calities, among which, in this country, the banks of the Delaware are the most noted. With this silicious substance, which is the basis of glass, we mix potash or soda; and, if the glass be coarse, alkaline ashes of any kind, such as kelp, barilla, &c. To these are added, in many instances, small quantities of lime and borax, to render the materials more fusible; and also, in some cases, some metallic oxide, which is used to increase the transparency and whiteness of the glass, or to impart to it a peculiar color. 2. Having mixed these materials carefully together, we proceed to apply heat. This, in the first instance, must not be sufficient to fuse the materials; otherwise, the alkali will be driven off, before it can be combined with the silica. At this stage, heat is used merely to expel the carbonic acid, and other gaseous and volatile matters, which would otherwise prove troublesome, by causing the materials to swell up while fusing. It is gradually increased, and the materials constantly stirred, for some hours, until they unite into a soft adhesive mass. The alkali having gradually combined with the silicious earth, the homogeneous mass, thus formed, is called frit, and the process by which it is procured, fritting. The frit is now placed in a crucible, made of the most refractory clays and sand, and deposited in the melting furnace. A quantity of old glass is commonly placed upon the top of the frit, and the heat of the furnace is raised to its greatest height, at which state it is continued thirty or forty hours. During this period, the materials become perfectly united, and form a transparent uniform mass, free from specks and bubble. The whole is then suffered to cool a little, by slackening the heat of the furnace, until it acquires sufficient tenacity to be wrought. 3. Glass is wrought by blowing, casting, and moulding. Blowing is one of the most interesting operations connected with this manufacture. The workman is provided with a long iron tube, one end of which he thrusts into the melted glass, turning it round until a certain quantity, sufficient for the purpose, is gathered, or adheres to the extremity. The tube is then withdrawn from the furnace, and the lump of glass which adheres is rolled upon a smooth iron table, and the workman blows strongly with his mouth, through the tube, by which the glass is gradually inflated, like a bladder. The inflation is assisted by the heat, which causes the air and moisture of the breath to expand with great power, and enables the workman to give the glass any form he may choose. Whenever the glass becomes so stiff, from cooling, as to render the inflation. difficult, it is again held over the fire, to soften it. If window glass is to be formed, the blowing is repeated, until the globe is expanded to the requisite thinness. It is then received by another workman, upon an iron rod, while the blowing-iron is detached. It is now opened at its extremity; and, by means of the centrifugal force communicated by rapid whirling, it is spread into a smooth, uniform sheet, of equal thickness throughout, excepting a protuberance at the centre, where the iron rod was attached. This protuberance is called a bull's eye; and being in the centre, no very large squares can be obtained from these circular plates. Hence, when larger sizes are wanted, it is usual to blow them in the form of cylinders, not more than four feet long. When plates of still larger dimensions are made, it must be effected by casting. The glass is melted in great quantities, in large pots or reservoirs, until it is in a state of perfect fusion, in which condition it is kept for a long time. It is then drawn out, by means of iron cisterns, of considerable size, which are lowered into the furnace, filled, and raised out by machinery. The glass is poured out, from these cisterns, upon tables of polished copper, of large size, having a rim elevated as high as the intended thickness of the plate. In order to spread it perfectly, and to make the two surfaces parallel, a heavy roller, of polished copper, weighing five hundred pounds or more, is passed over the plate, resting on the rims, at the edges. The glass, which is beginning to grow stiff, is pressed down and spread equally, the excess being drawn before the roller, until it falls off at the extremity of the table. When ornamental forms are to be impressed upon the surface of glass vessels, it is effected by using metallic moulds, into which the glass, while in a melted state, is blown, until it receives the impression on the outside. This process has been essentially improved, by the manuturers of American glass, who, in moulding, subject the material to pressure on the inside and outside (at the same time) of different parts of a mould, which are brought suddenly together by mechanical power. 