upon the tin leaf, in such a manner as to sweep off the redundant quicksilver, which is not incorporated with the tin; leaden weights are then placed on the glass; and in a little time the quicksilvered tin-foil adheres so firmly to the glass, that the weights may be removed without any danger of its falling off. The glass thus silvered is a common looking-glass. About two ounces of quicksilver are sufficient for covering three square feet of glass.

It is generally believed, that the art of making looking-glasses, by applying to their back surface a metallic covering, is a very modern invention. Muratori expressly says, that glass specula, such he means as are now in use, are not of any great antiquity.Seræ autem antiquitati novimus fuisse specula, quorum usus nunquam desiit; sed eorum fabricam apud Italos unice forsan Veneti per tempora multa servarunt et adhuc servant: quæ tamen alio translata nunc in aliis quoque regnis floret *. -The authors of the French Encyclopedie+ have adopted the same opinion, and quoted a Memoir, printed in the twenty-third volume of the Academy of Inscriptions, &c.-Il est d'autant plus étonnant que les anciens n'aient pas connu l'art de rendre le verre propre à conserver la représentation des objets, en appliquant l'etain derriere les glaces, que les progrés de la découverte du verre furent, chez eux, poussés fort loin.-Mr. Nixon, in speaking of the glass specula of the ancients, says, "before the application of quicksilver, in the con. struction of these glasses, (which I presume is of no great antiquity), the reflection of images by such specula must have been effected by their being besmeared behind, or tinged through with some dark colour, especially black." I have bestowed more time in searching out the age in which the applying a metallic covering to one side of a looking-glass was introduced, than the subject, in the estimation of many, will seem to deserve; and, indeed, more than it deserved in my own estimation: but the difficiles nuga, the stultus labor ineptiarum, when once the mind gets entangled with them, cannot be easily abandoned: one feels, moreover, a singular reluctance in giving up an unsuccessful pursuit. The reader would pardon the introduction of this reflection, if he knew how many musty volumes I turned over, before I could meet with any information which could satisfy me, in any degree, on this subject; I am not yet quite satisfied; though I take the liberty to say, in opposition to Muratori, and the other respectable authorities which I have quoted, that the applying a metallic covering to look. ing-glasses is not a modern invention; it is probable it was known in the first century, if not sooner; and it is certain, I apprehend, that it was known in the second.

The Romans, before the time of the younger Pliny, not only used glass, instead of gold and silver, for drinking vessels, but they knew how to glaze their windows with it, and they fixed it in the walls of their rooms to render their apartments more pleasant. Now a piece of flat glass, fixed in the side of a room, is a sort of looking-glass, and if the stucco into which it is fixed be of a dark colour, it will not be a very bad one. And hence I think the Romans could not fail of having a sort of glass specula in use : but this, though admitted, does not come up to the point; the question is, whether they covered the posterior surface of the glass with a metallic plate? It has been observed before, that the Romans knew how to make a paste of gold and quicksilver; and it appears from Pliny also, that they knew how to beat gold into thin leaves, and to apply it in that state both on wood and metal: now there is a passage in Pliny, from whence it may be collected, that the Romans began in his time to apply a coat of metal to glass specula, and that this coat was of gold. The passage occurs in the very place where Pliny professes to finish all he had to observe concerning specula*. An opinion, says he, has lately been entertained, that the application of gold to the back part of a speculum, renders the image better defined. It is hardly possible that any one should be of opinion, that a plate of gold put behind a metallic speculum, could have any effect in improving the reflected image; but supposing Pliny (whose transitions in writing are often abrupt) to have passed from the mention of metallic to that of glass specula, then the propriety of the observation relative to the improved state of the image is very obvious. If we suppose the Romans, in Pliny's age, to have simply applied some black substance to the back surface of the glass, or even to have known how to put tin behind it, yet the observation of the image being rendered more

* Atque ut omnia de speculis peragantur hoc loco-Optima apud majores fuerant Brundusina stanno et ære mixta. Prælata sunt argentea. Primus fecit Praxiteles, magni Pompeii ætate. Nuper credi cœptum certiorem ima ginem reddi auro apposito aversis. Hist. Nat. 1. xxxiii. s. xlv.

distinct by means of gold, might have been made with more justicė than is generally supposed; for Buffon is of opinion, that a look. ing-glass made with a covering of gold and quicksilver, would re. flect more light than one made in the ordinary way with tin and quicksilver*; and hence Pliny's expression, certiorem imaginem reddi auro apposito aversis, will be accurately true.

Alexander Aphrodiseus flourished towards the end of the second century; he wrote several works in Greek, and among the rest two books of Problems; one of his problems is this t:

Δια τι τα ύελινα κατοπίρα λαμπεσι αγαν ;
Why are glass specula so very resplendent?

The only part of the answer which we are concerned with, is,
Οτι ενδοθεν ανία χριεσι κασσιτερω.

Because they besmear the inside of them with tin.

