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
among the ancient Greeks and Romans generally meant carbonate of soda, sometimes carbonate of potash. The Arab chemists, however, clearly described nitrate of potash. In the works attributed to Geber and Marcus Græcus, especially, its characters are represented. Raymond Lully, in the thirteenth century, mentions sal nitri, and evidently alludes to saltpetre, and Roger Bacon always meant nitrate of potash when he wrote of nitre. It was not, however, until the seventeenth century that the term acquired the definite meaning which we attach to it.
At the beginning of that century there was much discussion as to the formation of nitre, as it had been held that the acid which combined with the alkali was ready formed in the atmosphere. Glauber was the first to argue that vegetables formed saltpetre from the soil. Stahl taught that the acid constituent of nitre was vitriolic acid combined with phlogiston emanating from putrefying vegetable matter.
After gunpowder had become a prime necessity of life, saltpetre bounded upwards in the estimation of kings and statesmen. In France in 1540 an Edict was issued commissioning officials called “salpêtriers” in all districts who were authorised to seek for saltpetre in cellars, stables, dovecotes, and other places where it was formed naturally. No one was permitted to pull down a building of any sort without first giving due notice to the salpêtriers. The “Salpêtrière” Asylum in Paris recalls one of the national factories of nitre. During the French Revolution citizens were “invited” to lixiviate the soil and ceilings of their cellars, stables, etc., and to supply the Republic with saltpetre for gunpowder. The Government paid 24 sous, 1s., a pound for the nitre thus procured, though, as this was no doubt paid in assignats, it was cheap enough. It was estimated that 16,000,000 lbs. a year were thus provided.
Petroleum.
Under the name of naphtha and other designations petroleum has been known and used from the earliest times. The Persians were the first, as far as is known, to employ it for lighting, and also for cooking. They likewise made use of it as a liniment for rheumatism. So in this country, a kind of petroleum was sold as a liniment under the name of British oil; and in America, long before the great oil industry had been thought of, petroleum was popular as a liniment for rheumatism under the name of Seneca Oil.
Asphalt, or Bitumen of Judæa, was used by the Egyptians for embalming. Probably they reduced its solidity by naphtha. Naphtha was employed by Medea to render the robe which she presented to her rival Glauca inflammable, and this legend is given to account for the name of Oil of Medea, by which petroleum was anciently known. It was no doubt the principal ingredient in the Greek Fire of the middle ages.
Petroleum has been called by many other names. Oil of Peter or Petre was a common one, meaning, like petroleum, simply rock oil. Myrepsus, in the thirteenth century, refers to it as Allicola. The monks called it sometimes oil of St. Barbarus, and oil of St. Catherine.
Dioscorides said naphtha was useful as an application in dimness of sight. Two centuries ago it was occasionally given in doses of a few drops for worms, and was frequently applied in toothache. Petroleum Barbadense, Barbadoes tar, had some reputation in pectoral complaints in the seventeenth and eighteenth centuries, and was admitted into the P.L. as the menstruum for sulphur in the balsamum sulphuris Barbadense.