ments, is occupied by three substances, called humors, which serve, like a lens, to refract the rays of light, and to form an image on the retina. The humors are of different refractive powers, and have been supposed, by their successive action on the light, to correct any irregularity which might otherwise take place.*

CHAPTER XI.

POTTERY AND PORCELAIN.

CERTAIN clays have a property, which renders them extremely valuable in the arts. They soften in water, and allow themselves to be kneaded and formed into moulds. These moulds, when exposed to heat, acquire a durable hardness, which fits them for being used

* It is not certain that Dollond received the first hint for his great invention from observing the structure of the eye; and it now seems probable, that the resemblance referred to in the text is rather apparent, than real. Dr. Brewster has discovered that the human eye is not achromatic; the deviation of the differently-colored rays being too small to require any correction. That no provision for such correction exists in the eye, is inferred from the fact, that, if we shut up all the pupil, except a portion of its edge, or look past the finger, held near the eye, till the finger almost hides a narrow line of white light, we shall see a distinct prismatic spectrum of this line, containing all the different colors; an effect which could hardly take place, if the eye were achromatic.

An improvement, which was recently made by M. Guinand, of Switzerland, in the manufacture of flint glass for lenses, has contributed, nearly as much as the invention of Dollond, to restore the refracting telescope to use. The oxide of lead, which has been used, of late, much more freely than formerly, in making glass, because it improves its whiteness and lustre, serves to impair the value of the substance for optical purposes, by preventing that parallel arrangement of layers, of different densities, which is necessary, in order to prevent flaws. This difficulty has been so far obviated, that M. Guinand has produced objectglasses for telescopes, nearly twelve inches in diameter, and free from flaws; and it is stated, that, while out of ten objectglasses, made of English or French flint glass, only one, or two, at most, were found serviceable, eight or nine, out of the same number, made by M. Guinand, turned out good. His method is unknown.

either in buildings, or in forming utensils and vessels. Brick, tiles, and the lining of furnaces, are familiar specimens of this art. They are formed of coarse common clay, which is a mixture of argillaceous earth and sand, and are apt, from the iron which the clay contains, to turn red in burning. Earthen and crockery ware form other wellknown specimens,-the first being composed of common clay, similar to that of which bricks are made, and the latter of the purer and whiter clays, in which iron exists only in small quantities. Porcelain, the most beautiful and expensive kind of pottery, is formed only from argillaceous minerals, of extreme delicacy, united with silicious earths, which serve to render them semi-transparent, by means of its vitrification. The clay, which is used in pottery, causes the operation to differ essentially from that employed in making glass. There, the substances are softened by heat, and wrought at a high temperature; whereas, in this case, they are wrought while cold, and afterwards hardened by heat. It is impossible to explain all the details which enter into the manufacture of the different kinds of pottery. We shall merely describe those principles and processes which are common to them all, referring the reader, as in the last Chapter, to 'Bigelow's Technology.'* These may be arranged under the several heads of, I. Preparing the clay. II. Moulding, or shaping. III. Burning. IV. Printing. V. Glazing.

I. Preparing the clay. This consists in adding to it, in certain cases, a portion of silicious earth, which serves to increase the firmness of the ware, and to render it less liable to shrink and crack, on exposure to heat; and likewise partially to vitrify its surface. In the common clays, this artificial mixture of silica is unnecessary, since they have it in sufficient quantities already. In the finer clays, it is always required. The materials being mixed with water, are formed into a

* See note on page 201.

paste, which is thoroughly beaten and kneaded, to render it ductile, and to drive out the air. This operation is performed in different ways, according to the fineness of the fabric. In the celebrated manufactory of Mr. Wedgewood, in England, the clay is brought into a state of minute division, by machinery. A series of iron blades, or knives, are fixed to an upright axis, and made to revolve in a cylinder, intersecting or passing between another set of knives, which are fixed to the cylinder. The clay, being sufficiently divided by the continual intersection of these blades, is transferred to a vat, and agitated with water, till it assumes the consistence of pulp, so thin, that the stony and coarser particles can subside, after a little rest, to the bottom, leaving the finer clay suspended. This last is then poured off, and suffered to subside; after which, it is passed through sieves of different fineness, and becomes sufficiently fine for use. To this pulp is then added about one fifth as much pulp of pounded flint, prepared in the same manner; and the mixture is then exposed to evaporation, by a gentle heat, until the superfluous water is dissipated, and the mass reduced to a proper consistency to work. To produce a uniformity in the thickness of the material, it is taken out, in successive pieces, which are repeatedly divided, and struck or pressed together, until every part becomes blended with the rest.

