applied, as PORCELAIN CLAY, which consists of alu. min and silex, with a little mica, and is found in Corn. wall, Saxony, Japan, and China. It is of a reddish white, is supposed to be formed from the decomposition of felspar, and is used in the manufacture of Porce lain or China PIPE CLAY is of a greyish or yellowish white. POTTERS' CLAY is found of various shades of yellow, gray, green, and blue. The most durable kind of bricks are made of a YELLOW CLAY, containing some iron, and a considerable portion of silex.

BOLE, or Bole Armeniac, or the Armenian Bole, is a soft, friable fat earth, of a pale red colour, easily pulverized, and which adheres to the tongue; esteemed a good drier, and used in many diseases externally and internally. Bole of the Levant is an earth nearly

of the same nature. Some think that it is the Levant bole which passes among us for the armeniac.

There is also a medicinal substance of a brown or chocolate colour, called JAPAN EARTH, very austere upon the palate, seeming to melt like bole in the mouth, and leaving somewhat of a sweetish taste behind it. This substance is now known to be a dry extract prepared from the wood of a species of sensitive plant, the Mimosd Catechu, and is called CATECHU. According to Mr. Davy, who analyzed it, 200 grains of picked Catechu from Bombay, contained 109 grains of tannin, 68 extractive matter, 13 mucilage, 10 resi duum of sand and calcareous earth. The same quan. tity from Bengal yielded tannin 97 grains, extractive matter 73, mucilage 16, and residual matter, being sand and a little calcareous and aluminous earth 14.

BITUMINOUS AND INFLAMMABLE
SUBSTANCES.

NAPHTHA is a transparent yellowish white sub. stance, fluid as water, and exceedingly light. It is insoluble in spirits of wine, feels greasy, has a penetrating odour, and burns with a bright flame, leaving scarcely any residuum. It dissolves the essential oils of

thyme and lavender, and resins, but not gums: it is not decomposed by distillation, yet, if long exposed to the air, it thickens and degenerates into Petroleum. This fragrant mineral oil issues from white, yellow, and black clays in Persia and Media. PETROLEUM is a brown, greenish, or blackish semitransparent substance, found dropping from rocks, or issuing from the earth in Modena, and various other parts of Europe and Asia. It seems to be naphtha changed by exposure to the atmosphere. MINERAL TAR is petro leum, farther altered by the air, having assumed the colour and consistency of tar. ASPHALTUM is produced by a still farther exsiccation, and is known in three different states of solidity, called Cohesive or Mineral Pitch; Semicompact or Maltha; and Compact or Asphaltum. MINERAL TALLOW is a white substance resembling tallow, but one-fifth lighter, and is found in Finland and Persia. MINERAL CAOUTCHOUC, or Elastic Bitumen, resembles in its elastic properties the substance from which it is named. JET is a substance of a full black, harder, and less brittle than asphalt, and according to the opinion of Fourcroy it is indurated asphalt, and to Wiedenman it is a species of coal. Jet is manufactured into ornaments like amber. CANNEL COAL appears to be next to jet, in gradation of purity of the compound mineral bituminous substances; the varieties of bitumens, however, are almost innumerable; some of the most curious of which are described in Mawe's mineralogy of Derbyshire. The elementary principles of bitumens are carbon and hydrogen, and from the vestiges and exuvia of animals which usually accompany them, it is inferred that animal matter has contributed to their formation.

PIT-COAL is the most general and useful of all fossil and inflammable bodies. Various and contradictory are the opinions respecting the origin of its formation, some ascribing it to animals and others to vegetables. COAL, according to Mr. Hatchett, means nothing more than carbon oxydized in a certain degree, and may be formed either in the humid or the dry way. Wood is converted into coal by sulphuric acid, and

Pit

burns like mineral coal devoid of bitumen. BITUMEN is supposed to be a modification of the resinous and oily parts of vegetables, produced by some process of nature operating slowly on immense masses. Coal may also have been formed from vegetables by the agency of sulphuric acid, with which it abounds. Coal, by dry distillation, yields carbonat of ammonia, or smelling salts, and an empyreumatic oil in the state of tar, called COAL TAR. If performed in a proper oven, it becomes charred, and forms COKE, which, burning without smell, flame, or smoke, is preferred to coal itself where great heats are required. This same process also yields CARBURETTED HYDROGEN GAS, which is used for Gas Lights, with which the metropolis of the British empire is now to be illuminated by act of Parliament. BOVEY COAL is of a brownish black colour, of a lammellar texture, and consists of wood impregnated with bitumen or petroleum, and containing pyrites and alum.

