The nature of bleaching

The best supported view is

residue. The solution is alkaline. powder is still a subject of dispute.

that it is a compound of Ca Cl + CaO CIO, that is, of chloride of calcium and of hypochlorite of lime with some caustic lime.

Testing Bleaching- Powder. One of the most convenient methods of testing bleaching-powder is that of Dalton, depending upon the conversion of the protosulphate of iron into the persulphate, at the expense of the oxygen of water decomposed by the chlorine of the bleaching-powder. For this purpose, we take 4 atoms of good crystals of protosulphate of iron (Fe OSO, 7 HO), or 69.5 grs. previously well dried and pulverized, and dissolve them in two ounces of distilled water, with the addition of a few drops of SO, placed in a stoppered bottle. We then weigh out 50 grains of the specimen of bleachingpowder to be tried, and add it, in small portions at a time, to the copperas solution, shaking the mixture between each addition, and gently applying heat. As every atom of protoxide of iron requires half an atom of oxygen to convert it into the peroxide, this is equivalent to half an atom of chlorine in the bleaching-powder, or 2.25 grs. of Cl. When 4 atoms of the protoxide have been peroxidized, it is equivalent to 2 atoms of chlorine in the powder, or 9 grains by weight of Cl. The peroxidation of the solution is judged of by frequently taking out a drop with the end of a rod, placing it on a piece of white porcelain, or spotting it on blotting paper, and then adding a drop of a solution of red prussiate of potash. As soon as a blue precipitate of Prussian blue ceases to be formed, we infer that the whole of the protoxide, in the 69.5 grs. of sulphate of iron has been peroxidized. We then weigh the residue of the bleaching-powder. Suppose it amounts to 15 grs., then this amount, deducted from 50 grs., shows that 35 grs. of the bleaching-powder contained 9 grs. of chlorine. We have then the proportion, 35: 100 9 257 the quantity of chlorine contained in 100 grains of the sample.

Carbonate of lime, calcareous spar, CaO CO, 6.25. Chalk, limestone, marble, marl, oolite, travertine. Primary form an obtuse rhomboid 680 modifications of crystalline form have been described - - all of which are referable to rhomboids, prisms, and pyramids. When the angle of the summit is replaced by three or six planes, resting on the primary planes or edges, so as to obliterate the primary faces of the crystal, 3-sided or 6-sided pyramids are produced. Lustre splendent and vitreous, hardness 3. Specific gravity 2.721. Arragonite is a dimorphous form of calcareous spar. Crystal right rhomboidal prism. Hardness 3.75, specific gravity 2.931. It has essentially the same composition as calcareous spar; but sometimes, however, contains carbonate of strontian in minute quantities.

Carbonate of lime may be obtained pure by precipitating chloride of calcium with carbonate of ammonia, washing and burning. It may be procured in a light form by washing it, and allowing it to dry spontaneously on the filter; 1 part of carbonate is soluble in 10,601 parts of cold and 8834 of boiling water, and in 65,246 parts of water containing alkaline salts in solution (Fresenius). It is soluble in water impregnated with CO2; when heated to a high temperature, it loses its CO. Under pressure, when ignited, it melts and assumes the appearance of granular limestone (Hall), some CO, being evolved. Carbonate of lime

enters very extensively into the constitution of the globe under the forms of primary limestone (Tyree marble, Parian marble), mountain or transition limestone under the coal, coal limestone and chalk.

Sulphate of lime, CaO, SO, 2HO gypsum, plaster of Paris, selenite, alabaster. Crystal, right oblique prism. Hardness 2, specific gravity 2.31. This mineral occurs principally in the new red sandstone formation, and also in the tertiary formations round Paris. When heated to 225° it loses 1 atoms of water, the rest at 400°, and on being pounded and mixed with water, it again combines with water, and thus sets, and is useful in forming casts. This mineral occurs also destitute of water, crystallized in rectangular prisms. Specific gravity 2·899. Hardness 2.75

to 3. Phosphate of lime, Bone earth, Guano earth, 8CaO 3PO, or 2(3CaO PO5) + HO 2CaO PO+зHO, obtained from calcined bones or guano, by dissolving in HCl, and precipitating the solution by NH. Is an excellent manure.

