532

CARBONATES AND BORATE OF MAGNESIUM.

of sodium, sealing the tube, and then allowing the two solutions to mix. Crystals of carbonate of magnesium are deposited slowly.

Magnesia alba, the common white magnesia of the shops, is made by precipitating a boiling solution of the sulphate of magnesium by a hot solution of carbonate of sodium. The sulphate of magnesium is allowed to remain slightly in excess, otherwise the precipitate contains a little carbonate of sodium. It is deposited as a white, light, bulky powder, which is composed of hydrate of magnesia (MgH,,) combined with a quantity of hydrated carbonate (MgCO,,H,→), the amount of which may vary from 2 to 4 atoms to 1 atom of the hydrate of magnesia; it is very sparingly soluble in water. A quantity of carbonic acid is expelled from the mixture during the preparation of this compound.

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Dolomite, when its structure is crystalline, usually consists of carbonate of magnesium and carbonate of calcium in the proportion of I atom of each (Mga 2 €Ð ̧), though sometimes the proportion of carbonate of calcium considerably exceeds 1 atom. A solution of sulphate of calcium decomposes carbonate of magnesium at ordinary temperatures, and thus spring water originally charged with sulphate of calcium may, by filtration through a bed of dolomite, become impregnated with sulphate of magne

sium.

A very pure carbonate of magnesium is manufactured from dolomite by a process introduced by Pattinson. In this operation the mineral is finely ground and sifted, and exposed to a low red heat for 2 or 3 hours, by which the carbonate of magnesium is decomposed. It is then introduced into a strong iron cylinder lined with lead, where it is mixed with water, and carbonic anhydride is forced in under a pressure of 2 or 3 atmospheres, till it ceases to be absorbed; the carbonate of magnesium becomes dissolved as acid carbonate, leaving the carbonate of calcium: the clear liquid, when boiled, deposits the carbonate of magnesium, which is drained, and dried in a stove at a low temperature.

By mixing a solution of nitrate of magnesium with an excess of a saturated solution of the acid carbonate of potassium, and allowing the solution to stand for some days, a remarkable double salt is deposited in regular crystals, composed of (MgCO„KHEO ̧ 4 H2O), but which is decomposed by redissolving it in water. The corresponding salt of sodium is more stable.

A native borate of magnesium [3 (Mg 2 B→), B,,, or 3 MgO, 4 BO], named boracite, is found crystallized in cubes; it is rendered electric by heat.

(690) SILICATES OF MAGNESIUM.-Silica and magnesia may

SILICATES AND PHOSPHATES OF MAGNESIUM.

533

be artificially combined in many proportions. A large number of minerals are formed, either wholly or partially, of the silicates of magnesium. Olivine or chrysolite [2 (MgFe)O, Sie,] is a crystallized orthosilicate, usually of a green colour, obtained from basaltic and volcanic rocks; it frequently accompanies masses of meteoric iron. Talc is a very soft slaty mineral, which has a formula [2 (MgO, SiO,). 2 MgO, 3 Sie,]. Steatite, French Chalk, or Soapstone, is (MgO, Si✪, . 2 MgO, 3 Sie). Picrosmine is a hydrated metasilicate, (2 MgSiÐ ̧. H2O). Meerschaum is another hydrated silicate, of which the formula is (2 MgO, 3 Si✪,.4 H2O). Serpentine (2 [(MgFe)✪, Si✪2] . MgO, 2 H2O) is another hydrated silicate of magnesium, in which a portion of the magnesia is often displaced by protoxide of iron. Serpentine frequently occurs in compact masses, which take a high polish, and from the beauty of its variegated colours, it is often employed for ornamental pur

poses.

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It is readily attacked by acids, and occurs in sufficient abundance to be employed as a source of the salts of magnesium.