SULPHUR is a simple, inflammable, acidifiable, brittle, and yellow substance, of which there are two kinds; the common sulphur extracted from pyrites and the volcanic brimstone. When sulphur is placed on inflamed coals, it burns with a bluish flame, and emits a pungent suffocating vapour. If this vapour be condensed over water, it forms sulphuric acid or vitriol. COMMON SULPHUR, or Brimstone, is that used in gunpowder, and for other purposes. It is the native sulphur found near volcanoes, and cast into cylindrical rolls. In this state it often contains a little arsenic. This kind is used in whitening silken, woollen, and cotten stuffs. Sulphur, either native or combined with other minerals, is found in almost every country. FLOWER OF SUL PHUR is the purest part of common sulphur, gained by evaporation, or sublimation, by heating it in pots for that purpose, and collecting the powder in the head of the cucurbit, where it adheres. The flower of sulphur has long been known in medicine as the only specific for the itch, and other cutaneous diseases.

VITRIOL is a vague term formerly applied to se veral metallic salts, but now wholly disused. It pro

perly signified some combination of sulphur and metal. The ancients gave the name of calcitis, or calcanthum, to native vitriol, or that which acquires its full perfec tion in the earth. Some moderns take this calcitis to be the same with the colcothar, brought from Sweden and Germany, the best whereof is of a brownish red colour, and a vitriolic taste, and dissolves easily in water, and when broken is of the colour of shining cop. per. Formerly the quality of vitriol was designated by its colours, which are very deceptive. Blue or Roman and Cyprus vitriol is the modern sulphat of copper, or copper dissolved in sulphuric acid and crystallized. Green vitriol is the sulphat of iron, or copperas of commerce, which is often both white and green; white vitriol is the sulphat of zinc. The sulphat of iron was also called green copperas, the sulphat of zinc, white copperas, and the sulphat of copper, blue copperas. The use of sulphat of iron, which is generally prepared from martial pyrites, found in coals, &c. is very exten. sive in dyeing, calico printing, writing ink, &c.; and is even used to give a head to beer or porter.

OIL OF VITRIOL, now more properly called Sulphuric Acid, is prepared by burning sulphur in close vessels.

GEMS AND PRECIOUS STONES.

GEM is a common name for all precious stones or jewels.

The DIAMOND was by the ancients called Adamant. It is a precious stone, the first in rank, value, hardness, and lustre, of all gems. The goodness of diamonds consists in their water, or colour, lustre, and weight. The most perfect colour is the white. Their defects are veins, flaws, specks, &c. In Europe the lapidaries examine the goodness of their rough diamonds by day. light. In the Indies it is done by night. Dr. Wall, in the Philosophical Transactions, seems to have found an infallible method of distinguishing diamonds from other stones: a diamond, with an easy slight friction in the

dark, with any soft, animal substance, as the finger, woollen, silk, or the like, appears luminous in its whole body; and if you keep rubbing it long, and then expose it to the eye, it will remain so for some time. Diamonds are found in the East Indies, in Golconda, Visapour, Bengal, and the Island of Borneo. There are four mines, or rather two mines and two rivers. The miners work quite naked, except a thin linen cloth before them; and they have also inspectors to prevent their concealing the stones; which, however, they frequently find means to do, by swallowing them when they are not observed. Diamonds have also been found in the Brazils, and hence the terms oriental and occidental diamond; the latter is esteemed the least valuable, but the constituent principle of both is the same, they are all pure carbon, and perfectly inflammable, as Newton concluded.

CRYSTAL, Quartz, or Kock Crystal, is a trans. parent stone, white like a diamond, but much inferior in lustre and hardness. It is used for vases, urns, mirrors, &c.-The ancients knew little of its nature. Pliny speaks of it as hardened petrified water. Several mountains of Europe, and some of Asia, produce rock crystal. Madagascar is also said to yield more than all the world beside. Its perfection consists in lustre and transparency that with straws, dust, clouds, &c. is little valued. It is frequently found hexagonal (or with six sides): the edges inimitably fine and accurate. It is cut and engraved in the same manner as common glass, of which it was the prototype. F. Francisco Lana, in describing the formation of diamonds, observes, that in the Val Sabbia is a spacious round of a meadow, bare of all herbs, wherein crystals are generated all sexangular. The country people thought them produced from dews, because, being gathered over night, others would arise in a serene and dewy sky. He observed that there was no mark of any mine thereabout, and concluded them produced by the plenty of nitrous steams, which, while it hindered the vegetation of those places, might coagulate the dew falling thereon. Rohault observes, that crystal, diamond, &c, must