dered fusible by a due mixture of alkali, which acts as a flux to the silex, and renders the whole transparent.*

Are these the only articles necessary to form glass?

In Holland, and some other parts of Europe, glass is manufactured with alkali and sand only; but in England, flint-glass is made by a mixture of red lead with those substances; which gives the glass great weight, and makes it more useful for all common purposes.

What are the chief properties of alumine?

ALUMINE, or pure clay, is soft to the touch; adhesive to the tongue; emits a peculiar odour

* Glass cannot be made without great heat, as the alkali retains the last portions of carbonic acid and water with singular obstinacy, and it is only at a very high temperature that the alkali fuses, and then it prefers the silex; for it is one of the laws of nature, (to which there are few exceptions,) that, in order that two bodies may become chemically united, one of them must be in a state of fluidity. † Metallic oxides have the property of making glass more fusible. The oxide of lead is serviceable in this way when mixed with glass. Plate and crown glass contain no lead, but the former has generally lime combined with it.

The common bottle glass is made with a large portion of the ashes of vegetables, or soap-boilers' waste ashes, instead of pure alkali. The portion of iron, which is generally found in vege table substances, gives it the green colour.

For the best flint glass, rather more alkali is used than is necessary to flux the sand; and when the whole is in fusion, the fire is continued so as to volatilize the superabundant quantity. If an excess of alkali be left in the glass, it will attract water from the atmosphere, and in a short time assume a fluid state.

Professor Seigling, having left a bottle of liquor silicum undisturbed eight years, found transparent rock crystal formed in it, hard enough to give fire with steel. See an account of a similar result in Nicholson's Journal, vol. i. 217.

when moistened; forms a paste with water; will unite with most acids;* and acquires great hardness, and contracts in the fire. Its specific gravity is 2..

In what state is alumine found native?

This earth acquired its name from its being the basis of alum ; it has also been called argil, as it it is the principal part of all clay;§ it is found in a state of crystallization in the sapphire, and is united to the oxides of iron in the ochres; it also forms a part of the precious stones.

What are the chief uses to which alumine has been applied?

Alumine in the state of common clay is em

* Alumine is soluble in every acid, and in solutions of the caustic fixed alkalies. Sir Humphrey Davy has made many attempts to decompose this earth; and though there are reasons to believe that he has succeeded in part, the experiments were not so decisive as they were with the other earths. Elem. Chem. Phil. page 355.

† Alumine is completely fusible per se into a very hard vitreous kind of substance, hard enough to scratch glass; but sùfficient heat cannot be procured to effect its fusion without oxygen gas. Saussure found that it required 18.900° of Wedgwood to fuse it; but when mixed with certain proportions of silex and lime, it fuses readily.

Pure alumine may be procured by decomposing common alum with carbonate of ammonia. It has not been found native in a state of purity any where, except at Halle în Germany.

Alu

Common clay is a mixture of alumine and silex. It frequently contains metallic oxides, chalk, and other earths. mine united to the oxides of iron, is plentifully procured in Staf fordshire and Derbyshire; in which state it is called raddle, an article very useful in colour-making. Fullers' earth is alumine combined with very fine silex. It is owing to the affinity which alumine has for greasy substances, that this article is so useful in scouring cloth. Hence pipe-clay is frequently used for the same purpose.

ployed for various purposes, on account of its aptitude for moulding* into different forms, and its property of hardening in the fire; such as for making bricks, earthen-ware, porcelain,‡ crucibles, &c. &c.

Is alumine employed in forming any chemical combinations?

Aluminous earth is employed in different combinations by the dyer and the calico-printer, as à mordant for fixing various colours; and upon the continent it is artificially combined with sulphuric acid, in order to form alum,§ though we

* Earthen-ware, according to the Old Testament, was known at an early period to the Jews; and the potter's wheel, there spoken of, was probably the same simple machine as is used at the present day to form round vessels with plain surfaces. The making of porcelain has long been known in China and Japan; but it was accidentally discovered in Europe by a chemist, in the beginning of the 18th century. It was so esteemed by the Romans, that, after the taking of Alexandria, a porcelain vessel was the only part of the spoil retained by Augustus. Dr. Thomson, vol. ii. 286.

† Alumine is of inestimable value for securing the bottoms and sides of canals and reservoirs of water, and composes in a great measure those tenacious earths called arable soils.

The beautiful colours which are seen upon porcelain and carthen-ware are given by metallic oxides. Purple is given by gold; red by the oxide of iron; yellow by the oxide of silver; green by copper; blue by cobalt; and violet by manganese.

