The true character of magnesia and its salts was not clearly understood until Joseph Black unravelled the complications of the alkaline salts by his historic investigation, which became one of the most noted epochs of chemistry by its incidental revelation of the combination of the caustic alkalies with what Black termed “fixed air,” subsequently named carbonic acid gas by Lavoisier in 1784. When Black was studying medicine at Edinburgh a lively controversy was in progress in medical circles on the mode of action of the lithontriptic medicines which had lately been introduced. Drs. Whytt and Aston, both university professors, were the leaders in this dispute. Whytt held that lime water made from oyster shells was more effective for dissolving calculi in the bladder than lime water prepared from ordinary calcareous stone. Alston insisted that the latter was preferable. Black was interested, and his experiments convinced him of the scientific importance of his discoveries. He postponed taking his degree for some time in order to be sure of his facts. His graduation thesis, which was dated June 11, 1754, was entitled “De humore acide cibis orto et magnesia alba.” His full treatise, “Experiments upon magnesia alba, quicklime, and some other alkaline substances,” was published in 1756. It had been previously believed that the process of calcining certain alkaline salts whereby caustic alkalies were produced was explained by the combination with 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. [Illustration: JOSEPH BLACK LECTURING (AFTER JOHN RAY) (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 magnesia and left the mercury free, thus proving that the earth was an oxide of a metal. In 1830 Bussy isolated the magnesium by heating in a glass tube some potassium covered with fragments of chloride of magnesium, and washing away the chloride of potassium formed. Magnesium in small globules was left in the tube. The metal is now prepared on an industrial scale either by electrolysis, or by fusing fluor-spar with sodium. At present the uses of magnesium and of its derivatives are infinitesimal in comparison with the vast quantities available in deposits, as in dolomite, and in the sea. NITRE