Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/236

Rh 218 MAGNESIUM parent crystals, MgClo.6H 2 O, very hygroscopic, very readily soluble in water, and (like the anhydrous salt) soluble also in alcohol. When dehydrated by heat it loses acid (HC1), so that the residue is largely contaminated with oxychlorides, MgCL.xMgO. Certain of these are industrially important as cements, used by dentists and otherwise. A concentrated solution of the chloride, made into a paste with the ignited (CO 2 -free) oxide, in a few hours hardens into a stone susceptible of polishing (Sorel). To prepare the anhydrous chloride, the crystallized or dissolved hydrated salt is evaporated with sal-ammonia to dry ness, and the residue (a stable double salt) is ignited in platinum. Pure chloride, MgClo, remains as a clear liquid, solidifying on cooling into a compact aggregate of flexible crystal -leaves of a mother- of-pearl lustre. From the chloride the metal can be pro duced, either by electrolysing the fused substance, or (more conveniently) by Caron and Deville s method : 600 grammes of the chloride, 480 grammes of powdered fluor-spar, and 230 grammes of finely cut-up sodium are mixed, and thrown, small portions at a time, into a red-hot crucible, which is then well covered, When the reaction, which is violent, is over, the contents are well stirred with an iron rod to cause the small metal-beads to unite into bigger lumps, which, after cooling, are picked from the broken-up mass. Sonstadt (who, along with Mellor, was the first to prepare magnesium industrially) substitutes for plain MgCl 2 the double chloride of potassium and magnesium, obtainable synthetically from the hydrated components without the use of sal-ammoniac, or simply by ignition of pure carnal- lite. To purify the crude metal Sonstadt and Mellor distil it &quot; per descensum &quot; out of an iron crucible provided with a pipe piercing the bottom and reaching up to almost the lid ; an atmosphere of coal-gas is established to prevent oxidation of the metal in this operation, as also in the subsequent casting of the ingot. The metal magnesium has the colour of silver, and re mains unchanged in dry air ; in ordinary air it tarnishes a little more readily than zinc does. It is malleable and ductile, but has little tenacity. The specific gravity is 1 75 ; thus it is considerably lighter than even aluminium, whose specific gravity is 2 6. It fuses and distils at about the same temperatures as zinc. It is generally sold in the form of thin ribbon, being used for the easy production of highly intense light. The ribbon kindles readily in a candle flame, and then continues burning most intensely by itself, the solid oxide produced radiating out abundant light. A wire 3 millimetre (= 012 inch) thick gives out the light of 74 stearine candles weighing 100 grammes (-f ft&amp;gt; avoirdupois) a piece (Bunsen and Roscoe). It used to be employed for photographing at night, but is now super seded by the electric light. Magnesium has the excep tional property of combining (at a bright red heat) directly with nitrogen gas into Mg 3 N* (Briegleb and Geuther), a greenish-yellow powder, which, when heated in steam, yields magnesia and ammonia, NH 3. Of magnesium salts the most important is the sulphate, EPSOM SALT (q.v.). This salt serves as a raw material for the preparation of two or three medicinally important substances, especially magnesia alba. When epsom salt solution is precipitated by carbonate of soda, the salt MgOC0 2, first formed, loses carbonic acid and takes up water, forming a precipitate of the approximate composi tion 3MgCOo.Mg(pH) 2 + 2 to 3 times H 2 O. When pre pared by cold precipitation it forms a very light, when in the heat a somewhat denser, white powder (magnesia alba levis and ponderosa). This substance holds an important position in the history of chemistry, having served, in the hands of Black, to prove finally the individual existence of magnesia as something&quot; distinct from lime, and helped him in establishing the true relation between &quot;caustic&quot; and &quot; mild &quot; alkalies. Before his time the &quot; causticity &quot; was supposed to go into the &quot;mild&quot; substance from the fire; Black showed that it is owing to the loss of a ponderable substance, which he called &quot;fixed air,&quot; that is, carbonic acid. It was in this memorable research that for the first time in chemistry the balance was used for the precise determination of quantitative relations. Magnesia, the oxide MgO, is produced by the gentle ignition of magnesia alba. It is a white powder, absolutely infusible and non-volatile, and not reducible by charcoa^ and is used medicinally. The gently ignited oxide com bines very slowly with water into the practically insoluble hydrate MgO.H 2 O. Magnesia when boiled with sugar- water dissolves into a solution of saccharate, and in this form is sometimes administered medicinally in lieu of plain magnesia. Magnesia alba (also the normal carbonate) dissolves rather largely in carbonic acid water. Accord ing to R. Wagner, one part of MgOCO 2 dissolves in 760 parts of water saturated by carbonic acid under 1 atmo sphere pressure. Under 6 atmospheres pressure it requires only 76 parts of carbonic acid water for its solution. Dinneford s &quot; fluid magnesia &quot; is a solution of such bicar bonate of magnesia. The bicarbonate solution, when allowed to stand in air, deposits crystals of hydrated nor mal carbonate, MgC0 3 + 3 or 5H 2 0. Magnesia preparations play a great part in therapeutics. The oxide and basic carbonate (also the dissolved forms of saccharate and bicarbonate) are used in small doses as anti-acids, in larger ones as very mild purgatives, for children more especially. For the latter purpose, however, the sulphate is generally preferred as acting far more ener getically. The nauseous bitter taste of the salt can be concealed, to some extent, by acidification of its solution with dilute sulphuric acid. Citrate of magnesia, being exceptionally free of the &quot;JBittererde&quot; taste, was introduced some thirty years ago by the French as a pleasant substi tute for Epsom salt, and it has since come much into fashion everywhere, although, weight for weight, it is far less efficient than the sulphate. The preparation of dry soluble citrate offering difficulties, the French originally dispensed it exclusively in the dissolved form of &quot; Limon- ade au citrate de magne sie,&quot; a flavoured, decidedly acid solution of the salt, rendered effervescent by addition of some bicarbonate of soda immediately before corking up. In England it is generally preferred to offer the dry in gredients of the &quot;limonade&quot; in the form of &quot;granular effervescent citrate of magnesia.&quot; Magnesia alba is pounded up with an excess of citric acid crystals and a few drops of water to produce a paste of amorphous acid citrate, which is dried at a temperature below 30 C. At higher temperatures the salt would pass into a crystalline, insoluble, and consequently therapeutically valueless modi fication. The citrate is mixed with bicarbonate of soda, citric acid, and sugar, made into a &quot; dough &quot; with alcohol, granulated, and dried. The granules, when thrown into water, dissolve with effervescence. We must not omit to state here that much of what is sold under the name is a mere concoction in which Epsom salt figures as &quot;citrate.&quot; To test a solution for magnesia, remove whatever can be precipitated by means of sulphuretted hydrogen or sul phide of ammonium ; then eliminate lime, baryta, and strontia by precipitation with carbonate of ammonia in the presence of sal ammoniac. The magnesia remains dis solved, and can be precipitated (and detected) by addition of phosphate of ammonia (or soda) and free ammonia ; the salt FO 4 MgNH 4 .GH O gradually separates out as a crystal line precipitate. This method of course fails when the magnesia is present from the first as phosphate ; but we cannot here enter into a consideration of this or any other exceptionally difficult case. (w. D.)