this lake the ground is perfectly barren, not producing even a blade of grass; and the earth is so full of a saline matter, that after falls of rain or snow, it concretes in white flakes on the surface like the natron of Hindostan. On the banks of this lake, in the winter season, when the falls of snow begin, the earth is formed into small reservoirs six inches high: when these are filled with snow, the hot water from the lake is thrown upon it, which, together with the water from the melted snow, remains in the reservoir, to be partly absorbed by the earth, and partly evaporated by the sun; after which there remains at the bottom a cake of sometimes half an inch thick of tincal or crude Borax, which is taken up and reserved for use. It can only be made in the winter season, because the falls of snow are indispensibly requisite, and also because the saline appearances upon the earth are strongest at that time. When once it has been made on any spot, it cannot be made again on the same until the snow has fallen and dissolved three or four times, when the saline efflorescence will appear as before.

Elastic Marble.

48. In 1816, Dr. Mitchell exhibited to the New York Philosophical Society a specimen of American elastic marble, measuring four feet in length, three inches in breadth, and one inch in thickness. The slab was of a snowy whiteness, of a grained structure, and of remarkable flexibility. He had received it from the quarry in Pittsfield, Massachusetts. Since the receipt of this extraordinary sample, another one, of far more considerable size has been procured by Mr. Meyher, from Stockbridge. This he is preparing for Dr. Mitchell's Cabinet of Mineralogy. The dimensions of this stone are as follow breadth, one foot and ten inches, length,

five feet, and thickness, two inches; making a mass of two thousand six hundred and forty cubic inches of elastic marble. This slab, when shaken, undulates sensibly backwards and forwards; when supported at the two extremities, the middle forms a curve of about two inches from a horizontal line: and when turned over, recovers itself, and inclines as much the other way. It has many other curious properties.

Natural History of the Pearl.

49. The production of the pearl is one of those mysterious operations of nature which the ingenuity of man has not yet been able to unravel. The Arabs, with whom the pearl was an article of great traffic, entertained a notion (which they had from the Brahmins) that when it rained, the animal rose to the surface to catch the drops which turned into pearl. By some of the natives they are considered to be formed of certain mineral substances carried to the banks by the river which is opposite to them; by others, they are supposed to be formed from dewdrops in connexion with sun-beams which was pretty nearly the opinion entertained by Pliny and the ancient naturalists.

Some have thought them to be an accretion within the body of the animal of the super-abundant matter with coats over the inside of the shell, called mother-of-pearl, and to which it is very common to find little knobs adhering, precisely like pearls, but not of a clear water. Others again, consider them as the effect of disease or injury, like bezoars and other stones found in various animals, pearls being generally composed of lamellæ or coats, formed round a foreign nucleus. In the early ages of the Christian era, the people who lived on the borders of the Red Sea were acquainted with the method of forcing certain shell-fish to produce pearls; as the Chinese at present do the Mytilus Cygneus, the swan muscle, by throwing into the shell, when it opens, five or six minute mother-of-pearl beads strung on a thread. In the course of a year these are found covered with a pearly crust, which per

fectly resembles the real pearl. It is supposed that if sharp pointed wires be thrust through the shells of certain species of muscles and oysters, the animal protects itself from being injured and galled, by throwing off a substance which coats them over with little round knobs resembling pearls. Beckman says, that "Linnæus once showed him, among his collection of shells, a small box filled with pearls, and saidHos uniones confeci artificio meo; sunt tantum quinque annorum, et tamen. tam magni." They were deposited, the professor adds, near the Maja Margaritifera, from which most of the Swedish pearls are procured.

Natural History of Gems.

50. Gems or precious stones are sometimes found of regular shapes, and with a natural polish; and sometimes of irregular shapes, and with a rough The first sort may be considered as of the the pebble kind, and are said to be found near the beds of rivers, after great rains: the others are found in mines, and in the clefts of rocks.

coat.

The gems of the first sort were what the ancients most usually engraved upon: these are commonly called Intaglios; and they are mostly of a long oval figure, inclining to a point at each end, convex as well on the engraved face, as on the others, with a ridge running from end to end on the under side, which is hereby, as it were, divided into two faces; both which are also, though not so distinctly, parted from the upper face, by another ridge running quite round the oval.

