MAGNESIA, in chemistry, an earth, the properties of which were not fully known till Dr. Black, about the middle of the last century, investigated its na ture. In the pursuit, the Doctor was led to the important discovery of the carbo nic acid gas. Magnesia had, before his time, been frequently confounded with lime; he, however, by the most accurate experiments shelved that it possessed properties different from all the other earths. Although magnesia exists in great abundance in combination with other substances, it has never been found perfectly pure in nature. It is an ingre dient in many fossils ; and several of the salts, which it forms by combination with the acids, are found in mineral springs, and in the water of the ocean. From these combinations magnesia is obtained by different artificial processes. Mr. Murray mentions the sulphate of magne sia, or Epsom salt, as well adapted to this purpose. One part of this salt is to be dissolved in twenty of water, and the so lution filtered ; to this is added, while hot, a solution of pure potash or soda, as long as precipitation is produced. The alkali combines with the sulphuric acid,and the magnesia is separated : being insolu ble in waterAt falls down in white pow der : it is then washed in water till the fluid comes off tasteless. This earth ex ists under the form of a white spongy powder, soft to the touch, without smell, and having a slightly bitter taste. Its spe cific gravity is 2.3. It slightly changes vegetable colours to a green. Magnesia, when quite pure, is infusible, though ex posed to the most intense heat : even in the focus of the very powerful burning mirror, or in the heat excited by oxygen gas, it cannot be melted. When made into a paste with water it contracts like alumina, if exposed to a sudden heat. It is almost insoluble in water. There is no action between magnesia and hydro gen, or and very little between it and phosphorus. It combines readily with the acids, and with them forms neu tral salts. Of these the greater number are soluble or crystallizible, and have a bitter taste. It'does not enter into com bination with the fixed alkalies, but in combination with some of the other earths, it is fusible by means of a very strong heat. With lime, in certain pro portions, it forms a greenish yellow glass. It is much used in medicine as a gentle laxative, and as an absorbent to destroy acidity in the stomach. It is also employ ed to aid the solution of and gummy substances, as camphor and opium in water. We shall notice only a few of its combinations.
Magnesia combines with sulphur either in the dry or humid way, forming there by a sulphuret of magnesia. The solid sulphuret of magnesia decomposes rapid ly when exposed to the air.
Sulphate of magnesia is a compound of sulphuric acid and magnesia, and is found in sea water, and in many mineral springs. Those at Epsom once afforded a large part of what was used in com merce ; hence the name of Epsom salt. Now indeed it is commonly obtained from sea-water. The bittern water, or, as it is usually called, the mother water of common salt, that is, the water which remains after the crystallization, con sists chiefly of sulphate of magnesia. The constituent parts are, according to Bergman, Sulphuric acid 33 Magnesia 19 Water 48 100 • But Mr. Kirwan gives a different result.
In crystals. Dry.
Sulphuric acid 29.35 63.32 Magnesia 17.00 36.68 Water .. . . . 53.65 100 100 — — Sulphate of magnesia is formed by pass ing sulphurous acid through water, in which magnesia is diffused. At first it is in a state of powder, which is gradually dissolved, and by exposure to the air, it deposits crystals, and passes into sulphate of magnesia. It consists of Sulphurous acid Magnesia 16 Water 45 100 Carbonate of magnesia, or the magnesia alba, of the physicians, is a very ant compound. The manufacture of this on the large scale is thus conducted. Instead of the pure sulphate of magnesia, the bittern, or liquor remaining after the crystallization of sea salt is used, and the magnesia is precipitated by carbonate of potash. When properly prepared it is perfectly white, nearly or wholly tasteless, and very sparingly soluble in water. The magnesia of commerce is composed of nureroy. Kirwan.
Carbonic acid . . . 48 34 Magnesia 40 45 Water 12 21 100 100— — When common carbonate of magnesia is exposed to a moderate heat, it is de. composed : its carbonic acid disengaged. It loses about half its weight, and the magnesia remains nearly pure.
Under the magnesian genus of fossils are comprehended, not only those it which magnesia is the ingredient which is present in largest proportion, but those also in which, though in a smaller propor• tion, there exist the characters in some measure peculiar to this genus. These are softness, unctuosity, and being in ge. neral destitute of hardness, lustre, and transparency, which are conspicuous in many of those which belong to the sili cious and argillaceous genera. Magne sian fossils have usually a green colour, more or less deep.