as lead; and as a mixture consisting of a large portion of tin with a small one of lead, cannot easily be distinguished from a mass of pure tin; the temptation to adulterate tin is great, and the fear of detection small. In Cornwall, the purity of tin is ascertained, before it is exposed to sale, by what is called its coinage: the tin, when smelted from the ore, is poured into quadrangular moulds of stone, containing about 320 pounds weight of metal, which, when hardened, is called a block of tin; each block of tin is coined in the following manner:-" The officers appointed by the Duke of Cornwall assay it, by taking off a piece of one of the under corners of the block, partly by cutting, and partly by breaking; and if well purified, they stamp the face of the block with the impression of the seal of the Duchy, which stamp is a permission for the owner to sell, and at the same time an assurance that the tin so marked has been purposely examined, and found merchantable *.” This rude mode of assay, is not wholly improper; for if the tin be mixed with lead, the lead will by its superior weight sink to the bottom, and thus be liable to be discovered, when the bottom cor. ner of the block is examined. But though the seal of the Duchy may be some security to the original purchasers of block tin, it can be none at all to those foreigners who purchase our tin from Holland; for, if we may believe an author of great note—“ in Holland every tin founder has English stamps, and whatever his tin be, the inscription, block tin, makes it pass for English +." This foreign adulteration of English tin may be the reason that Musschenbroeck, who was many years professor of natural philosophy at Utrecht, puts the specific gravity of what he calis pure tin equal to 7320, but that of English tin, and he has been followed by Wallerius, equal to 7471 ‡; for it will appear presently, that such sort of tin must have contained near one-tenth of its weight of lead.

* Borlase's Nat Hist. of Cornw. p. 183.

+ Newman's Chem. by Lewis, p. 89.

Musschen. Ess. de Phys. 1739. French Trans. Wallerii Min. vol. I. p. 154. There is a very good Table of Specific Gravities, published in the second volame of Musschenbroeck's Introductio ad Philosophiam Naturalem, 1763, in which the author does more justice to English tin, putting the weight of a cubic foot of the purest sort equal to 7295; avoir. oua. One specimen of the purest sort of Malacca tin gave 7331, and another 6125 ounces a cubic foot, which is the lightest of all the tins which he examined.

Weight of a cubic foot of English tin, according to different authors.

From the following experiments it may appear probable, that not one of these authors, in estimating the specific gravity of tin, has used the purest sort, but rather a mixture of that with lead, or some other metal.

A block of tin, when it is heated till it is near melting, or after being melted, and before it becomes quite fixed, is so brittle that it may be shattered into a great many long pieces like icicles, by a smart blow of a hammer*: tin in this form is called by our own manufacturers grain tin, by foreigners virgin tin, or tears of tin; and they tell us, that its exportation from Britain is prohibited under pain of death. The tin which I used in the following experiments, was of this sort, but I first melted it, and let it cool gra. dually; a circumstance, I suspect, of some consequence in de. determining the specific gravity not only of tin, but of other metals. I have put down in the following table, the specific gravity of this tin, and of the lead I mixed with it by fusion, and of the several mixtures when quite cold; the water in which they were weighed was 60o.

Weight of a cubic foot of lead, tin, &c.

* This property is not peculiar to tin; I have seen masses of lead which, under similar circumstances, exhibited similar appearances; and it has been observed, that zinc, when heated till it is just ready to be fused, is brittle.

Ency. Fran. and Mr. Baumé calls it "étain en roche, à cause que sa forme

Blocks of tin are often melted by the pewterers into small rods; I think the rods are not so pure as the grain tin; at least, I found cubic foot of the specimen I examined, weighed 7246 ounces: but even this sort exceeds in purity any of the kinds examined by the authors above mentioned. Chemistry affords cer. tain methods of discovering the quantity of lead with which tin is alloyed, but these methods are often troublesome in the applica tion; an enlarged table, of the kind of which I have here given a specimen, will enable us to judge with sufficient precision of the quantity of lead contained in any mixture of tin and lead, of which we know the specific gravity. Pewterers, however, and other dealers in tin, use not so accurate a method of judging of its purity, but one founded on the same principle; for the specific gravities of bodies being nothing but the weights of equal bulks of them, they cast a bullet of pure tin, and another of the mixture of tin and lead, which they want to examine, in the same mould; and the more the bullet of the mixture exceeds the bullet of pure tin in weight, the more lead they conclude it contains.

