The cost of the pro( ess of reduction by the hot-blast," observes Mr. Weale, in his Dictionary of Terms of Art,' " being so much less than that with the eold-blast. the ulti mate value of the former is acourse also partly dependent upon the quality or the prtIdiice. (Jul this head notch di&r ellCe of Opinion has often been manitested, and with all the earnestness usually displayed in the advocacy of self-interest. The value of each prlwes., trust, no doubt, arise from the completeness of the fusion produced, and the separation etrected between the iron, and the impurities combined with it in the ()re. The hot-blast furnace the fusion more readily than the cold-blast, but admits a larger combination of cinders with the ore ; and the advantage which has been taken of this theility of adulteration. in order to reduce the cost of produetion, has doubtless led to the introduction into the market of many qualities of hot-blast iron. which are inferior in strength to that made with the cold-blast, The results of some of the most carefully-conducted experiments which have been made upon the strength of east-iron, and published in the sixth volume of the new series of "Memoirs of the Literary and Philosophical Society of Manchester," show that the transverse strength of the cold-blast iron tried was about per eent. greater than that of the hot-blast. The experiments here referred to were made upon rect angular bars 1 inch square, and 4 feet 6 inches long between the supports. The mean average breaking-weights, placed at the middle of these bars, were— In 21 samples or hot-blast iron . . . 443.5714 lbs.
In 22 samples of cold-blast iron . . 430.0000 Cast iron, which is scarcely malleable at any temperature, is generally so hard as to resist the file, and is extremely brittle ; however. it is equally permanent, in many applica tions. a ith is less liable to rust, and being, easily cast into various forms by melting, is much cheaper. Indeed, the labour of wrought-iron, if applied to many of the purposes to which •‘st-iron is used, would be incredible, and in some eases insurmountable.
The uses to which cast-ircm is now applied, are so nume rous, that it is quite impossible to particularize them. It is used extensively in the wheel-work of every department of machinery, in crane-work, in iron bridges, in beams, and pillars, for buildings, and in numerous articles of manu facture. It is employed in the construction of works of the greatest magnitude, and of the most minute character. The immense iron-girders of a railway bridge are made of east iron. and offering a striking contrast to the delicate ornaments of the drawing-room fabricated of the same useffil metal. Of lace y ears, the perfection to which the art of moulding in iron has been brought is almost incredible, in particular, we may mention, beautiful specimens of ornamental railings, chimney-pieces, figures imitative of ancient sculptures, &e.
Cast-iron is reduced into wrought or bar-iron, or forged iron, by divesting it of several fOreign mixtures with which it is incorporated. The varieties of wrought-iron are the fol lowing: hot-short iron is so brittle when heated, that it will not bear the weight of a small hammer without. breaking to atoms, but is malleable when cold, and very thsible in a high temperature; cold-short iron possesses the opposite qualities, and is with difficulty fusible in a strong heat, and though capable w bile hot of being beaten into any shape, is when cold very brittle, and but slightly tenacious. The iron in general use, which though, in a chemical point of view, not entirely pure, is so far pertbct, that it possesses none of these defects; its principal properties are the !Idiom ing: 1st, AV hen applied to the tongue. it has a styptic taste, and emits a pecu liar smell when rubbed: 2nd, Its specific gravity varies from '7.6 to '7.8 ; a cubic foot of it weighs about 5E.i0lb. avoirdu pois : 3rd. It is attracted by the Magnet or loadstone, and is itself, in one of its ores, the substance which constitutes the loadstone. It is also capable of acquiring itself the attrac tion and polarity of the magnet in various Nvays ; iron, how ever, that is perfectly pure, retains the magnetic virtue only a very short tithe; 4th. It is malleable in every temperature, which, as it rises, increases the malleability. It cannot, ho• ever, be hammered out so thin as gold or silver, nor even as copper. Its ductility is very great. and its tenacity such, that an iron wire than the twelfth of an inch in diameter is capable of supporting, without breaking, 5491lb. avoirdupois : 5th, It melts at about 158° of Wedge wood : Gth, It combines very readily with oxygen ; when exposed to the air, its surface is soon tarnished, and is gra dually changed into a brown or yellow colour, usually called rust: this change takes place more rapidly, in proportion as it is more exposed to moisture.
" Between the cast-irons made in different parts of Great Britain, there are characteristic differences. The StatIbrd shire metal runs remarkably fluid, and makes fine sharp castings. The Welsh is strong, less fluent, but produces bar iron of superior quality. The Derbyshire iron also forms excellent castings. and may be worked with care into very good I ar-iron. The Scotch iron is very valuable for casting into hollow wares, as it affords a beautiful smooth skin front the moulds, SO remarkable in the castings of the Carron com pany. in Sterlingshire. and of the Phoenix founder• at Glasgow. The Shropshire iron resembles the Statffirdshire in its good qualities."—Dr. (Ire, Dict. of Arts.