Estimation. Lime is usually separated from its solution either in a neutral or alkaline state by oxalate of ammonia; the ignited precipitate is CaOCO2, every 61 parts of which contain 3 of lime.

MAGNESIUM. Mg 1.5.

Magnesium was discovered by Davy, in 1808, by exposing moistened magnesia to the action of galvanism in contact with mercury. Bussy showed, in 1828, that it might be obtained by passing the vapour of potassium over the chloride of magnesium, or by heating them in a dry state in a tube. White solid metal like silver, heavier than water, melts at no very high temperature, and volatilizes at a white heat.

Magnesia, oxide of magnesium, calcined magnesia, MgO 2.5. About 1700, magnesia alba was exposed for sale by a Roman canon. In 1707, Valentini found it in the lixivium from nitre, and in 1709, Slevogt observed it in the mother ley of nitre. In 1755, Dr. Black distinguished it from lime. Magnesia is found united with acids as a sulphate in Epsom springs, as a carbonate in magnesian limestone, as a silicate in serpentine, nephrite, meerschaum, hornblende, asbestus, talc. It is most conveniently

prepared by igniting the carbonate of magnesia, which is obtained in a natural state in Madras; but in this country is usually obtained from bittern or sea water, from which as much as possible of the common salt has been crystallized out. The bitter mother liquor contains muriate of magnesia, which is to be precipitated by means of an alkaline carbonate, and washed. When this carbonate is calcined, the result is pure magnesia. It requires a lower temperature to remove the CO, than lime. Magnesia is a soft white powder, with little taste, and no smell, specific gravity 2.3 (Kirwan), 3-2 (Karsten), 307 (Richter), converts vegetable blues to green, When calcined magnesia is pure, it dissolves entirely without effervescence in acids, and when a quantity of sal ammoniac is added to the solution, the magnesia is not precipitated by caustic ammonia. It is permanent at a red heat, but volatilizes at very elevated temperatures (Liebig). It is used in this form as a mild aperient and corrector of acidity. Hydrate of magnesia, MgOHO 3.625. 1 part of magnesia dissolves in from 53 to 56,000 parts of hot and cold water, being equally soluble in both (Fresenius). The hydrate may be prepared by dissolving the sulphate of magnesia in water, adding caustic soda or potash; throwing the precipitate on a filter, washing it and drying it in a gentle heat. Hydrate of magnesia occurs in nature in the form of pearly scales, or crystallized in low 6-sided prisms, and at Hoboken, in New Jersey, in needles. Specific gravity 2.35; contains 68 per cent. of magnesia and 31 of water. It dissolves in acids without effervescence.

Salts of Magnesia. - Magnesia neutralises the acids, and forms salts, most of which are soluble in water, and crystallizable. They are decomposed by the carbonates of potash and soda, and also by caustic potash and soda. Ammonia precipitates them only partially in consequence of the formation of a double soluble salt of magnesia and ammonia. If a salt of ammonia be added to a solution of a magnesian salt, then an excess of caustic ammonia, on the addition of phosphate of soda, the ammonia phosphate of magnesia falls in granular crystals, which attach themselves to the sides of the glass, and if lines be drawn with a rod on the sides of the glass, the salt will be deposited in the course of those lines. (Dr. Wollaston.) When a magnesian salt, tinged with some nitrate of cobalt, is fused before the blow-pipe, it assumes a flesh colour.

4-sided prisms,

Chloride of Magnesium- Mg Cl 6 HO. with nearly square bases, deliquescent in the air. It dissolves in half its weight of cold water, or one quarter its weight of boiling water, and twice its weight of alcohol of specific gravity 817. Taste bitter. At 223° this salt begins to part with its acid, and at a red heat the acid is driven off, and magnesia remains.