Pewter is a mixed metal; it consists of tin united to small portions of other metallic substances, such as lead, zinc, bismuth, and the metallic part, commonly called regulus of antimony. We have three sorts of pewter in common use; they are distinguished by the names of plate-trifle-ley. The plate pewter is used for plates and dishes; the trifle chiefly for pints and quarts; and the leymetal for wine measures, &c. Our very best sort of pewter is said to consist of 100 parts of tin, and of 17 of regulus of antimony, though others allow only 10 parts of regulus to 100 of tint; to this composition the French add a little copper. Crude antimony, which consists of nearly equal portions of sulphur and of a metallic substance, may be taken inwardly with great safety; but the metallic part, or regulus, when separated from the sulphur, is held to be very poisonous. Yet plate pewter may be a very in. nocent metal, the tin may lessen or annihilate the noxious qualities of the metallic part of the antimony. We have an instance somewhat similar to this in standard silver, the use of which has never

ressemble à des stalactites;" he says also, that its exportation is prohibited, but that he does not see the reason for the prohibition, as it is not more pure than Cornish tin; and in this observation he is right, it is nothing but Cornish tin in a particular form. Chym. par M. Baumé, vol. III. p. 422

* Med. Trans. vol. I. p. 286.

+ Pemb. Chem. p. 322.

been esteemed unwholesome, notwithstanding it contains near onetwelfth of its weight of copper. Though standard silver has always been considered as a safe metal, when used for culinary purposes; yet it is not altogether so, the copper it contains is liable to be corroded by saline substances into verdigris. This is frequently seen, when common salt is suffered to stay a few days in silver saltcellars, which have not a gold gilding; and even saline draughts, made with volatile salt and juice of lemons, have been observed to corrode a silver tea-spoon, which had been left a week in the mixture.

The weight of a cubic foot of each of these sorts of pewter is,

If the plate pewter be composed of tin and regulus of antimony, there is no reason to expect, that a cubic foot of it should be hea vier than it appears to be; since regulus of antimony, according to the different ways in which it is made, is heavier or lighter than pure tin. A very fine silver-looking metal is said to be composed of 100 pounds of tin, 8 of regulus of antimony, 1 of bismuth, and 4 of copper. The ley pewter, if we may judge of its composition by comparing its weight with the weights of the mixtures of tin and lead, mentioned in the table, contains not so much as a third, but more than a fifth part of its weight of lead; this quantity of lead is far too much, considering one of the uses to which this sort of pewter is applied; for acid wines will readily corrode the lead of the flagons, in which they are measured, into sugar of lead; this danger is not so great with us, where wine is seldom sold by the measure, as it is in other countries where it is generally sold so, and their wine measures contain, probably, more lead than ours do. Our English pewterers have at all times made a mystery of their art; and their caution was formerly so much encouraged by the legislature, that an act of parliament was passed, rendering it unlawful for any master pewterer to take an apprentice, or to em. ploy a journeyman, who was a foreigner. In the present improved state of chemistry, this caution is useless; since any one tolerably skilled in that science, would be able to discover the quality and quantity of the metallic substances, used in any particular sort of pewter; and it is not only useless now, but one would have thought it must have been always so; whilst tin, the principal ingredient,

was found in no part of Europe in so pure a state, nor in so great plenty as in England.

Borlase and Pryce, who have written so minutely on the method of preparing the tin in Cornwall, are both of them silent, as to any operation the tin undergoes subsequent to its coinage; nor do they say any thing of its being mixed with other metallic substances previous to its coinage; but assure us, that the tin, as it flows from the ore, is laded into troughs, each of which contains about three hundred pounds weight of metal, called slabs, blocks, or pieces of tin, in which size aud form it is sold in every market in Europe. Foreigners, however, in general assert, that our tin as exported is a mixed metal; and the French Encyclopedists in par ticular (article Etain) inform us, on the authority of Mr. Rouelle, that the virgin tin is again melted and cast into iron moulds of half a foot in thickness; that the metal is cooled very slowly; that when cold it is divided horizontally into three layers; that the uppermost, being very soft pure tin, is afterwards mixed with cop. per, in the proportion of 3 pounds of copper to 100 of tin; that the second layer, being of a harsher nature, has 5 pounds of lead added to 100 of the tin; and that the lowest layer is mixed with 9 pounds of lead to an hundred of the tin; the whole is then re. melted, and cooled quickly; and this, they say, is the ordinary tin of England: and Geoffroy had formerly given much the same account*. There is, probably, no other foundation for this report, but that pewter has been mistaken for tin, these metals being some. times called by the same name; and fine pewter being sometimes made from a mixture of 1 part of copper with 20 or 30 parts of tin.

The mixture generally used for the tinning of copper vessels, consists of 3 pounds of lead, and of 5 pounds of pewter; when a finer composition is required, ten parts of lead are mixed with sixteen of tin; or one part of lead with two of tin: but the proportions in which lead and tin are mixed together, even for the same kind of work, are not every where the same; different artists having dif

fusores aperto furni ostiolo, metallum in forinas quasdam ex arena paratas diffluere sinunt, ibique in massas grandiores concrescit. Superior stanneæ massæ pars adeo mollis est et flexilis, ut sola elaborari nequeat sine copri miscela, trium scilicet librarum super stanni libras centum. Massæ pars media binas tantum cupri libras recipit. Infima vero adeo fragilis est et in tractabilis, ut cum hujus metalli centum libris plumbi libras octodecim consociare oporteat. Geoff. Mat, Med. vol. I. p. 282.