"But great as is the advantage of this first step of the iron-making, the iron is not yet fit for a knife. It is cast iron. It cannot be worked by the hammer, or sharpened to a cutting edge; and so it must be made into malleable iron, a kind of iron, which, instead of melting in the fire, will soften, and admit of being hammered into shape, or united by the process of welding.

"The methods, by which this is accomplished, vary; but they in general consist in keeping the iron melted in a furnace, and stirring it with an iron rake, till the blast of air in the furnace burns the greater part of the carbon out of it. By this means, it becomes tough; and, without cooling, is taken from the furnace and repeatedly beaten by large hammers, or squeezed through large rollers, until it becomes the bar iron, of which so much use is made, in every art of life.

"Bringing it into this state requires great force; and the unaided strength of all the men in Britain could not make all the iron which is at present made, though they did nothing else. Machinery is therefore resorted to; and water-wheels, steam-engines, and all sorts of powers, are set to work, in moving hammers, turning rollers, and drawing rods and wires through holes, till every workman can have the particular form which he wants. If it were not for the machinery that is employed in the manufacture, no man could obtain a spade for less than the price of a year's labor; the yokes of a horse would cost more than the horse himself; and the farmer would have to return to wooden ploughshares, and hoes made of sticks with crooked ends. There would be labor enough, then, as we have already shown: but the people could not live upon the labor only; they must have profitable labor.

"After all this, the iron is not yet fit for a knife,—at least for such a knife as an Englishman may buy for a shilling. Many nations would, however, be thankful for a little bit of it, and nations, too, in whose countries there is no want of iron ore. But they have no knowledge of the method of making iron, and have no fur

naces or machinery. When our ships sail among the people of the Eastern islands, those people do not ask for gold. 'Iron, iron!' is the call; and he, who can exchange his best commodity for a rusty nail, or a bit of iron hoop, is a fortunate individual.

"We are not satisfied with that, in the best form, which is a treasure to those people, in the worst. We must have a knife, not of iron, but of steel, a substance that will bear a keen edge, without either breaking or bending. In order to get that, we must again change the nature of our material.

"How is that to be done? The oftener that iron is heated and hammered, the softer and more ductile it becomes; and as the heating and hammering forced the carbon out of it, if we give it the carbon back again, we shall harden it; but it happens that we also give it other properties, by restoring its carbon, when the iron has once been in a ductile state.

"For this purpose, bars or pieces of iron are buried in powdered charcoal, covered up in a vessel, and kept at a red heat for a greater or less number of hours, according to the object desired. There are niceties in the process, which it is not necessary to explain, that produce the peculiar quality of steel, as distinguished from cast iron. If the operation of heating the iron in charcoal is continued too long, or the heat is too great, the iron becomes cast steel, and cannot be welded; but if it is not melted in the operation, it can be worked with the hammer, in the same manner as iron.

"In each case, however, it has acquired the property upon which the keenness of the knife depends; and the chief difference, between the cast steel and the steel that can bear to be hammered, is, that cast steel takes a keener edge, but is more easily broken.

"The property which it has acquired is that of bearing to be tempered. If it be made very hot, and plunged into cold water, and kept there till it is quite cooled, it is so hard that it will cut iron, but it is brittle. In this state, the workman brightens the surface, and lays

the steel upon a piece of hot iron, and holds it to the fire till it becomes of a color which he knows from experience is a test of the proper state of the process. Then he plunges it again into water, and it has the degree of hardness that he wants.

"The grinding a knife, and the polishing it, even when it has acquired the requisite properties of steel, if they were not done by machinery, would cost more than the whole price of a knife, upon which machinery is used. A travelling knifegrinder, with his treadle and wheels, has a machine, but not a very perfect one. The Sheffield knifemaker grinds the knife, at first, upon wheels of immense size, turned by water or steam, and moving so quickly, that they appear to stand still; the eye cannot follow the motion. With these aids, the original grinding and polishing cost scarcely any thing; while the travelling knifegrinder charges twopence for the labor of himself and his wheel, in just sharpening it.

