Magnesia.
The first use of carbonate of magnesia medicinally was in the form of a secret medicine which must have acquired much popularity in the beginning of the eighteenth century. It was prepared, says Bergmann, by a regular canon at Rome, sold under the title of the powder of the Count of Palma, and credited with almost universal virtues. The method of preparation was rigidly concealed, but it evidently attracted the attention of chemists and physicians, for it appears that in 1707 Valentini published a process by which a similar product could be obtained from the mother liquor of “nitre” (soda) by calcination. In 1709 Slevogt obtained a powder exactly resembling it by precipitating magnesia from a solution of the sulphate by potash. Lancisi reported on it in 1717, and in 1722 Hoffmann went near to explaining the distinction between the several earthy salts, which in his time were all regarded as calcareous.
Hoffmann’s process to obtain the powder was to add a solution of carbonate of potash to the mother liquor from which rough nitre had been obtained (solution of chloride of magnesium), and collect the precipitate. This being yielded by two clear solutions gave to the carbonate of magnesia precipitated the name of Miraculum Chemicum.
Magnesia was the name of a district in Thessaly, and of two cities in Asia Minor. The Greek “magnesia lithos,” magnesian stone, has been frequently applied to the lodestone, but this can hardly have been correct, as the magnesian stone was described as white and shining like silver. Liddell and Scott think talc was more probably the substance. The alchemists sometimes mention a magnesia, but the name seems to have been a very elastic one with them. The Historical English Dictionary quotes the following reference to the word from “Norton Ord. Alch.,” 1477:—“Another stone you must have ... a stone glittering with perspicuitie ... the price of an ounce conveniently is Twenty Shillings. Her name is Magnetia. Few people her knows.”
Paracelsus uses the term in the sense of an amalgam. He writes of the Magnesia of Gold. In Pomet’s “History of Drugs,” 1712, magnesia meant manganese. Hoffmann, 1722, first applied the name to oxide of magnesia, adapting it from the medical Latin term, magnes carneus, flesh magnet, because it adheres so strongly to the lips, the fancy being that it attracts the flesh as the lodestone attracts iron.
Hoffmann’s observations on magnesia and its salts, which were published in the first quarter of the eighteenth century, were very intelligent, and undoubtedly it was he who first distinguished magnesia from chalk. He says “A number of springs, among which I may mention Eger, Elster, Schwalbach, and Wilding, contain a neutral salt which has not yet received a name, and which is almost unknown. I have also found it in the waters of Hornhausen which owe to this salt their aperient and diuretic properties. Authors commonly call it nitre; but it has nothing in common with nitre. It is not inflammable, its crystallising form is entirely different, and it does not yield aqua fortis. It is a neutral salt similar to the arcanum duplicatum (sulphate of potash), bitter in taste, and producing on the tongue a sensation of cold.” He further states that the salt in question appears to proceed from the combination of sulphuric acid with a calcareous earth of alkaline nature. The combination “is effected in the 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 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.
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.