페이지 이미지
PDF

CHEMISTRY AS A SCIENCE. Art, XIII.

IBON.

It is probable that our readers could imagine there was some skill and some power in chemical analysis, when we pointed out the mode in which oxygen and hydrogen had been detected, and why they were at present considered as simple and elementary substances. But their respect for that skill and power may probably decrease, when we state, that the iron, with which they are so familiar, is also considered by the chemist as a simple and elementary substance, in which his art can work no further change. The fact is, that the art of the chemist, or rather of the metallurgist, has already been employed to make iron, and he has not yet discovered any means of further decomposing •it. Iron, wrought iron, however, is a simple undecompounded substance, and is seldom or never found in this state, but is made by art. Large masses of it have certainly been found nearly pure, in Siberia and in South America; but -all the iron of commerce is obtained by a chemical process from some species of earths or stones,; which are usually called iron ores. The metal, in combination with other substances, is, indeed, extensively diffused; and as it is the most useful, so it is certainly the most abundant of all the metals. Few mineral bodies, stones, or earths, are quite free from iron; the water of riversand of springs generally contain it. and traces of it are perceptible in the juices of vegetables, and in the fluids and solids of animals. When found mixed with earths, or other matters forming stones, it'is usually in the state of an oxide, or combined with oxygen, and itis principally from these substances that it is obtained for the arts. Perhaps not a single part of the globe is to be found wholly destitute of iron^ and certainly there is no civilized part of the world, of any extent, where it is not manufactured from its ores. The iron of Russia and Sweden has long been celebrated for its good qua

lities, and that of England and France, if not quite so good, is probably more extensively worked. The chemist never makes iron as he makes oxygen, and hydrogen, because iron is already made for him. We shall describe, therefore, the mode of making iron in those large laboratories, iron-works.

The first step, in order to extract iron from those ores in which it is most generally found, is to roast them. This is done by laying the ore, broken into pieces, in a kiln, mixed with small coals; or simply by laying the iron stones,mixed with coals, in alternate strata, in a heap, and then the coal is burnt till.all the fuel is consumed, as is the practice in making lime. This separates the volatile parts with which the ore may be combined, and the stones are much more easily broken. The iron stone is found, after this process, to consist principally of oxide of iron and clay; and when iron ores, as is the case with the rich ores of Cumberland,are found native, in the state of pure oxide, this first part of the process is not requisite. After the one has been thus reduced to the state of an oxide mixed with clay, it is melted in large furnaces, from sixteen to thirty feet high, of various shapes. Near the bottom of these furnaces is a hole for the nozzle of a large pair of bellows, which are worked either by steam or water, and keep up a constant blast. The furnace is open at top, and has more than one hole at different parts, which can be opened at pleasure to draw off the metal, or to remove the earthy substances which are to be separated from it. Charcoal or coke, with lighted brushwood, is first thrown into the furnace, and when the whole has got heated, baskets of ore and of fuel, either coal or coke, but genenerally in this country coke, and on the Continent charcoal, are thrown alternately into the furnace. At the same time, a sabstance is added which is caUcdJiu.r, and is different accordingto the nature of the ore ; but, in general, this flux is lime-stone. The reason for adding this is, that, in most cases, the earth mixed with the iron is alumina or clay, which of itself cannot be melted, but which is easily fused when mixed with a certain portion of lime. By adding the lime-stone, therefore, in proportions which the eye and hand of the workman generally determine, the two earths melt, and form a sort of glass, separating from the metal, the oxygen of which, at the same time, combines with the charcoal or coke employed, and flies off as carbonic acid gas; while the metal, by reason df its greater weight, sinks, in a melted state, to the lowest part of the furnace, and is drawn off into moulds prepared for its reception. Thus, chemically speaking, there aTe here several decompositions and combinations. First, the ore is decomposed, and the clay unites with the lime of the lime-stone; which being in its turn, in general, a combination of lime and carbonic acid, gives up the latter, which flies off. Then, again, the coal or coke undergoing combustion, takes the oxygen from the ore, and separates from it, also in the state of carbonic acid gas, while a portion of the carbon combines with the iron. The contents of the furnace are separated into three distinct portions: one flies off as gas; another swims at the surface, and is a sort of glass, composed of all the earthy and foreign matters of the ore; and the third, thcmetal, sinks to the bottom. This process, after being once commenced, is carried on for years without interruption, the fires being never extinguished, and the furnaces being never allowed to cool. There is an incessant addition of the material, and an incessant production of iron. The'quantities and proportions of fuel, of ore, and the nature of the flux which must be employed, are all regulated in practice, according to the nature of the ore, and the qualities of the iron to be obtained; and depend on circumstances, which are always acted on with success, though not easily described. The iron, however.which has thus run out, is not pure iron, from the two boxes, K K, and impart it to the two wooden rollers, 6 G, each of which has a pattern out on it, and stamps it on the calico, as it revolves with a regular motion round the large roller, D; 11 are rollers which guide and stretch the blanket. Within each of the colour boxes there is a roller contrived for feeding the blankets, and L shows the wooden rollers for guiding and stretching the blankets, four of which' belong to each; M i s the roll of wbi te ealico; and N is the calico after being printed of the three patterns on the three rollers. Twenty pieces of calico, fastened together by I the ends, each containing 28 yards, are in general printed by Ibis machine at one operation, it being so contrived that they pass over the different rollers in one uninterrupted line till the whole are finished. It prints at the rate of about seven yards of calico in a minute.