"As iron is with us almost as plentiful as stone, we do not think much about it. But there is a great deal to be done, much thinking and inventing, before so simple a thing as a shilling knife could be procured; and without the thinking and the inventing, all the strength of all the men that ever lived never could procure it; and without the machinery to lighten the labor, no ingenuity could furnish it at a thousand times the expense."

CHAPTER X.

THE MANUFACTURE OF GLASS.

THE metals afford a striking instance of the power of human ingenuity and labor, in imparting value to substances, which, in their natural state, are worthless. This appears still more conspicuously in the case of glass and pottery. The ores, from which metals are

extracted, are frequently scarce, and can only be obtained at an immense expense of labor and time. The materials, of which glass and pottery are formed, exist, in abundance, near the surface of the earth, and in almost every neighborhood. Glass is composed of silicious sand or pounded flint, mixed with potash or soda; which latter materials are readily obtained by burning vegetable substances. Pottery is a species of brick; and, like it, is formed of clay, kneaded together, then moulded into the requisite forms, and hardened by heat.

The value of these substances, which are thus created from the most worthless materials, must be obvious to every one. They are equally important to science and the arts, and in domestic life. Take glass, for example. Without it, the astronomer would be unable to construct those instruments, with which he traces the movements, and counts the number, of the stars. The naturalist would be prevented from exploring the minute recesses of plants, and animals, and unorganized matter, by which means he ascertains their structure, and brings to light the myriads of animated beings with which they are peopled. Age and infirmity would be destitute of the means for repairing defects of vision, and thus be deprived of the innumerable benefits and pleasures which it was intended to afford. Our habitations, instead of being irradiated with the clear light of day, would enjoy only the faint beams which could find their way through oiled paper, horn, or mica. The druggist would no longer be able to examine his medicines, nor the chemist his experiments, through the sides of transparent vessels. Mirrors to reflect the person would not now be found in every cottage, but would be a luxury known only in the palace, or the mansion of the rich; and, instead of drinking-glasses and decanters, which now ornament every table, at a trifling expense, we should have to content ourselves with drinking-cups of tin or pewter, and jugs of stone.

The art of making glass was probably first suggested by the sight of native crystal. It is certainly ancient, but was probably confined, among the Oriental nations, as well as by the Greeks and Romans, to the manufacture of ornaments and utensils. Its use in windows, and, what is hardly less important, in optical instruments, is a modern invention.* At what time glass was first introduced into the windows of churches is not absolutely known; but probably not before the third or fourth century. In the windows of private dwellings it was very sparingly employed, till the seventeenth century; and a hundred years have not elapsed, since many houses in England and our own Country were without windows, or lighted only through oiled paper, horn, &c. At present, owing to chemical knowledge and greater division of labor, the art is so perfect, that glass vessels and windows can be afforded by the humblest individuals. We propose, in this Chapter, briefly to explain the several processes employed in making glass, referring the reader, for a more detailed account, to Bigelow's Technology,'t from which much of the matter of this Chapter is abridged.

Glass.-1. Glass, as we have already said, is composed of flint, fused with some alkaline substance. The flint is found in the sand of certain situations, or is obtained by heating common flints or quartz, red hot, and then plunging them into cold water. They turn white, fall to pieces, and are then pounded and sifted, before being mixed with the alkali. Pure silicious sand, however, is to be preferred, since it is already pulverized; and it may be found in several lo

* Glass windows are said to have been discovered in the buildings at Pompeii, formed of glass which must have been blown.

† Since the above was written, Dr. Bigelow's work, above alluded to, has been revised by the author, and, with many additions and alterations, republished as a part of THE SCHOOL LIBRARY,' (of which it forms volumes xi. and xii. of the larger series,) with the title, 'The Useful Arts considered in connexion with the Applications of Science,'