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in the whiting, and a part of the whiting taken up in itsroom. The liquor is then poured off, more of the coppersolution added, and this repeated, till the matter appearsof the due colour, after which it is spread on large piecesof chalk, and laid in the fun to dry. Boyle observes thatthe process often miscarried, and that heating the liquorbefore it is poured on the whiting, has been found to con-tribute to its success. It is still however, as I am inform-ed, very apt to fail, in the hands of the most skilful work-men, the preparation, instead of a fine blue, turning outof a dirty green.

From the liquor poured off" in making verditer, consist-ing of the nitrous acid saturated with the whiting, greatpart of the acid is recovered, by evaporating the waterypart, and adding the remaining thick matter in thedistillation of the next quantity of aqua fortis. Theacid may be extracted also either from the solution ofcopper or silver, and the metals recovered, the silver byfusion without any addition, the copper by the addition ofinsiammable matter. The following process is given forthis purpose in the French memoirs for the year 1728, asthe communication of an experienced artist.

The copper solution is put into a copper vessel placed ina furnace, and evaporated to about half: the vessel isthen filled up with more of the liquor, and the evapora-tion continued till the fumes begin to smell of aqua fortis.The acid, being already satiated with copper, does notact on the vessel, or so little, that du Fay fays he hasteen one vessel bear almost constant AVork for near a year :the vessel should be raised out of one plate, not formedof pieces, for if it is rivetted or soldered, the liquor willsoon make its way through the junctures, as I have oftenobserved in this and other solutions of the fame kind. Ondecanting off the liquor, a portion of silver is found at

the