Micaceous iron, another variety of the oxide, is found near Dunkeld.

Red iron ore, or hæmatites, gives a red streak, and is not magnetic. It abounds at Ulverstone in Lancashire, and affords from 60 per cent. to 80 per cent. of excellent iron.

Argillaceous iron stone is a very abundant, though a poor ore of iron; yet as it accompanies coal, it is the principal source of the great quantities of the metal manufactured in England. It affords from 25 to 30 per cent. of malleable iron. Its reduction is accomplished by the aid of limestone and coal; the former producing a fusible compound with the argillaceous part of the ore; the latter serving as fuel, and also reducing the oxide to the metallic state.

It is said, that the average annual manufacture of iron in Europe amounts to fifteen million quintals, of which five millions are manufactured in Britain, and nearly the same quantity in France.

Mr. Brande next adverted to the properties of pure or malleable iron, to the varieties of cast iron and steel, and to the composition of the oxides, chlorides, and salts of this metal. Their uses in calico printing, dying, in the manufacture of prussian blue and other pigments, were also pointed out; for writing ink the following proportions of materials were deemed best :-3 oz. of finely bruised galls; 1 oz. of green vitriol; 1 oz. of logwood shavings; 1 oz. of gum arabic; 1 oz. of bruised cloves; 1 pint of distilled vinegar; 1 pint of distilled water. These materials are to be put together in a bottle, and set in a warm place for ten days, being well shaken every day. The coarser parts being allowed to subside, the ink is poured off; and its quality is materially improved by dissolving a stick of Indian ink and ten grains of corrosive sublimate in each quart; an ounce of brown sugar may also be added if the ink is intended to be used in the copying press.

In reflecting upon the numerous uses to which iron has been applied, Mr. Brande said, he had now only alluded to a few of the most important of them. In considering its influence in extending the dominion of man, we are naturally led to ask for some information respecting its discovery, and some account

of the persons who, by inventing the methods of extracting it from its ores, and working it, conferred upon the human race a benefit so great and unlimited. The fact is, that the era of this important discovery is unknown, though doubtless, in our hemisphere, of very remote origin. Indeed it is only by tradition, or by digging up some of the rude instruments of our warlike forefathers, that we discover that at one period the useful metals were unknown, and that men endeavoured to supply their want by the use of flints and other hard stones.

Gold, silver, and copper, being found as already explained, in the metallic state, and especially the two former, have been long employed, and were used before any idea had been entertained of extracting iron from its brittle and stubborn ores. It is curious to draw a comparison, involving this discovery; and we are enabled to do so with much accuracy, by consulting the records of savage nations. To these tribes, the felling of a tree with a stone hatchet, was employment, we are told, for a month; and to hollow out a canoe consumed so many years, that, according to Borde, it frequently began to rot before it was finished. To clear a small field of timber, says Robertson in his History of America, was a work which required the united efforts of a whole tribe.

Agriculture, even when the strength of man is seconded by that of the animals which he has subjected to the yoke, and augmented by the instruments which iron furnishes, is still a work of great labour; and it is with the sweat of his brow that he renders the earth fertile. But when people were destitute of these advantages, cultivation was almost impossible; and we accordingly observe them depending on fishing and hunting for subsistence, rather than upon the fruits of the earth.

The records of Holy Writ inform us that in the time of Moses, iron was used for cutting instruments, and the furnaces and ores are mentioned. In Homer, this metal is alluded to, as by no means common. A spear of iron was among the prizes which Achilles offered at the celebration of the funeral rites of Patroclus. None of the weapons of war, however, were at that time made of iron, they were a species of bronze, an alloy of copper and tin.

By the processes of fusion, puddling, and rolling, described in my last Lecture, iron is separated from the combination in which it exists in the ore, and is obtained in the various states of cast or pig iron, and of forged, bar, or malleable iron. The characters of cast iron, by which it is most evidently different from bar iron, are its brittleness and fusibility; circumstances dependant upon extraneous matters, which by chymical union with the pure metal thus modify its characters. These substances are chiefly carbon, and the metallic bases of some of the earths, forming compounds which have hitherto been but little examined, but which promise to become important. Independent, however, of composition, the mechanical structure of different kinds of iron is extremely various, a subject which has been very ingeniously investigated by Mr. F. Daniell, and of which he has given an account in the Journal of Science and the Arts. Thus, gray cast iron, submitted to dilute muriatic acid, seemed made up of numerous bundles of minute needles. Bar iron, which had undergone the operation of puddling and rolling, presented fibres running parallel to its length; and indeed, if we tear bars of malleable iron asunder we shall observe their fibrous texture.

