In this state of uncertainty was the public mind till the year 1725, when it was thought that a recipe, older than that previously mentioned, might possibly be discovered. Accordingly, in that year, in the month of January, the following is said to have been transmitted to the publisher of the “Œkonomische Encyclopedie,” by Dr. John George Weygand, of Goldingen, which is reported to have belonged to Dr. Matthew Pauli, of Dresden, then deceased; with which the last named gentleman had etched, on glass, arms, landscapes, and figures of various kinds. We find, that in it, very strong acid of nitre was used, which entirely disengages the acid of sparry fluor, though the vitriolic acid has been commonly employed, and figures thus produced will appear as if raised above the plane of the glass. This sparry fluor is found abundantly in Derbyshire, as well as in the mines of Germany. Theophrastus is the first who notices the effect of sparry fluor, by observing that there are certain stones which, when added to silver, copper, and iron ores, become fluid. It appears that Cronstedt was the first systematic writer who gave it a name. When _spiritus nitri per distillationem_ has passed into the recipient, ply it with a strong fire, and when well dephlegmated, pour it, (as it corrodes ordinary glass,) into a Waldenburg flask; then throw into it a pulverised green Bohemian emerald, otherwise called _hesphorus_, (which, when reduced to powder and heated, emits in the dark a green light,) and place it in warm sand for twenty-four hours. Take a piece of glass, well cleaned, and freed from all grease by means of a ley; put a border of wax round it, about an inch in height, and cover it equally all over with the above acid. The longer you let it stand, so much the better; and at the end of some time the glass will be corroded, and the figures which have been traced out with sulphur and oil varnish will appear as if raised above the plane of the glass. HYDROMETERS. The Hydrometer is an instrument for admeasuring liquids; by it the strength or specific gravity of different fluids is discovered, by the depth to which it sinks in them. It has been chiefly used for discovering the contents of different salt waters, without analysis, and is now almost entirely used by persons connected with the spirit-trade, to ascertain the different degrees of strength, and what alloy they will bear; hence its utility to the manufacturer and the excise-officer is apparent. The laws respecting the comparative weight of different fluids, as well as of solid bodies immersed in them, was first discovered by that great geometrician Archimedes. It may be far from improbable that Archimedes constructed that instrument himself; and if it should appear that he did, it must have happened two hundred and twelve years before the Christian era.