colour on being exposed to a gentle heat, as has been already noticed with respect to glass containing copper1. The former when taken from the crucible is generally of a pale rose-colour, but sometimes colourless as water, and does not assume its ruby colour till it has been exposed to a low red-heat, either under a muffle or in the lamp. Great care must be taken in this operation, for a slight excess of fire destroys the colour, leaving the glass of a dingy brown, but with a blue transparency like that of gold-leaf. These changes of colour have been vaguely attributed to change of oxygenation in the gold; but it is obviously impossible that mere exposure to a gentle heat can effect any chemical change in the interior of a solid mass of glass, which has already undergone a heat far more intense. In fact I have found that metallic gold gives the red colour as well as the oxide, and it appears scarcely to admit of a doubt, that in a metal so easily reduced, the whole of the oxygen must be expelled long before the glass has reached its melting-point. It has long been known that silver yields its colour to glass while in the metallic state, and everything leads one to suppose that the case is the same as to gold.

There is still one other substance by means of which I find it is possible to give a red colour to glass, and that is a compound of tin, chromic acid, and lime; but my trials do not lead me to suppose that glass thus coloured will ever be brought

into use.

With respect to the production of artificial gems, they are now made abundantly of almost every shade of colour, closely approximating to those which occur in nature, excepting in hardness and refractive power. They are formed by fusing what is called a base with various metallic oxides. The base varies in composition: thus, M. Fontanieu makes his by fusing silica with carbonate of potash, carbonate of lead and borax. M. Donault Wieland's consists of silica, potash, borax, oxide of

[At the recent meeting of the British Association for the Advancement of Science, held at Cambridge (June 1845), M. Splittgerber exhibited specimens of glass into the composition of which gold entered as a chloride. These specimens were white, but upon gently heating them in the flame of a spirit-lamp, they became a deep-red. If again the same reddened glass is exposed to the heat of an oxygen blowpipe, it loses nearly all its colours, a slight pinkiness only remaining.]

.ead, and sometimes arsenious acid. Hence the base differs but little in composition from glass. By fusing the base with metallic oxides, the former acquires various tints. Thus with oxide of antimony the oriental topaz is prepared; with oxide of manganese and a little purple of cassius, the amethyst; with antimony and a very small quantity of cobalt, the beryl; with horn silver (chloride of silver), the diamond and opal: the oriental ruby is prepared from the base, the purple of cassius, peroxide of iron, golden sulphuret of antimony, manganese calcined with nitre and rock crystal.]

SEALING-WAX.

WRITERS on diplomatics mention, besides metals, five other substances on which impressions were made, or with which letters and public acts were sealed, viz. terra sigillaris, cement, paste, common wax, and sealing-wax'. The terra sigillaris was used by the Egyptians, and appears to have been the first substance employed for sealing2. The Egyptian priests bound to the horns of the cattle fit for sacrifice a piece of paper; stuck upon it some sealing-earth, on which they made an impression with their seal; and such cattle only could be offered up as victims 3

Lucian speaks of a fortune-teller who ordered those who came to consult him to write down on a bit of paper the questions they wished to ask, to fold it up, and to seal it with clay, or any other substance of the like kind. Such earth seems to have been employed in sealing by the Byzantine emperors: for we are told that at the second council of Nice, a certain person defended the worship of images by saying, no one believed that those who received written orders from the emperor

1 Gattereri Elem. Artis Diplom. 1765, 4to, p. 285.

2 It is singular that Pliny denies that the Egyptians used seals, lib. xxiii. c. 1. Herodotus however, and others, prove the contrary; and Moses speaks of the seal-rings of the Egyptians. See Goguet.

3 Herodot. lib. ii. c. 38.

4 Lucian. in Pseudomant.

and venerated the seal, worshiped on that account the sealingearth, the paper, or the lead '.

Cicero relates that Verres having seen in the hands of one of his servants a letter written to him from Agrigentum, and having observed on it an impression in sealing-earth (cretula), he was so pleased with it that he caused the seal-ring with which it was made to be taken from the possessor. The same orator, in his defence of Flaccus, produced an attestation sent from Asia, and proved its authenticity by its being sealed with Asiatic sealing-earth; with which, said he to the auditors, as you daily see, all public and private letters in Asia are sealed: and he showed on the other hand that the testimony brought by the accuser was false, because it was sealed with wax, and for that reason could not have come from Asia3. The scholiast Servius relates, that a sibyl received a promise from Apollo, that she should live as long as she did not see the earth of the island Erythræa where she resided; that she therefore quitted the place, and retired to Cuma, where she became old and decrepid; but that having received a letter sealed with Erythræan earth (creta), when she saw the seal she instantly expired*.

No one however will suppose that this earth was the same as that to which we at present give the name of creta, chalk; for if it was a natural earth it must have been of that kind called potters' clay, as that clay is capable of receiving an impression and of retaining it after it is hardened by drying. That the Romans, under the indefinite name of creta, often

1 Act. iv. ap. Bin. tom. iii. Concil. part. i. p. 356. Whether the yn σημαντρὶς, however, of Herodotus and the πηλός of Lucian and of the Byzantine be the same kind of earth, can be determined with as little certainty as whether the creta, called by some Roman authors a sealingearth, be different from both.