Steel is a compound also of iron and carbon, generally made by heating bar iron in contact with charcoal; so far therefore it resembles cast iron; but it does not afford siliceous earth or manganese on solution. The different kinds of steel contain very different proportions of carbon; and it seems principally to this circumstance, that their different qualities are to be referred. But another cause of variation is referable to what is called tempering, and by this process the hardness of steel is very curiously modified; when a steel instrument is heated and plunged into water it becomes very hard, but likewise very brittle; and as different cutting instruments require very different degrees of hardness, it becomes necessary again to soften them down to a proper point. Files are the hardest steel instruments used, and are heated red hot and plunged into water. If a file be now again heated to a particular degree, it softens, or is tempered, and acquires a varying hardness at different thermometrical points. If the steel in

this state be polished, we observe that its surface undergoes various changes of colour; first it becomes pale yellow; then bright yellow; then reddish brown; then bluish, and light blue, then full blue; and lastly purple and black. The cause of this change in the hardness of the steel is referable to change of texture; and the great alteration it suffers has been rendered curiously evident by Mr. Daniell. A bar of steel was broken in two and heated red; one part suffered to cool spontaneously, the other quenched in water. The former was easily acted upon by muriatic acid, the latter with much greater difficulty. The former had a fibrous and wavy texture; the latter was like worm-eaten wood, compact, and not striated.

In noticing the singular properties exhibited by different kinds of steel and iron, Mr. Brande mentioned the mode of imitating Damascus sword blades, and the manufacture of stub iron.

Tin is a metal, the mineralogical history of which is extremely simple, for there is only one ore, which is the native oxide, or tin stone. It is found crystallized in octohedral and in four-sided prisms, resulting from their elongation. It often presents curious macles, or twin crystals. A ferruginous variety of oxide of tin forms the wood tin ore. Those who are curious in regard to the history of this ore, I must refer to a valuable dissertation upon it by Mr. Phillips, in Vol II. of the Geol. Trans.

Tin, geologically speaking, is an old metal; its veins are cut by those of copper. Cornwall may be considered as the richest tin district in the world. It traverses granite and slate, accompanied by quartz, &c. Sometimes immense masses of the ore have been raised, and it is curious that particular varieties of crystallization belong to particular mines. Thus Penandrae near Redruth abounds in twin crystals: PolgarthHuel Fanny, and other mines, are renowned for particular crystalline forms. The ores are reduced by simple ignition with charcoal.

Pure tin approaches silver in appearance. Its specific. gravity is 7,9. It is malleable and melts at 442° Fahrenheit.

There are two oxides of tin, the gray and the white. The former contains 100+ 14 oxygen; the latter, 100 + 28. These oxides are soluble in fixed alkalies. Tin is not easily oxidized by air and moisture; hence tin plate, which is an alloy of iron and tin, is much more durable than iron.

The two oxides combine with acids; the salts containing the protoxide have a strong attraction for oxygen; and hence precipitate many of the metals in a metallic state, or otherwise rob them of oxygen. Nitromuriate of tin is used by diers of scarlet.

ART. XIII. On the Ventilation of Covent Garden Theatre. ON a former occasion (Vol. IV. p. 383) we adverted to the

ingenious and effectual manner in which the ventilation of Covent Garden Theatre has been effected. At the desire of many of our readers, we applied to Mr. Harris for permission to examine the various contrivances, and to inquire into their efficacy, which was most liberally granted; and we are indebted to the Marquis de Chabannes, under whose directions the ventilation has been perfected, for the description and plates annexed.

We have before stated, that the principle of ventilating and regulating the temperature of this large building depends upon the constant admission of fresh air into the lower parts of the theatre, cold or warm, as the season requires, while the heated and foul air is continually carried off by tubes connected with heated cylinders. The gas lights are well contrived, so as to co-operate in this ventilation, and thus, while the house is admirably lighted, the whole of the burned air is conducted away, and none of that closeness and suffocating sensation is perceived which common lamps are apt to produce, and which is more especially occasioned by ill-devised gas illumination.

The following description of the plates will sufficiently explain the most important parts of the apparatus by which the temperature of the building is regulated, and by which a