Alum is generally made artificially in France. Chaptal, a chemist of some eminence, has formed a considerable establishinent for the manufacture of this very useful article, and has erected immense buildings for preparing sulphuric acid by the combustion of sulphur, totally different from those usually appropriated to such purposes in England. See Annales de Chimie, tome iii.

In England it is not necessary to form alum by art, as the alum-slate is found in great abundance. It is gotten on the seacoast of the north-east part of Yorkshire, from Whitby to

possess the compound ready formed in its native

state.

Why do potters employ a mixture of alumine and silex for earthen-ware?

In making earthen-ware a due proportion of both these earths is necessary; for if alumine*

Stockton, a distance of about 50 miles. The slate, when gotten, is broken to pieces by the aid of fire, and afterwards further acidified, by being frequently moistened, and by exposure to the air. When the efflorescence has taken place, it is put into lixiviating vessels for the extraction of the salt. The saline liquor is then boiled down to the proper strength for crystallization; previous to which is added a portion of alkali, to saturate the superabundant acid, and to favour the crystallization. Indeed, alum cannot be made without a portion of ammonia or potass, as it is a triple salt. All alum is either a sulphate of alumine and potass, or a sulphate of alumine and ammonia. Urine is generally used in Scotland, and sulphate of potass in Yorkshire.

I have been informed by one of the most considerable manufacturers of alum, that the account of the English alumworks published in 1678, by Daniel Colwall, Esq. corresponds exactly with the present practice. This paper may be seen in the 12th vol. of the Philosophical Transactions, page 1052.

The first English alum-work was established at Gisborough in Yorkshire, in the reign of Queen Elizabeth, by Sir Thomas Chaloner, who engaged workmen from the Pope's alum-works to superintend it; and kept those workmen, till his manufacture was brought to perfection, notwithstanding the bulls and anathemas which His Holiness issued in abundance against him.

One of the most ancient manufactures of alum was at Roche, a city of Syria; whence the name of Roche-alum. See Additional Notes, No. 24.

* For making pottery or earthen-ware, the clay is beaten in water; by which the fine parts are suspended in the fluid, while the coarser sink to the bottom of the vessel. The thick liquid is further purified by passing through hair and lawn sieves of different degrees of fineness; and is afterwards mixed with another liquor of about the same density, consisting of ground flints. This was the composition of the white stone ware, about 40 years ago the staple manufacture of the potteries of this kingdom; and it is also that of the finer earthen-wares at present in use, though in different proportions, and with various improvements,

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.

introduced by the ingenuity of succeeding manufacturers. This mixture is then dried in a kiln, and, after being beaten to a proper consistence, becomes fit for being formed by the workman into dishes, plates, bowls, &c. The fine white and cream-colour earthen-wares now made in England are fired twice: the first time to give them the requisite hardness; and in that state they are called biscuil: they are then dipped in a vitreous composition, and, being subjected to a second burning, acquire a coating of true glass, thence called a glaze. If they are finished with painting in enamel, it is necessary to pass them a third time through the fire: One of the ingredients of this glass (see Note † page 120) being oxide of lead, the workman, whose hands are constantly immersing in the mixture, is subject to paralysis, unless due precaution be taken. To prevent it, the manufacturers have of late years assigned to such men a varied employment about their ovens, and furnished them with a dress to wear at the glazing tub, and throw off when they leave it, and a water cistern, soap, and towels near them, that they may be more certain, when employed in the glazing, to wash their hands before they go to their meals. It is feared, however, that an unhappy opinion of the efficacy of spirituous liquors does more mischief to this class of workmen than any other circumstance; for, attributing to the effect of lead whatever slight disorder occurs, they have recourse, in the first instance, to them as specifics, in such cases, and thus acquire the habit of an immoderate use of what probably affords them a temporary relief, but does not fail to produce a permanent and destructive disease. This is, I believe, the sole opprobrium which falls upon the potter's art; yet even this may be greatly diminished, if not entirely removed, by the precautions above mentioned however, as it is desirable to preclude the use of lead altogether, the Society for the Promotion of the Arts has offered a premium for a substitute for this glaze, or for a mode of using it which would not subject the men to these dangers.

The white and the brown stone wares are passed only once through the fire, for at a certain period of the heat they are made to undergo a partial vitrification at the surface, by the fumes of muriate of soda; this salt, being thrown into the oven, and the pieces of ware so disposed as to receive the fumes of it on every part of their surface. This method of glazing earthen