to the Senſes, that Copper was capable of giving the deepeſt and fineſt Blue imaginable) was made, was a volatile alkaline Spirit: And where Nature could find, in the Bowels of the Earth, any Thing analogous to a volatile urinous Alkali, produced by Chemiſtry, was a Queſtion not eaſily anſwered. The particular Salt of the mineral Waters ſeems to approach, indeed, ſomething to a Menſtruum of this Kind; and Dr. Hoffman has proved, that it is at leaſt much fitter to be claſſed with the Alkalies than with the Acids. But the Syſtem of the Colours of the blue Gems being from Copper, muſt ſtand upon a very precarious Baſis, if there could be found no other Menſtruum than one we are ſo very uncertain about, to ſtrike their Colour from that Metal.
Copper, however, is, in Truth, perhaps the fartheſt of all the Metals from being ſubject only to the Power of one appropriated Menſtruum; and a Courſe of Experiments on it, have now ſhewn me, that we need not have Recourſe to ſo uncertain a mineral Subſtance as this latent Alkali, for producing a Blue from it; but that Menſtruums of another Kind, even Acids, and thoſe the very Acids, whoſe Principles are the commoneſt of all others in the Earth, can afford us the ſame Colour; and are every where to be found in great Abundance. Gold is ſoluble only in Agua regia; for all the other Menſtruums that are talked of for it, have a genuine Sea-ſalt for their Baſis, and are therefore only ſo many
