CHAPTER V. OF EARTHS. WHAT are the principal characters of a simple earth? The earths are incombustible bodies, and in general are unalterable in the fire. Till lately they were thought to be unsusceptible of decomposition;* are insoluble in water, or nearly so, when combined with carbonic acid;† and are of a specific gravity never exceeding five times that of water. How many earths are there? There are nine distinct earths known at present, viz. silex, alumine, zircone, glucine, yttria, barytes, strontian, lime, and magnesia.‡ * Baron Born, many years ago, was of opinion that the earths were compound bodies. Sir Humphrey Davy has now succeeded in decomposing most of them; and from his experiments we have reason to suspect that they are all metallic oxides. Bergman classes all substances, except metals, with the earths, which require more than one thousand parts of water for their solution. We have, however, another test to distinguish them, when held in solution by acids, viz. the prussian alkali. Prussiate of potass, and indeed prussiates in general, have the property of precipitating all metallic oxides from their solutions; yet, excepting yttria, it has no effect on any of the earths. To a cursory observer the earths appear to be infinitely diversified; so much so, that he would probably think the dif ferent kinds are innumerable. However, notwithstanding the Why are some of these earths called alkaline earths? Barytes, strontian, and lime,* are called alkaline because they agree with alkalies in taste, causticity, solubility in water, and in their effect on vegetable colours. Magnesia agrees with the alkalies in the latter property only. What is the nature of silex, and what are its chief properties? varied appearance of the earth under our feet, of that of the furrows of the field, 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. This may fairly be adduced as one instance of the infinite skill of the Deity, as it "bespeaks an artist master of his work, acquainted with his materials." To give a still greater variety to the works of Nature, these earths are endowed with an affinity for acids and metallic oxides, whence arise the spars, gems, and precious stones of every colour and every species. "The unfruitful rock itself, impregn'd by thee, In dark retirement forms the lucid stone." * Barytes, strontian, lime, and magnesia, are found in nature always combined with acids. None of the combinations are very hard, as either of them may be scratched with a knife, The acids which are found united by nature with these alkaline earths, are generally the carbonic, the sulphuric, the fluoric, the boracic, and the phosphoric. Barytes, strontian, and lime, differ from the alkalies, in their infusibility by fire, in forming insoluble compounds with carbonic acid, and in being insoluble in alcohol. As these are the common properties of the earths, we have sufficient reason to reject the proposal of some of the French chemists, who are desirous of transferring these substances from the earths to the class of alkalies. SILEX,* or pure flint, is insoluble in water,† and in every acid except the fluoric; it endures the strongest heat without alteration; but, when mixed with soda or potass, becomes fusible in a strong fire into glass. Its specific gravity is 2.65. In what state is silex found native? Silex is found in almost all solid mineral substances, particularly in gravel, sand, quartz, and * Pure silex may be procured for chemical experiment by fusing common flint stones with three or four times their weight of potash, dissolving the product in water, and then taking up the alkali by the addition of an acid, which will precipitate the silex, which is to be well washed for use. The siliceous stones should be previously heated red in a crucible, and plunged in that state into cold water. This will render them brittle, so that they may easily be reduced to powder before they are mixed with the potash. Sir Humphrey Davy has not yet succeeded in exhibiting the base of this earth in a separate state, though from the result of his experiments, there is great reason to believe that silex, like the other earths, is a metallic oxide. El. Ch. Phil. part i page 362. We cannot by any means dissolve silex in water :--Nature, however, by some wonderful and unknown process, contrives to dissolve it, even copiously, so as to form stalactites and other incrustations. In Iceland there is a boiling fountain which spouts water 90 feet in the air, and deposites in falling so great a quantity of siliceous earth, that it forms around its base a sort of solid cup, which surmounts and envelops it.-See note, p. 49. This earth has also been found in solution in the Bath waters, and in some other spring water. According to Saussure, quartz or silex may be fused by a heat equal to 4043° of Wedgwood; but this is a temperature so far above any heat that can be procured by common means, that I have not thought it necessary to alter the text. Mr. Hare also fused it by means of a stream of oxygen and hydrogen gases. .. It is a curious fact, that if barytes be fused with this earth, the compound will be soluble in all the acids. Sand sets into a less bulk when wet, and does not afterwards contract by heat: This property may be taken advantage of in constructing reverberatory furnaces, &c. 118 EARTHS. [Chap. 5. flint, of which it forms nearly the whole substance.