2 Orat. in Verrem, iv. c. 9. In the passage referred to, some instead of cretula read cerula. I shall here take occasion to remark also, that in the Acts of the Council of Nice before-mentioned, instead of πŋλòv some read Kηpóv: but I do not see a sufficient reason for this alteration, as in the before-quoted passage of Lucian it is expressly said, that people sealed kηp ĥ πnλ. Reiske himself, who proposes that amendment, says that nov may be retained. Stephanus, however, does not give that meaning to this word in his Lexicon. Pollux and Hesychius tell us, that the AtheLians called sealing-earth also púπov.

3 Orat. pro Flacco, c. 16.

4 Serv. ad lib. vi. Æneid. p. 1037.

understood a kind of potters' earth, can be proved by many passages of their writers. Columella speaks of a kind of chalk of which wine-jars and dishes were made1. Virgil calls it tough 2; and the ancient writers on agriculture give the same name to marl which was employed to manure land3. Notwithstanding all these authorities, I do not clearly comprehend how letters could be sealed with potters' clay, as it does not adhere with sufficient force either to linen, of which in ancient times the covers of letters were made, or to parchment; as it must be laid on very thick to have a distinct impression; as it is long in drying, and is again easily softened by moisture; and, at any rate, if conveyed by post at present, it would be crumbled into dust in going only from Hamburg to Altona. I can readily believe that the Roman messengers employed more skill and attention to preserve the letters committed to their care than are employed by our postmen; but the distance from Asia to Rome is much greater than that from Hamburg to Altona.

But may there not be as little foundation for the ancient expression creta Asiatica, Asiatic earth, as for the modern expression, cera Hispanica, Spanish wax? May not the former have signified a kind of coarse artificial cement? These questions might be answered by those who have had an opportunity of examining or only seeing the sigilla cretacea in collections of antiquities. We are assured that such are still preserved; at least we find in Ficoroni the representation of six impressions which, as he tells us, consisted of that earth. In that author however I find nothing to clear up my doubts; he says only that some of these seals were white; others of a gray colour, like ashes; others red, and others brown. They seem all to have been enclosed in leaden cases. Could it be proved that each letter was wrapped round with a thread, and that the thread, as in the seals affixed to diplomas, was drawn through the covering of the seal, the difficulty which I think occurs in the use of these earths, as mentioned by the ancients, 2 Georg. i. v. 179.

1 Lib. xii. c. 43.

3 Creta fossica, qua stercorantur agri.-Varro, i. 7. 8. It appears also that the nλo's of the Greeks signified a kind of potters' earth. Those who do not choose to rely upon our dictionaries, need only to read the ancient Greek writers on husbandry, who speak of ἀῤῥαγεῖ πηλῷ ἀργιλλώδει. See Geopon. x. c. 75. 12, and ix. c. 10. 4.

4 I piombi antichi. Roma 1740, 4to, p. 16.

would entirely disappear1. It seems to me remarkable that neither Theophrastus nor Pliny says anything of the Asiatic creta, or speaks at all of sealing-earth; though they have carefully enumerated all those kinds of earth which were worth notice on account of any use.

In Europe, as far as I know, wax has been everywhere used for sealing since the earliest ages. Writers on diplomatics, however, are not agreed whether yellow or white wax was first employed; but it appears that the former, on account of its low price, must have been first and principally used, at least by private persons. It is probable also, that the seals of diplomas were more durable when they consisted of yellow wax; for it is certain that white wax is rendered more brittle and much less durable by the process of bleaching. Many seals also may at present be considered white which were at first yellow; for not only does wax highly bleached resume in time a dirty yellow colour, but yellow wax also in the course years loses so much of its colour as to become almost like white wax. This perhaps may account for the oldest seals appearing to be of white, and the more modern of yellow wax. These however are conjectures which I submit with deference to the determination of those versed in diplomatics.

of

In the course of time wax was coloured red; and a good deal later, at least in Germany, but not before the fourteenth century, it was coloured green, and sometimes black. I find it remarked that blue wax never appears on diplomas; and I may indeed say it is impossible it should appear, for the art of giving a blue colour to wax has never yet been discovered; and in old books, such as that of Wecker, we find no receipt for that purpose. Later authors have pretended to give directions how to communicate that colour to wax, but they are altogether false; for vegetable dyes when united with wax become greenish, so that the wax almost resembles the hipstone; and earthy colours do not combine with it, but in melting fall again to the bottom. A seal of blue wax, not coloured blue merely on the outer surface, would be as great a rarity in the arts as in diplomatics, and would afford matter

1 Heineccius and others think that the amphora vitreæ diligenter gypsatæ, in Petronius, were sealed; but it is much more probable that they were only daubed over or closed with gypsum, for the same reason that we pitch our casks.