The double silicates of magnesium are still more numerous. Augite or pyroxene is one of these: it is a crystalline mineral, often found in basalt and lava, and is a silicate of calcium and magnesium, portions of which metals are often displaced by iron and manganese [(CaMgFeMn)O, Sie,]. Hornblende or amphibole is a silicate and aluminate of magnesium, calcium, and iron, with a variable proportion of the fluorides of calcium and potassium, (3 [(MgCaFe), Sie,]. 2 (MgCaFeMn) →, 3 Sie, . x (K¤a) F2). It occurs sometimes in dark green or black crystals, at other times massive, disseminated through many rocks, such as syenite and porphyry, and frequently in basalt and lava. Asbestos and amianthus commonly consist of a fibrous variety of amphibole.

(691) Triphosphate of magnesium and hydrogen (H,Mgʻ, PO 7 H2, or HO, 2 MgO, PO . 14 Aq) is an efflorescent, sparingly soluble salt, which crystallizes in fine tufts of six-sided acicular prisms, when a solution of a magnesian salt is mixed with the solution of the common phosphate of sodium.

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TRIPHOSPHATE OF MAGNESIUM AND AMMONIUM (Mg"H ̧N, PÐ ̧ 6 H2, or 2 MgO, H¿NO, PO.12 Aq=137+108), or triple phosphate, as it was formerly called, is a more important compound than the foregoing one. "It is prepared by mixing phosphate of sodium, mingled with chloride of ammonium, with a salt of magnesium; by agitation this compound is deposited in minute crystalline grains:" it furnishes a very delicate test of the presence of magnesium it is insoluble in water containing free ammonia or muriate of ammonia, but it is taken up in appreciable quantities

:

534

CHARACTERS OF THE SALTS OF MAGNESIUM.

by pure water. It is frequently met with as a constituent of urinary calculi, both in man and in the lower animals. Phosphate of magnesium and ammonium is readily soluble in acids; ammonia precipitates it from such solutions unchanged; when ignited, it parts with its water and ammonia, and glows like alumina and zirconia as it suddenly contracts its bulk. The ignited residue contains 35'7 per cent. of MgO, and 643 of P. It is frequently employed for the determination of the amount of magnesia in the course of analysis.

(692) CHARACTERS OF THE SALTS OF MAGNESIUM.-The salts of magnesium are colourless and have a bitter taste. Many of the magnesian minerals possess a silky lustre, and feel unctuous to the touch. Compounds of magnesium may be recognized before the blowpipe, by the pink tinge which they acquire when heated with nitrate of cobalt.

In solution they give no precipitate with the acid carbonates of the alkali-metals till boiled; but a white basic carbonate of magnesium when mixed with the normal carbonate of potassium or sodium, unless a salt of ammonium be present, which interferes with the precipitation. Phosphate of ammonium gives with them a white crystalline granular precipitate of double phosphate of magnesium and ammonium, which is easily soluble in acids. Oxalate of ammonium, mixed with sal ammoniac, gives no precipitate with the magnesian salts, neither do the soluble sulphates. The fixed alkalies throw down a white gelatinous hydrate of the earth, which is insoluble in excess of the precipitant. Lime-water produces a similar precipitate. Ammonia produces but a very incomplete precipitation of magnesia from its solutions; the gelatinous precipitate which it occasions becomes redissolved on the addition of a solution of chloride of ammonium, and a double salt of magnesium and ammonium is formed.

(693) CHARACTERS OF THE METALS OF THE FIRST GROUP (Metals of the Alkalies).-The salts of these metals when in solution are distinguished by the following characters:-1. By the absence of any precipitate on the addition of a solution of carbonate of potassium, or of sodium: in the case of lithium, if the salt exceed two per cent. of the solution, a precipitate of carbonate of lithium is liable to occur. 2. By the absence of any precipitate when sulphuretted hydrogen or hydrosulphate of ammonium is added to the solution. 3. By the occurrence of a precipitate with chloride of platinum in the case of salts of ammonium or of

ESTIMATION OF SALTS OF POTASSIUM AND SODIUM.

535

potassium ;* and by the formation of prismatic crystals of the double chloride of sodium and platinum when evaporated in the presence of salts of sodium.