but iroil combined with a> greater or less quantity of carbon, and is known under different names, according to the means used to obtain it. For our purpose, it will be enough to distinguish three great varieties; viz. white east iron, which is extremely hard and brittle, can neither be bored, bent, nor filed, and seems composed of a series of small crystals. Grey cast iron, which is much softer, and less brittle, may be cut or bored, and is,the sort of iron which is used for making artillery, and a variety of similar purposes ; and black cast iron, which is the softest and most fusible of the three. Th» Jast mentioned contains the most carbon, and the first the least. To separate the carbon, and obtain pure iron, the metal is first melted in a refining furnace, in which it is exposed to a continued blast of air, which separates a part of the carbon. After being exposed to this for about three or four hours, it is run out of the furnace, along with a large portion of the vitreous oxide of iron, which has been formed by the blast, and assists in depriving the metal of its carbon. The cake of metal, after being run out, is broken in pieces, and introduced into what is called the puddling furnace, where it is exposed, at the same time, to the action of flame and of blasts of air. As soon as the metal is molted, a man called a puddter begins to rake it about, and a more terrific task cannot well be conceived; at the same time, he occasionally throws water on it with a small iron dish. The water is decomposed by the melted mass, the hydrogen escapes, and the oxygen unites with a portion of the iron. As the metal is agitated, it gradually loses its carbon, its fusibility diminishes, it loses all cohesion, and becomes a loose, granulated mass. As this change is going on, bubbles of gas, which are carbonic oxide, burst from it with a blue flame. After this, the heat is raised, and the loose mass becomes a number of balls of an irregular shape, which consist of iron united with the \i

treous oxide. When it has thus become solid, it is lemoved from the furnace, and is beat by means of hammers driven by machinery, or it is passed through heavy rollers. It has now acquired a certain degree of malleability, but is yet very brittle when cold. It is again heated, .and again hammered or rolled; and every time this process is repeated the iron is improved; the hammering or the rolling, whichever may be adopted, makes the particles approach nearer each other, and drives away several impurities. After it lias undergone repeated hammerings or rollings, it is what . is called in commerce bar iron, or sheet iron, and is supposed by chemists to be iron nearly in a state of purity. It is of a bluish white colour, very hard, and may be made, •when converted into steel, harder than most substances. It is malleable, but cannot be hammered so thin as gold or silver, or copper; and it may be drawn into wires as fine as human hair. A wire, 0.078 of an inch in diameter, has been found capable of supporting 449.341bs. avoirdupois, without breaking.

