Cast Iron, when viewed under favorable circumstances, by the help of a microscope will be found to be a mechanical aggregation of molecules of iron and carbon; and the relative position of these particles may be illustrated by a pile of cannon balls as usually arranged in navy yards, each alternate ball being iron and carbon (charcoal).
If a mass of cast iron be heated until softened, and then puddled (squeezed), the carbon will be forced to the surface, and will there combine with the oxygen of the atmosphere, forming carbonic acid or carbonic oxide gases, and thus pass off. When all the carbon has been parted with, the mass is called Wrought Iron, and may then be welded, when at proper heat, but cannot be melted—the hottest blast furnace will not melt wrought iron. Wrought iron at red heat combines rapidly with oxygen, and becomes oxide of iron—thus a joint of stove-pipe thrown into a furnace will never melt, but by contact with atmosphere will change into oxide of iron, and thus be practically lost. This operation is technically called burning. If a piece of wrought iron be surrounded by carbon (charcoal) finely pulverized, and the whole enclosed in a sheet iron vessel to exclude the air, and this placed for a sufficient length of time in a furnace constructed for the purpose, the iron will imbibe an atomic quantity of carbon, and become Steel. This process is called cementation, and steel so made can be melted as readily as cast iron.
Thus it will be seen that both cast iron and steel are combinations of iron and carbon, and in the same proportions, but not in the same state of combination. In cast iron, the carbon and iron are a mere mechanical combination, while in steel the iron and carbon are combined chemically.—Wrought iron, when pure, is free from carbon, and its ductility, toughness, &c., are due to the absence of carbon, sulphur, phosphorus, and other substances, with which it is occasionally pervaded.
The French chemists are experimenting, and occasionally succeeding by accident, in causing heated iron to take the carbon from carbonic acid and other gases containing carbon, and thus becoming steel more rapidly, and at less cost, than when made by the process of cementation. Mr. Dixon, of Jersey City, has succeeded in making steel direct from the Adirondack iron ore, while Peter Cooper, Esq., Mr. Dickinson, and others, are manufacturing wrought iron direct from the iron ores of New-Jersey without first forming the pig or cast iron, and of course at less expense, as the saving of fuel is very great.
The process of case-hardening, or changing the immediate surface of iron utensils into steel, is readily performed by covering their surfaces with such organic substances as contain carbon as a constituent, and then subjecting them to high heat for short spaces of time—thus the roller of a paper or sugar mill may be case-hardened by a coating of prussiate of potash, or of leather chips, and then subject the whole to high heat, excluded from atmospheric influences. By this process the gelatine and other constituents of the leather are reduced to carbon, and this enters the surface particles of the iron, converting them into steel. Many hypotheses are offered for this action, and among others, that "the ultimate particles of matter are always in motion," admitting the ingress of particles travelling in smaller orbits between them. The friends of this hypothesis offer as proof, that a fresh cast sash-weight when broken is a gray mass, while one taken from an old building, and broken, is beautifully crystalline, from the centre to the outside, like speculum metal. A freshly drawn piece of tin pipe when suddenly bent opposite the ear gives no crackling sound, and if broken has no crystalline structure, but if left at rest for one hour it has both. Barbers often tell us that razors get tired of shaving, but if laid by for thirty days they will then shave well. By microscopic examination it is found that the tired razor, from long stropping by the same hand and in the same directions, has the ultimate particles or fibres of its surface or edge all arranged in one direction, like the edge of a piece of cut velvet; but after a month's rest, these fibres re-arrange themselves heterogeneously, crossing each other and presenting a saw-like edge, each fibre supporting its fellow, and hence cutting the beard, instead of being forced down flat without cutting, as when laid by. These and many other instances are offered by the friends of the hypothesis named, to prove that the ultimate particles of matter are always in motion, and they say that in the process of welding, the absolute momentum of the hammer causes an entanglement of orbits of motion, and hence a re-arrangement, as in one piece; indeed, in the cold state, a leaf of gold laid on a polished surface of steel, and stricken smartly with a hammer, will have its particles forced into the steel so as to permanently gild it at the point of contact.
The oxidation of metals is equally curious, and the length of time necessary for the formation of an infinitesimal coating of oxide is less than the one-thousandth of a second. This fact may be readily proved: a clean surface of steel, free from oxide, when brought in contact with mercury (quicksilver) will amalgamate, but if the least oxide be upon the surface no such effect will take place. Prepare a trough containing quicksilver, and place a bar of steel above it, and within one inch or less of the surface of the quicksilver—break this bar with a smart blow from a hammer, so that the blow which breaks it shall at the same time force the broken ends into the quicksilver, and although the time occupied by the ends in passing through one inch of atmosphere before reaching the surface of the quicksilver will be immeasurably short, still they will be so oxidized as not to amalgamate with the mercury; if, however, the bar of steel be confined at its ends below the surface of the quicksilver, and then be broken upwards, by a lever applied to its centre, the ends of the broken bar will be beautifully amalgamated before reaching the atmosphere above. The reason for the success of the last named experiment is doubtless due to the absence of oxide of iron, when broken beneath the surface of the mercury. J. J. MAPES.
New Weigh-lock at Albany.—A writer in the Courier and Enquirer gives a description of the great weigh-lock that has just been erected at Albany, for the purpose of ascertaining the tonnage of canal boats.
Heretofore long and vexatious delays have been the result, while now, a boat is brought into the lock, and in a time which would have scarcely sufficed in other days to have ascertained the weight of a small parcel, the unerring register on the beam, has registered its weight, and the record is on the books of the office. The weigh-lock is directly on the side of, and attached to the large canal, and is adapted to the use of such boats as shall hereafter be built, when the locks throughout the entire length of the canal shall be of the uniform enlarged size.