(see Chemistry Index, in this Supplement), is a valuable article to potters and dyers. To fit it for their use, it is first roasted and freed from the foreign mineral bodies with which it is united; it is then well calcined, and sold either mixed or unmixed with fine sand under the name of zaffer (zaffers); or it is melted with silicious earth and pot ash, to a kind of blue glass called smalt, which when ground very fine is known in commerce by the name of powder blue. All these articles, because they are most durable pigments, and those which best withstand fire, and because one can produce with them every shade of blue, are employed above all for tinging crystal and for enamelling; for counterfeiting opaque and transparent precious stones, and for painting and varnishing real porcelain and earthen and potters ware. This colour is indispensably necessary to the painter when he is desirous of imitating the fine azure colour of many butterflies and other natural objects; and the cheaper kind is employed to give a bluish tinge to new-washed linen, which so readily changes to a disagreeable yellow, though not without injury to the health as well as to the linen.

Professor Beckmann, in his History of Inventions, gives the following account of the paint prepared from cobalt: "About the end of the 15th century cobalt appears to have been dug up in great quantity in the mines on the borders of Saxony and Bohemia, discovered not long before that period. As it was not known at first what use it could be applied, it was thrown aside as a useless mineral. The miners had an aversion to it, not only because it gave them much fruitless labour, but because it often proved prejudicial to their health by the arsenical particles with which it was combined; and it appears even that the mineralogical name cobalt then first took its rise. At any rate, I have never met with it before the beginning of the 16th century; and Matthias and Agricola seem to have first used it in their writings. Frisch derives it from the Bohemian word kov, which signifies metal; but the conjecture that it was formed from kobalt, which was the name of a spirit that, according to the superstitious notions of the times, haunted mines, destroyed the labours of the miners, and often gave them a great deal of unnecessary trouble, is more probable; and there is reason to think that the latter is borrowed from the Greek. The miners, perhaps, gave this name to the mineral out of joke, because it thwarted them as much as the supposed spirit, by exciting false hopes and rendering their labour often fruitless. It was once customary, therefore, to introduce into the church-service a prayer that God would preserve miners and their works from kobalt and spirits.

"Regarding the invention of making an useful kind of blue glass from cobalt, we have no better information than that which Klorzech has published from the papers of Christian Lehmann. The former, author of an historical work respecting the upper district of the mines in Münster, and a clergyman at Scheibenberg, collected with..." with great diligence every information that respected the history of the neighbouring country, and died at a great age in 1688. According to his account, the colour-mills at the time when he wrote were about a hundred years old; and as he began first to write towards the end of the thirty years war, the invention seems to fall about 1540 or 1550. He relates the circumstance as follows: "Christopher Schurter, a glass-maker at Platten, a place which belongs still to Bohemia, retired to Nendeck, where he established his business. Being once at Schneeberg, he collected some of the beautiful coloured pieces of cobalt which were found there, tried them in his furnace; and finding that they melted, he mixed some cobalt with glass metal, and obtained fine blue glass. At first he prepared it only for the use of the potters; but in the course of time it was carried as an article of merchandise to Nuremberg, and thence to Holland. As painting on glass was then much cultivated in Holland, the artists there knew better how to appreciate this invention. Some Dutchmen therefore repaired to Nendeck, in order that they might learn the process used in preparing this new paint. By great promises they persuaded the inventor to remove to Magdeburg, where he also made glass from the cobalt of Schneeberg; but he again returned to his former residence, where he constructed a handmill to grind his glass, and afterwards erected one driven by water. At that period the colour was worth 7½ dollars per cwt. and in Holland from 50 to 60 florins. Eight colour mills of the same kind, for which roasted cobalt was procured in casks from Schneeberg, were soon constructed in Holland; and it appears that the Dutch must have been much better acquainted with the art of preparing, and particularly with that of grinding it, than the Saxons; for the Elector John George sent for two colour-makers from Holland, and gave a thousand florins towards the enabling them to improve the art. He was induced to make this advance chiefly by a remark of the people of Schneeberg, that the part of the cobalt which dropped down while it was roasting contained more colour than the roasted cobalt itself. In a little time more colour-mills were erected around Schneeberg. Hans Burghardt, a merchant and chamberlain of Schneeberg, built one, by which the eleven mills at Platten were much injured. Paul Nordhoff, a Frisian, a man of great ingenuity, who lived at the Zwittermill, made a great many experiments in order to improve the colour; by which he was reduced to so much poverty that he was at length forced to abandon that place, where he had been employed for ten years in the colour-manufactory. He retired to Annaberg, established there in 1649, by the alliance of a merchant at Leipzig, a colour manufactory, of which he was appointed the director; and by these means rendered the Annaberg cobalt of utility. The consumption of this article, however, must have decreased in the course of time; for in the year 1659, when there were mills of the same kind at more of the towns in the neighbourhood of mines, he had on hand above 8000 quintals." Thus far Lehmann."

Kofler says, that the Bohemian cobalt is not so good as that of Minaia, and that its colour is more like that of azurite. We trust, however, that the qualities of foreign cobalt shall soon be a matter of little importance to the British artist, as a rich mine of this mineral has lately been discovered near Penzance in Cornwall.