4. When, through any of these methods, the glass has been brought to the requisite form, it is next subjected to the operation called annealing, which consists in removing the glass into a furnace, whose heat is not sufficiently intense to melt it, and gradually withdrawing the article from the hottest to a cooler part of the annealing chamber, till it is cold enough to be taken out for use. This process is indispensable to the durability of the glass; for, if cooled too suddenly, it becomes extremely brittle, and flies to pieces upon the least touch of any hard substance. This effect is shown in the substance called Rupert's drops, which are made by cooling, suddenly, drops of green glass, by letting them fall into cold water. These drops fly to pieces, with an explosion, whenever the smaller extremities are broken. The Bologna phials, and some other vessels of unannealed glass, break into a thousand pieces, if a flint, or other hard and angular substance, is dropped into them. This phenomenon seems to depend upon some permanent and strong inequality of pressure; for, when these drops are heated so red as to be soft, and left to cool gradually, the property of bursting is lost, and the specific gravity of the drops is increased. 5. To annealing, succeed the processes of grinding and cutting. The object of the first is to polish the surface of the glass, so as to fit it for mirrors, &c. It is performed in a manner very similar to that employed in polishing marble, except that the glass, being a harder substance, requires more labor and nicety in the operation. The plate to be polished is first cemented to a table of wood or stone, with plaster of Paris. A quantity of wet sand or emery is spread upon it, and another glass plate, similarly cemented to another wooden surface, is brought in contact with it. The two plates are then rubbed together, until the surfaces have become materially smooth and plane. The emery which is first used is succeeded by emery of a finer kind, and the last polish is given by putty, or the oxide of iron, called colcothar. When one surface has become perfectly polished, the cement is removed, the plate turned, and the opposite side polished in the same way. In some cases, pure flint, reduced to powder, is substituted for emery. The advantage of it consists in the fact, that the mixture of glass and flint, which is left. after the operation, is valuable for forming fresh glass. Cutting is a species of grinding, by which successive portions of glass are ground away, and figures are given it, which appear as if made by the incision of a sharp instrument. An establishment for cutting glass contains a great number of wheels of stone, metal, and wood, which are made to revolve rapidly by a steam-engine, or other power. The first, or rough cutting, is sometimes given by wheels of stone, resembling grindstones. Afterwards, wheels of iron are used, having their edges covered with sharp sand or emery, in different states of fineness. The last polish is given by brush wheels, covered with a putty, which is an oxide of tin and lead. To prevent the friction from exciting so much heat, as to endanger the glass, a small stream of water continually drops upon the surface of the wheel. 6. Coloring. We have already stated, that the beautiful colors imparted to glass beads, enamelling, &c., were produced by various metallic oxides, melted with the other ingredients, and combining in intimate union with them. Another method of coloring is termed staining; and consists in applying some color superficially, which vitrifies on being exposed to heat, and incorporates with the substance of the glass. This art was employed very much by the ancients, in decorating the windows of churches, and cathedrals. It is often described as being no longer known ; but erroneously, since nothing has been lost but the art of communicating certain peculiar colors, which are found in the windows of the ancient cathedrals. Before dismissing the subject of glass, we may notice a combination, which has been invented within the last century, and which has proved of inestimable importance, in the construction of telescopes. When flint glass (that is, a very clear and comparatively soft glass, so called, because it was formerly made of pulverized flint) is formed into single lenses, for telescopes, it is found that the objectglass (that is, the one nearest the object) imparts colors to the image formed by light passing through it, owing to a difference in the refrangibility of the rays which compose white light. In this way, the vision becomes indistinct, and the inconvenience was so great, as to render the refracting telescope nearly useless; until an English artist, Dollond,* (who, like many English artists, was a man of science, as well as practical skill,) discovered a method of obviating it. This consists in making the objectglass in three separate parts, which fit exactly together, and which consist of different kinds of glass. The dispersion occasioned by the first glass is partially corrected by the second, and still more by the third; so that the light, at last, emerges from the lens perfectly colorless, and forms a distinct image; and hence it is called the achromatic (or colorless) glass. It is an interesting fact, that the human eye is constructed of lenses combined in a manner somewhat similar, and is supposed to have furnished to the artist the first hint of his invention. The cavity of the eye, that most perfect of all optical instru *For a notice of Mr. Dollond, see Difficulties,' vol. ii, being vol. xv. of Pursuit of Knowledge under |