The Greek word which I have here rendered besmear, does not clearly point out the manner in which the operation of fixing the tin upon the glass was performed. Pliny uses a Latin word (illitum) of exactly the same import as this Greek one, when he speaks of copper vessels being tinned; and as in that operation, tin is melted and spread over the surface of the copper, I see no difficulty in supposing, that the tin may have been, in the time of Alexander Aphrodiseus, melted and spread over the surface of the glass, when previously heated.

Having carried up the invention of covering glass specula with a metallic coating to the second century, we may be the more ready to admit that the Sydonians possessed this art, before Pliny wrote his Natural History: for in that work he not only praises them for their former ingenuity in various glass manufactures, but he addsand they had invented specula also‡.- Now there is some reason

* On pourroit trouver le moyen de faire un meilleur étamage, et je crois qu'on parviendroit en employant de l'or et du vifargent.-Hist. Nat. Buffon. Sup. tom. i. p. 451.

† ΑΛΕΞΑΝΔΡΟΥ ΑΦΡΟΔΙΣΕΩΣ ιατρικα απορηματα και φυσικα προβληματα, Parisiis, 1541.-If there be any doubt concerning the authenticity of these problems, I leave it to be discussed by the critics.

Aliud (vitrum) flatu figuratus, aliud torno teritur, aliud argenti modo cælatur, Sydone quondam iis officinis nobili, siquidem etiam specula excogitaverat. Hist. Nat. 1. xxxvi.

to think, that if the Sydonians had only invented the art of using a flat piece of glass as a speculum, without knowing how to give it a metallic coating, on which its excellency chiefly depends, they would not have merited the mention which Pliny makes of them; for their looking-glasses must have been inferior to the metallic mirrors then in use at Rome. There seems to be but one objection of any consequence to this conclusion: -had the method of giving a metallic covering to plates of glass been known at least to the Romans, (for it might have been known in Asia long before it was known in Italy), it seems probable, that the metallic specula would have fallen into general disuse, much sooner than there is cause to think they did, for it would have been much easier to make a looking-glass, than to polish a metallic mirror; and the image from the glass would have been superior to that from the metal, and on both accounts the mirrors would have become unfashionable.

The first mode of fixing a coat of tin on a looking-glass, I sus. pect to have beenthat of pouring the melted metal on the glass; and I have some reason, not now to be insisted on, to think, that this mode was not disused in the fourteenth century. - Baptista Porta lived in the fifteenth, and died towards the beginning of the sixteenth century; he gives us a very accurate description* of the manner in which looking-glasses were then silvered; it differs from that now in use only in this, that the tin-foil, when silvered, was taken up and gently drawn upon the glass. J. Maurice Hoffman published his Acta Laboratorii Chemici, in 1719; he there speaks t of a mixture of one part of tin with three of quicksilver, which some time ago, he says, was usually applied to the back surfaces of looking-glasses; although the Venetians did then make looking. glasses by pouring quicksilver upon tin-foil placed on the back surface of the glass. -This mode of silvering the glass was not then invented by the Venetians, as appears from what Baptista Porta had advanced above two hundred years before; though the mode of silvering the tin-foil, when laid upon the glass, was an improve. ment on that prescribed by Baptista Porta, just as the mode now in use is a great improvement on that practised by the Venetians in the time of Hoffman,

The men who are employed in silvering looking-glasses often

become paralytic, as is the case also with those who work in quicksilver mines; this is not to be wondered at, if we may credit Mr. Boyle, who assures us that mercury has been several times found in the heads of artificers exposed to its fumes*. In the Philosophical Transactionst, there is an account of a man, who having ceased working in quicksilver for six months, had his body still so impregnated with it, that by putting a piece of copper into his mouth, or rubbing it with his hanus, it instantly acquired a silver colour. This, though a surprising, is not a fact of a singular nature; it is well known that sulphur, taken inwardly, will blacken silver which is carried in the pocket; and I have somewhere read of a man whose keys were rusted in his pocket; from his having taken, for a long time, large quantities of diluted acid of vitriol. I remember having seen, at Birmingham, a very stout man ren. dered paralytic in the space of six months, by being employed in fixing an amalgam of gold and quicksilver on copper; he stood before the mouth of a small oven strongly heated; the mercury was converted into vapour, and that vapour was inhaled by him. A kind of chimney, I believe, has of late been opened at the farther side of the oven, into which the mercurial vapour is driven, and the health of the operator is attended to. The person I saw was very sensible of the cause of his disorder, but had not courage to withstand the temptation of high wages, which enabled him to continue in a state of intoxication for three days in the week, instead of, what is the usual practice, two.

[Bishop Watson.

CHAP. VI.

METALLIC PLANTS, OR TREES.

BEFORE We quit the very curious and interesting subject of metals, we may observe that many of these substances when minutely dissolved in a menstruum, and treated with a third substance that se parates them wholly or in part from the fluid in which they are thus contained, crystallize into the appearances of very beautiful

* Boyle's Works, vol. iii. p. 330,

+ 1665.