bosom of the earth." In another of his works Hoffmann distinguishes the magnesian salt from one of lime, showing particularly that the latter was but slightly soluble and had scarcely any taste. Crabs' eyes and egg shells he notes combine with sulphuric acid and form salts with no taste. The sulphate of this earth (Epsom salt) he found had a strong bitter taste.
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
