per was known long before the age of Pliny; it is mentioned by Aristotle, incidentally, when he is describing a method of rendering copper white, but not by tin; and from its great utility, it will probably never fall into disuse. We have ceased, indeed, since the introduction of glass mirrors, to use it in the way the ancients did; but it is still of great use amongst us, since the specula of reflecting telescopes are commonly made of it. Mr. Mudge has ascertainedt, not only the best proportion in which the copper and tin should be mixed together, but has found out also a method of casting the specula without pores. He observes, that the perfection of the metal, of which the speculum should be made, consists in its hardness, whiteness, and compactness. When the quantity of tin is a third of the whole composition, the metal then has its utmost whiteness; but it is at the same time rendered so hard that it cannot be polished without having its surface splintered and broke up. After many experiments, he at length found that fourteen ounces and one half of grain tin‡, and two pounds of copper made the best composition; an addition of half an ounce more tin rendered the composition too hard to be properly polished. The casting the metal so as that it may be compact and without pores, is a matter of the greatest consequence; he hit upon the manner of doing it by accident. His usual way of casting a speculum metal, was to melt the copper and to add the tin to the melted copper: the mass when cast was seldom free from pores. After having used all his copper in trying experiments to remedy this defect, he recollected that he had some metal which had been reserved, when one of the bells of St. Andrew had been re-cast: he added a little fresh tin to it, and casting a metal with it, it turned out free from pores, and in all respects as fine a metal as he ever saw. Upon considering this circumstance, he proceeded to form a metallic mass in the usual way, by adding tin to melted copper; this mass was porous, it was in the state of the bell-metal he had tried; and upon re-melting it, it became, as the bell-metal

Grain tin is worth ten or twelve shillings per hundred more than mine tin, because it is smelted from a pure mineral by a charcoal fire; whereas mine tin is usually corrupted with some portion of mundick, and other minerals, and is always smelted with a bituminous fire, which communicates a harsh, sulphureous, injurious quality to the metal. Pryce, Min. Cornu. p. 137.—Mr. Mudge probably used what is called grain-tin in the shops, or the purest sort, which is usually sold in pieces like icicles.

had done, compact and free from pores. He accounts for this dif ference by observing, that the heat necessary to melt copper, cal cines part of the tin; and the earthy calcined particles of the tin, being mixed in the mass of the metal, render it porous; but the composition of tin and copper, melting with less than half the heat requisite to melt the copper, the tin is not liable to be calcined in the second melting, as in the first. I am rather disposed to think, that the absence of the pores is to be attributed to the more perfect fusion of the metal: for I have observed at Sheffield, that the same weight of melted steel will fill the same mould to a greater or less height, according to the degree of fusion the steel has been in; if it has been in a strong heat, and thin fusion, the bar of cast steel will be an inch in thirty-six shorter than when the fusion has been less perfect. Upon breaking one of the bars, which had been made from steel in an imperfect fusion, its inside was full of blebs; a shorter bar of the same weight and diameter, which had been in a thin fusion, was of a closer texture. Now the mixture of tin and copper melts far easier than copper does, and it is likely, on that account, to be in a thinner fusion when it is cast.

It may deserve to be remarked, and I shall have no other opportunity of doing it, that the melting or casting of steel was introduced at Sheffield, about forty years ago, by one Waller from London; and was afterwards much practised by one Huntsman, from whom steel so prepared, acquired the name of Huntsman's cast steel. It was first sold for fourteen-pence, but may now be had for ten-pence a pound; it costs three-pence a pound in being melted, and for drawing ingots of cast steel into bars of the size of razors, they pay only six shillings for a hundred weight, and ten shillings for the same quantity when they make the bars into a size fit for small files, &c. The cast steel will not bear more than a red heat; in a welding heat it runs away under the hammer like sand. Before the art of casting steel was introduced at Sheffield, all the cast steel used in the kingdom was brought from Germany; the business is carried on at Sheffield with greater advantage than at most other places, for their manufactures furnish them with great abundance of broken tools; and these bits of old steel they pur, chase at a penny a pound, and melt them, and on that account they can afford their cast steel cheaper than where it is made altogether from fresh bars of steel.

CHAP. IV.

OF TINNING COPPER-TIN-PEWTER.

UNHAPPILY for mankind, the fatal accidents attending the use of copper vessels, in the preparation of food and physic, are too common, and too well attested, to require a particular enumeration or proof: scarce a year passes, but we hear of some of them, espe cially in foreign countries; and many slighter maladies, originating from the same source, daily escape observation, or are referred to other causes, in our own.