* It is also the chief ingredient of those rocks which constitute the most bulky material of the solid parts of our globe.† * Dr. Kirwan, in his Geological Essays, relates, that about the year 1760 the emperor of Germany, being desirous to know the length of time necessary to complete a petrifaction, obtained leave from the Sultan to take up and examine one of the timbers that supported Trajan's bridge over the Danube, some miles below Belgrade. The outer part of this timber, to the depth of half an inch, was found to be converted to an agale; the inner parts were slightly petrified, and the central were still wood. The agate is a precious stone composed of silex and alumine, but chiefly silex. Agate is brought from Germany in blocks. It is cut by means of diamond powder into leaves, for making caps for the pivots of mariners' needles to run in, and other purposes. Might it not be used advantageously for the balances of watches, instead of iron, which is liable to vary by magnetic influence and other causes? "A granite mountain, about 30 miles from the Cape of Good Hope, called the PEARL DIAMOND, rises out of the ground to the height of 400 feet, being half a mile in circumference, and formed of a single block of granite.”—Parkinson's Organic Remains. Silex also constitutes two thirds of the asbestus, so valued by the ancients for wrapping up the bodies of the dead before they were committed to the funeral pile. They discovered methods of drawing the fibres of the mineral into thread, and afterwards weaving it into cloth. In consequence of its incombustibility, it preserved the ashes of the body from mixing with those of the wood upon which it was laid to be burnt. The practice was, however, probably confined to the families of the opulent. So late as the year 1702 a funeral urn was discovered at Rome, in which were a scull and other remains of a human body, wrapped in a cloth of amianthus or flexible asbestus. The whole was deposited in the Vatican library. This mineral might be manufactured into paper; and, for documents of great importance, it might, perhaps, be worth while to be at the expense of preparing this incombustible paper, and of writing upon it with indelible ink. The methods of making it into paper or cloth, and of preparing proper ink for writing upon it, may be seen in Dr. Rees's Cyclopædia. The mineral What are the chief uses of silex? Silex is the most durable article in the state of gravel for the formation of roads; it is a necessary ingredient in earthen-ware, porcelain, and cements; is the basis of glass,* and of all vitreous substances; and is an indispensable article in many of our chemical furnaces and utensils. What is the use of silex in glass-making? Silex is the chief ingredient in glass. It is ren is found in Corsica; in the isle of Elba; in Sweden; and in Cornwall and the isle of Anglesea, in England. The Chinese form furnaces with this mineral. They grind it, mix it up with some kind of mucilage, and introduce it into moulds, the form and polish of which it assumes. These furnaces are very portable, and indestructible by fire. What kind of mucilage they employ is not known; but it is said that the mucilage of gum adraganth answers this purpose completely. *The manufacture of glass was known very early; but glass perfectly transparent was reckoned so valuable, that Nero is said to have given a sum equal to 50,000l. for two glass cups with handles. When the excavations were made in the ancient city of Pompeii, which was buried by an irruption of Vesuvius A. D. 79, the windows of some of the houses were found glazed with a thick kind of glass, not transparent. In others isinglass was substituted, split into thin plates.-Miss Starke's Letters from Italy. The best book on glass-making which I have seen, is an octavo volume, by H. Blancourt, with plates, 1699. But a scientific work, lately published in Paris by Loy zel, 'entitled "Essai sur l'Art de la Verrerie," is, I believe, the best modern work on this subject. It gives a detailed account of the different processes in this manufacture, and treats of each chemically. The manufacturing of pastes, or artificial gems, is a branch of the art of glass-making. The basis of these is a very hard and pure silex, obtained by melting pounded quartz with an alkali, with the addition of borax, nitre, and oxide of lead, different metallic oxides being added to imitate the colour of the different gems. See Journal de Physique, tom. xxviii. page 502. I would recommend to the perusal of the young reader, Fourcroy's account of the various uses of silex in vol. ii. of his System of Chemistry, page 190. It is beautifully written, and cannot fail to interest and instruct the pupil, |