II. Moulding, or shaping.-This is effected by throwing, pressing, or casting. In throwing, a piece of clay, of sufficient size to form the vessel, is placed upon a sort of table, or wheel, to which a rotary motion is given by the feet of the operator, or an assistant, and in some cases by a steam-engine. The potter begins to shape the clay with his hands, which are wet, to prevent its adhering. The rotary motion of the wheel gives it a circular form, and it is gradually wrought up to the intended shape, a tool being occasionally used to assist in the finishing. The handles, and other similar appendages, are made by forcing the clay, with a

piston, through an aperture of the requisite size and shape. When formed, the handles are cemented to the ware, by a thin mixture of the clay with water, which the workmen call slip.

As the wheel, or table, can only be used in making vessels of a circular shape, when the form is different, the vessel must be made by pressing or casting. In pressing, moulds are made of plaster of Paris, half of the figure being on one side of the mould, and the other half on the other side. These fit accurately together. The clay being first made into flat pieces, of the thickness of the article, one of them is pressed into one side of the mould, and the other into the other side. The superfluous clay being cut away, the two sides of the mould are brought together, to unite the two halves of the vessel. The mould being then separated from the clay, the article is complete, as to form.

In casting, the clay, in the state of pulp, sufficiently thin to flow, is poured into moulds made of plaster, by which the superfluous water being rapidly absorbed, the clay is deposited, to acquire sufficient solidity to preserve the shape communicated by the mould. When the vessels are finished, as to form, they are dried, either by exposure to the air and sun, or by artificial heat, in a room of the temperature of eighty or ninety degrees.

III. Burning.-This is necessary, to give the requisite hardness. Vessels merely dried after the manner of some rude nations, in ancient and modern times, are extremely frangible. The burning is performed in ovens or kilns; and, in the better kinds of ware, the articles are enclosed in cases or boxes of burnt clay, called saggars, and, by means of flame circulating among these cases, are raised to a red heat. The fire is kept up, from twenty-four to sixty hours, and the saggars suffered to cool before they are removed. On the intensity and duration of the heat, depend, of course, the strength and solidity of the ware. When taken from the oven, the ware is extremely bibulous, —that is, capable of absorbing a great deal of water.

This is the case with our common brown and red earthen ware, which absorbs water in such quantities, that vessels in this state are used as coolers, being kept saturated with water, which, as it passes constantly to the outer surface, generates cold by its evaporation. This property of absorbing water renders necessary the next operation, which is termed

IV. Glazing.-This consists in covering the vessel with a vitreous coating. For this purpose, ground flint is mixed with an equal proportion of some metallic oxide, such as the oxide of lead, if we wish a yellow glazing; oxide of tin, if we wish it white; and if black, oxide of manganese. These materials are pounded to an extremely fine powder, and mixed with water, to form a thin liquid. The ware is dipped into this fluid, and drawn out; whereupon, the moisture is absorbed by the clay, and the glazing particles remain upon the surface. These are afterwards melted, by exposing the vessel a second time to the heat of the kiln, and they then constitute a uniform and durable vitreous coating. In the European and American manufactories, they are obliged to burn the vessels, before glazing them, because the composition, of which the vessel is made, is not rendered capable, merely by drying, of resisting water, so as to bear dipping in the glazing fluid. In this respect, the Chinese have great advantages; their materials being such, that, after drying, they will bear immersion in liquid without injury. By this means, a great amount of fuel is saved. It is proper to add, that the ware commonly called stone ware does not require to be mixed with metallic oxides, but affords the materials of its own glazing, by a vitrification of its surfaces. When the furnace in which it is burnt arrives at its greatest heat, a quantity of muriate of soda, or common salt, is thrown into the body of the kiln. The salt rises in vapor, and envelopes the hot ware, and, by the combination of its alkali with the silicious particles on the surface of the ware, a perfect vitrification is produced. This glazing, consisting of an earthy glass, is insoluble