In consequence of some representations from the College of Health, the use of copper vessels in the fleets and armies of Sweden was abolished in the year 1754; and tinned iron was ordered to be substituted in their stead*. The Swedish government de. serves the greater commendation for this proceeding, as they have. great plenty of excellent copper in the mines of that country, but no tin. An intelligent surgeon suggested, in 1757, the probability of the use of copper vessels in the navy, being one of the causes of the sea scurvy, and recommended the having them changed for ves. sels of iron; he remarked, that of the 200 sail of ships which went to sea from Scarborough, most of them used iron pots for boiling their victuals, and that the symptoms called highly scorbutic, were never seen, except in some few of the larger ships in which copper vessels were used t. Notwithstanding this hint, and the example of Sweden, I do not know that any other European state has prohibited the use of copper vessels for the dressing of food on board their ships; but many of them have shewn a laudable attention to prevent its malignity, by inquiring into the best manner of covering its surface with some metallic substance, less noxious, or less liable to be dissolved than itself. This operation is usually called tinning, because tin is the principal ingredient in the metallic mixture, which is made use of for that purpose; and, indeed, since the year 1755,

• Mem. de l'Acad. de Prusse, par M. Paul, vol. IV. Dis. Prel. p. 63. + Medical Observ, by a Society of Phys. in Lond, vol. II. p. 1.

it has been frequently, in this country at least, used alone. In that year, The Society for the Encouragement of Arts, Manufac tures and Commerce, thought it an object deserving their attention, to offer a premium for the tinning copper and brass vessels with pure tin, without lead or any other alloy. There were several candidates for the premium; and since that time, the tinning with pure tin, and hammering it upon the copper, has become very general in England. But this mode of tinning does not appear to have been known, or at least it does not appear to have been adopted in other countries; for in the Memoirs of the Royal Aca. demy at Brussels, for the year 1780, M. l'Abbé Marci recom. mends, as a new practice, the tinning with pure block-tin from England; though, he says, block tin is a compound body, even as it is imported from England; but he thinks it a much safer co. vering for copper than what is ordinarily used by the braziers; and he gives some directions as to the manner of performing the operation. The Lieutenant General of the Police at Paris, gave it in commission to the College of Pharmacy, in 1781, to make all the experiments which might be necessary for determining-whether pure tin might or might not be used for domestic purposes, without danger to health? The researches which were made, in consequence of this commission, by Messieurs Charland and Bayen with great ability, were published by order of the French government; and they have greatly contributed to lessen the apprehen sions relative to the use of tin, which had been generally excited by the experiments of Marggraf, published first in the Berlin Memoirs for 1747. That gentleman, in pursuing an experiment of Henckel, who first discovered arsenic in tin, shewed, that, though there was a sort of tin which being fluxed from an ore of a particular kind, contained no arsenic, the East India tin, which is generally esteemed the purest of all others, contained a great deal of arsenic. M. Bosc d'Antic, in his works, which were published at Paris, 1780, sets aside the authority of Marggraf, Cramer and Hellot, relative to the existence of arsenic in tin; and is not only of opinion, that the Cornish tin does not conceal any arsenic in its substance, but that its use as kitchen furniture is not dangerous. Messieurs Charland and Bayen found that neither the East India, nor the purest sort of English tin, contained any arsenic; but that the English tin, usually met with in commerce, did contain arsenic; though in so small a proportion that it did not amount, in that species of tin

which contained the most of it, to more than one grain in an ounce; that is, it did not constitute more than one five-hundredth and seventy-sixth part of the weight of the tin, there being 576 grains in a French ounce. This proportion of arsenic is so wholly inconsiderable, that it is very properly concluded, that the internal use of such small portions of tin, as can mix themselves with our food, from being prepared in tinned vessels, can be in no sensible degree dangerous on account of the arsenic which the tin may contain. But though tin may not be noxious, on account of the arsenic which it holds, it still remains to be decided, whether it may not be poisonous of itself; as lead is universally allowed to be, when taken into the stomach. The large quantities of tin, which are sometimes given in medicine with much safety, and the con. stant use which our ancestors made of it in plates and dishes, before the introduction of china or other earthen ware, without experiencing any mischief, render all other proof of the innocent nature of pure tin superfluous. And hence it may be proper to add a few observations concerning the purity of tin.

The ores of metallic substances often contain more substances than that particular one from which they receive their denomination. M. Eller, of Berlin, had in his collection an ore, which contained gold and silver, and iron and quicksilver, closely united together in the same mass. Lead ore, it has been remarked, so often contains silver, that it is seldom found without it; it is often also mixed with a sulphureous pyrites, which is a sort of iron ore, and with black jack, which is an ore of zinc; so that lead, and silver, and iron, and zinc, are commonly enough to be met with in the same lump of lead ore. Tin ore, in like manner, though it is sometimes unmixed, is often otherwise; it frequently contains both tin, and iron, and copper. The fire with which tin ore is smelted, is sufficiently strong to smelt the ores of the metals which are mixed with it; and hence the reader may understand, that, without any fraudulent proceeding in the tin smelter, there may be a variety in the purity of tin, which is exposed to sale in the same country; and this variety is still more likely to take place, in spe cimens of tin from different countries, as from the East Indies, from England, and from Germany. This natural variety in the purity of tin, though sufficiently discernible, is far less than that which is fraudulently introduced. Tin is above five times as dear