We may indeed regard the first half of the eighteenth century as the most flourishing period of the glass industry in Bohemia and Silesia. At the end of that time the Bohemian town of Haida—at the present day the centre of more than one branch of the glass manufacture—rose to importance, thanks to the fostering care of Count Kinsky. But the industrial and commercial element now came more and more to prevail. Enterprising manufacturers like Franz Weidlich of Steinschönau exported to Spain and Portugal, and others supplied the Eastern market as far as the Indies with glass summarily decorated with ‘little wreaths cut with a small copper wheel with the aid of emery.’ This Eastern trade passed through Vienna, and meeting with every encouragement from Maria Theresa and from Joseph II., soon undermined the time-honoured monopoly of the Venetians in the Levant and in Persia. With the Western market it was otherwise. The German glass had to reach the Peninsula by way of the Flemish ports, Antwerp and Ostend. What we have known as the Spanish Netherlands were now in Austrian hands, and the new government was eager to promote the local industries. The energetic firm of the Bonhommes (see p. 242), long established at Liége and other neighbouring towns, competed successfully first with the German and then with the English glass-makers, just as formerly they had competed with the Italians, adopting in turn the methods of each.[216] But in addition to cutting or engraving with a wheel and scratching with a diamond, there is a third method by which the surface of glass may be removed. This is by means of hydrofluoric acid, the only re-agent by which glass is rapidly attacked. The discovery of this acid is usually ascribed to Scheele, the Swedish chemist (born 1742), and a date as late as 1771 is given to the discovery. But there is no doubt that the special virtues of the fumes that are given off when fluor-spar is heated in sulphuric acid were known before this time.[217] We have seen how Sandrart, writing before 1675, mentions that his contemporary Henry Schwanhart engraved glass by means of a ‘_corrosiv_,’ and the statement is repeated with picturesque details by Doppelmayr. By covering part of the glass with a varnish and exposing the rest to these acid fumes, Schwanhart produced a smooth pattern on a dead ground. Certain calligraphic inscriptions on plates of glass, preserved in German museums,[218] were probably engraved in this way, but at the time the process did not come into general use. At a much later period hydrofluoric acid has been largely employed in England and elsewhere for engraving on glass. Still more recently this method has given way to the sand-blast. These are both, however, purely industrial processes that have little to do with art.