates, sulphates, and muriates are the most frequent. Chalk, limestone, and marble, are all included in the term carbonate of lime. Few salts are more copiously disseminated than the sulphate of lime, particularly in the vicinity of Paris, and hence its name Plaster of Paris. Of the native mu

riates, muriate of lime occurs with rock-salt, and muriate of magnesia is found in abundance in sea-water; and muriate of soda not only exists in immense quantities in the ocean, but vast mountains in different parts of the world are entirely formed of this salt. Nitrate of potash, known by the more familiar name of nitre or salt-petre, is collected in various parts of the globe. Phosphate of lime, which is the basis of all animal bones, exists native in Hungary, and composes several entire mountains in Spain. Mountains of salt were probably formed in very remote ages, and by processes of which we can form no idea. It may be supposed, however, that these changes have been slow and gradual, for several of the native salts exhibit marks of regularity and beauty in their crystallization, which cannot be imitated by art.

QUESTIONS.-1. To what substances is the name acid given? 2 To what do most acids owe their origin? 3. How do they form salts? 4. What is said of the division of acids? 5. How is sulphuric acid procured? 6. Muriatic acid? 7. What is carbonic acid? 8. How may you obtain carbonic acid gas? 9. What are some of the properties of this gas? 10. Why do fatal accidents often happen from the burning of charcoal? 11. How may it be destroyed at the bottom of wells? 12. What is said of the number and uses of the acids? 13. How are the different salts known from each other? 14. How may salts be separated from their water of solution 15. To what changes are crystallized salts liable on exposure to atmospheric air? 16. What native salts are mentioned? 17. What is said of salt mountains?