(694) Estimation of Potassium and Sodium.-If the relative proportions of the potassium and sodium be not required, their combined weight is usually ascertained in the form of sulphates. They may in most cases be readily obtained in this condition by treating the solution with sulphuric acid, evaporating to dryness, and fusing the mass in a platinum crucible in which a fragment of carbonate of ammonium is suspended. The excess of sulphuric acid is thus readily dissipated, and the amount of the acid combined with the potassium and sodium, is determined by precipitation with chloride of barium. When ammonium salts are present with those of potassium and sodium, its amount may be determined by distilling off the ammonia in the manner already described (626).

In order to determine the quantity of potassium and sodium in a mixture of the salts of the two metals, they should be converted into the state of chlorides, and heated to low redness to expel moisture and all ammoniacal salts, allowed to cool, and weighed; a certain proportion of these mixed chlorides (ten or twelve grains will suffice) is then mixed with an excess of the double chloride of platinum and sodium, evaporated to dryness over a steam bath, and the excess of the chloride of platinum and sodium removed by washing with alcohol of specific gravity o.860. The crystalline residue is collected on a filter and weighed. One hundred parts contain 30'53 of chloride of potassium, and correspond to 15'98 of potassium, or to 19:26 of anhydrous potash. The quantity of chloride of sodium is obtained by deducting the weight of the chloride of potassium from that of the mixed chlorides employed.

(695) The conversion of the alkaline metals into the condition of chlorides, previous to precipitation by the perchloride of platinum, if they are not already in that form, is rather troublesome. They may be first converted into sulphates by evaporating the solution with a slight excess of sulphuric acid, and igniting the residue; the sulphates thus obtained are to be dissolved in water and mixed with a solution of chloride of barium in slight excess. The sulphuric acid is thus precipitated as sulphate of barium, and the alkalies are converted into chlorides; but the excess of barium in the liquid must still be got rid of. A mixture of caustic ammonia

* Rubidium and cosium would also be found in this precipitate (607, 608), if either of these metals were present.

536

CHARACTERS OF THE METALS OF THE SECOND GROUP.

and of the sesquicarbonate of ammonium is therefore added to the solution after it has been filtered from the sulphate of barium. The excess of barium is thus thrown down as carbonate, and the carbonate of barium may then be removed by filtration. Once more the solution is to be evaporated to dryness in a platinum dish, and the residue gently ignited to expel the ammoniacal salts. The remaining mass now contains nothing but the mixed chlorides of sodium and potassium.

(696) CHARACTERS OF THE METALS OF THE SECOND GROUP (Metals of the Alkaline Earths, including Magnesium) :-1. The salts of these metals when in solution give a white precipitate on the addition of solution of carbonate of sodium or of potassium. —2. They yield no precipitate with hydrosulphate of ammonium nor with sulphuretted hydrogen.-3. Lime-water occasions no precipitate, except in cases in which the magnesian salts are present, or in which the solution contains free carbonic acid.

(697) Separation of the Alkaline Earths from the Alkalies.Supposing a solution to contain salts of the alkalies and of the alkaline earths, the quantities of each base may be determined in the following manner :-An excess of a mixture of ammonia and sesquicarbonate of ammonium is added to the solution; the ammonium thus combines with the radicle of the acid previously in union with the earths, whilst the carbonic acid converts the earths into carbonates; the liquid is filtered from the precipitate, then evaporated to dryness, and heated to expel the ammoniacal salts. The dry residue is then washed with water, which dissolves out the salts of the alkali metals: and from this liquid the proportions of potassium and sodium can be ascertained in the manner already described (694). A little magnesia is apt to accompany the salts of the alkali-metals: its presence may be detected and its quantity determined by the addition of lime-water to the solution; hydrate of magnesia is precipitated, and may be collected, weighed, and added to the amount obtained from the portion which was precipitated as carbonate. The precipitation must be effected in a stoppered bottle, to exclude the carbonic acid of the atmosphere, which would precipitate a portion of lime with the magnesia. The excess of lime may be got rid of by the addition of oxalic acid, which occasions a precipitate of oxalate of calcium that can be separated on a filter, but need not be weighed. The earthy carbonates must now be dealt with in the following manner :

(698) Separation of Barium, Strontium, Calcium, and Magnesium from each other.-The alkalies having been separated in the manner just described, the carbonates of the metals which in