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 the salt of an acrid principle derived from the fire. Now it was shown that something was lost in the process; that the calcined alkali weighed less than the salt experimented with. The something expelled Black proved was an air, and an air different from that of the atmosphere, which was generally supposed to be the one air of the universe. He identified it with the "gas sylvestre" of Van Helmont, and named it "fixed air." Magnesia alba first appeared in the London Pharmacopœia of 1787 under that name.

(From a print in the British Museum.)

The oxide of magnesia was believed to be an elementary substance until Sir Humphry Davy separated the metal from the earth by his electrolytic method in the presence of mercury. By this means he obtained an amalgam, and by oxidising this he reproduced