The stone most commonly found engraved is the beryl. The next is the emerald; and then the jacinth. The chrysolite is but rarely found engraved; as are also the crystal, or oriental pebble, the garnet, and the amethyst.

Of the beryl there are three species; the red, inclining to orange colour, transparent and lively; the yellow, of an ochre colour, and the white, commonly called the chalcedony, of the colour of sheer milk. These two last have less life than the first.

The emerald is green, nearly of the colour of stagnated

water; sometimes tolerably clear, but, for the most part full of black and white specks.

The jacinth is of a deep tawny red, like very old Port wine, but lively and transparent.

The chrysolite is of a light green grass colour, and is supposed to have been the beryl of the ancients, transparent, but not lively.

The crystal or oriental pebble is harder and more lively than the common rock crystal; is of a silvery hue, and but very little inferior to the white sapphire.

The garnet is of the same colour as the jacinth, but more inclining to the purple, and not so lively.

The amethyst is of a deep purple, transparent and lively. The following is a general table of what are usually called precious stones.

The beryl, red, yellow, or white; emerald green; jacinth, of a deep tawny red; chrysolite, of a light grass-green; crystal, or oriental pebble, of a silvery white; garnet, of a deep red claret colour; amethyst, purple; diamond, white; ruby, red or crimson coloured; emerald, of a deep green; aqua marina, of a bluish sea green, like sea water; topaz, of a ripe citron yellow; sapphire, of a deep sky blue, or of a silver white; cornelian, red or white; opal, white and changeable; vermillion stone, more tawny than the jacinth.

All these stones are more or less transparent; the following are all opaque.

The cat's-eye, brown; red-jasper, called also thick cornelian, of the colour of red ochre; jet, black; agates, of various sorts; blood-stone, green, veined or spotted with red and white; onyx, consisting of different parallel strata, mostly white and black; sardonyx, of several shades of brown and white; agate-onyx, of two or more strata of white, either opaque or transparent; alabaster, different strata of white and yellow, like the agate-onyx, but all opaque; toad's-eye, black; turquoise, of a yellowish blue inclining to green; lapis lazuli, of a fine deep blue.

Of most of the species beforementioned there are some of an inferior class and beauty. These are commonly called by jewellers occidental stones: they are mostly the produce of Europe, and found in mines or stone quarries; and are so named in opposition to those of a higher class, which are always accounted oriental, and supposed to be only produced in the East.

The onyx, sardonyx, agate-onyx, alabaster of two colours or strata, as also certain shells of different coats, were frequently engraved by the ancients in relief: and these sorts of engravings are commonly called cameos. They also some

times ingrafted a head, or some other figure in relief of gold, upon a blood stone.

Besides which there are some antiques, mostly cornelians, that are covered with a stratum of white. This stratum has by some been looked upon as natural; but it was really a sort of coat of enamel that was laid on.

The stones esteemed the best for engraving upon, were the onyx and sardonix; and next to them, the beryl and the jacinth. The ancients engraved most of their stones, except the onyx and the sardonyx, just as they were found; their natural polish excelling all that can be done by art; but the beauty of the several species of onyx could only be discovered by cutting.

The merit both of intaglios and cameos depends on their erudition or the goodness of the workmanship, and on the beauty of their polish.

The antique gems of Greek work are the most esteemed; and next to them the Roman ones, in the times of the higher empire.

"Lapidaries employ a considerable quantity of diamond in powder, which they use with steel instruments, to divide pebbles and precious stones. The small pieces of diamond of which the powder is made, are worth twenty-eight shillings a carat. The use of the diamond in this way is very extensive. Had nature withheld the diamond, the pebble, the agate, and a variety of other stones, would have been of little value, as no other substance is hard enough to operate upon them. In this way rock crystal from Brazil is divided into leaves, and ground and polished with diamond dust for spectacles, and other optical instruments."

CHAPTER III.

METEOROLOGY.

SECTION I.

GENERAL PRINCIPLES.

WHATEVER is engendered in the air which surrounds us, and which appears to be beyond the