The various combinations of iron do not fall to be now treated of, but, for the sake of our youthful readers, we mean to say a few words of one of its combinations, though at the hazard of repeating whatis already very familiar, probably, to our better informed readers. The combination to which we allude is known to them all under the name of steel, which is considered by chemists as a subcarburet of iron. Although the conversion of iron into steel has been practised from a very remote period, the principles on which that conversion was effected, and the chemical difference between the two substances, were not known till the close of the last century. Bergman was the first chemist who made any approach towards explaining correctly the nature of the chemical change which iron undergoes when it becomes steel. Before his time, it had been supposed that the difference was owing to plunging the metal in water, to fixing fire in it, or to com

bining it with sulphureous and oily particles. Bergman showed that all cast iron and steel contain carbon; and it has since been satisfactorily proved, that these different varieties of the same metal owe their different qualities to the carbon with which they are combined. According to this theory wrought iron is a simple substance; steel is iron combined with a small portion of carbon; and cast iron is iron combined with a still greater proportion. Thus, if purified or bar iron be bedded in pounded charcoal, in a covered crucible, and kept for a certain number of hours in a strong red heat, the iron gains a small quantity in weight, and acquires the properties of steel. A French chemist, M. Morveau, made steel by combining iron and diamond together; and steel may, by repeated heatings and hammerings, again have the carbon expelled and be converted into iron. The reader may, perhaps, inquire here, why it is then, if cast iron and steel are both iron combined with carbon, that manufacturers do not make steel in the first instance, without going through the whole process we have described to refine the iron before converting it into steel? The reason of this is, that cast iron in that state is generally contaminated with several other matters besides carbon, which are driven off by the same means as it is driven off. But it happens, that the very pure ore of Cumberland can be, and is converted into steel in the first instance; and in the furnaces of that part of the kingdom, by lengthening or shortening the duration of the fire, malleable iron, steel and cast iron are all obtained from the same furnace, one after another. According to this theory, we see the reason of the black spot which nitric acid leaves when dropped on steel, while when dropped on iron the spot is whitish green. In the steel the carbon is made visible. The length to which this Article has now reached obliges us to postpone its conclusion till next week.

[graphic][merged small][merged small]

We concluded our -last Article with a description of the mode in which the calico is dried: and after this process is completed, the pieces are passed, by means of a winch, through water, in which a little cow-dung is mixed. The object of this part of the process is to removeAtyof the mordant which has not actually combined, and which would otherwise stain the white or unprinted part. Perhaps, too, as it has been found that cowdung contains a substance like bile, a quantity of animal matter is imparted by this process to the cotton, which contributes to heighten the colours. After the dunging, which takes from five to forty minutes to perform, the pieces are taken to the river or water wheel to be effectually washed, after which they are passed through tepid water, to satisfy the workman that every impurity is removed. They are then put into a copper boiler containing pure water, into which a sufficient quantity of madder has been broken; a fire is made under the copper, and the pieces are constantly turned, either by a hand-winch or by a steam-engine, so as to pass everypart of them

through the liquor till it boils, and in some cases for a quarter of an hour after it boils. When the mechanical part of the work is done by a steam-engine, the ends of several pieces are fastened together, and they are kept in constant motion through the madder-vessel; and the introduction of this mode has been found to save a great deal of labour, as where it is employed one man is enabled to attend three dyeing-vessels at one time. After the goods have been passed through the madder-copper, they are carried to a boiler containing wheat-bran and water, in which they are winched for a considerable time to purify the white ground from the discolouration of the madder. Although this process impairs the intensity of the colours, no other means of purifying "the White ground is known which is so effectual. If this is found not sufficient to make the white clear, the pieces are laid on the grass for a few days, where they undergo a second, though a partial bleaching. Or, which is now the more common method, the pieces are immersed in a weak -solutioaof oxymuriate of potash or soda, which effects in a few minutes what formerly required as many days, besides making it not necessary for the calico-printers to have so much ground as they formerly employed.

There is another part of calicoprinting called resist work, which is conducted in the following manner: Resist paste is made by taking a salt of copper, either the nitrate, muriate, or sulphate, but the sulphate is preferred, and mixing it either with flour paste, with gum, or with pipe-clay and gum; and this paste is printed on the calico in any shape or pattern which it is desired shall not be made blue by dipping the pieces in the blue .or indigo vat. Hence the name of the process, to resist the dye. It has been found that indigo, in its oxygenized state, has no affinity for cloth; and an indigo vat is formed by such a mixture of lime and sulphate of iron as will de-oxidize the

« 이전계속 »