Sulphate of Magnesia, Epsom salt, Bitter salt, Sul catharticus amarus, English sult, Seidlitz salt. MgO SO, 7 HO 15.375,

right prisms-with nearly square bases, the angles deviating from right angles, being 90° 30 and 89° 30'. Specific gravity 1.66. Taste, intensely bitter. Grew described the salt in 1675, and in 1723 Brown described the process for obtaining it. It occurs in sea water and in springs, as at Epsom, Seidlitz, Saidschütz. It may be prepared from magnesian limestone, by digesting with dilute sulphuric acid until effervescence ceases, and then crystallizing out the sulphate of magnesia, after filtration. It may be freed from iron by boiling it with an excess of magnesia. Water at 60° dissolves its own weight of this salt. If pure, Epsom salt should give no precipitate with sulphohydret of ammonia if it becomes black it is a proof of the presence of iron, which may be separated by calcining the salt, dissolving in water, and crystallizing. It may be made by dissolving magnesia alba in SO. Sulphate of magnesia is employed as an aperient, and as an antidote to salts of lead. It forms with sulphate of potash a double salt MgOSO 3 KOSO ̧ + 6HO.

Carbonate of Magnesia, Mg O CO2, as it occurs in nature, is destitute of water, but as prepared artificially it contains water. Magnesite is sometimes found amorphous; at other times in rhombohedral crystals, resembling carbonate of lime. When a

salt of magnesia is boiled with carbonate of potash the precipitate diffused through water, and a current of CO, passed through it, crystals are deposited, consisting of MgO CO, зHO. By exposure to the air this salt loses water, and becomes Mg O CO, HO.

Magnesia alba of the shops is obtained by precipitating a soluble salt of magnesia with carbonate of soda or potash. Light magnesia is obtained by boiling equal parts of sulphate or muriate of magnesia and carbonate of potash or soda in at least ten times their weight of water. · Heavy magnesia is the result of the previous experiment, when only half the amount of water indicated is used. Magnesia alba consists of 3 (Mg O CO2) + MgO HO+ 3 HO or, is a mixture of carbonate of magnesia with hydrate of magnesia. The exact solubility of this salt in water is not known; but when in solution it is sometimes used as a medicine. This is promoted by an additional quantity of carbonic acid, although there is no evidence of the existence of a bicarbonate. It reddens litmus.

Magnesia occurs in nature united to silica, as in olivine (MgO SiO), Nephrite (MgO 1SiO HO), Picrosmine (MgO 2SiO) Magnesite (MgO 3SiO 2HO), Anthophyllite 3 (MgO 2SiO) FeO 2 SiO. Estimation.-Magnesia is separated from its solutions by phosphate of soda and caustic ammonia, a large quantity of sal ammoniac being previously added to the solution. When ignited the precipitate is phosphate of magnesia, 7 grains containing 24 Mg O.

THE earths 'proper are ten in number,—viz. Alumina, Glucina, Zirconia, Thoria, Didymia, Lantana, Ceria, Yttria, Terbia, Erbia. The only important earth is alumina, many of the others being imperfectly known. They are all precipitated white by NH, and also by soda; alumina and glucina, being soluble in that alkali. They are all precipitated by NH, SH, not as sulphurets, but as oxides, SH being evolved. They are not precipitated by SH.

ALUMINUM.

Al. 1.75 obtained by heating chloride of aluminum in a platinum crucible with potassium, the lid of which is held down by a wire. The metal resembles in appearance platinum; less fusible than cast-iron; non-conductor of electricity; burns at a red-heat with great splendour, and is converted into alumina.

Alumina, Al, 0, 6·5. Argil, Clay, Oxide of Aluminum, occurs nearly pure in corundum, crystallized in acute rhomboids, or 6-sided prisms: specific gravity 3.95. Alumina is obtained in a pure form from a solution of alum by precipitation, by means of caustic ammonia. It falls in the form of a thick jelly. This is thrown on a filter, and washed with hot water, till the liquor passing through is not precipitated by chloride of barium. When dry, it is a white powder, without taste and smell, but adhering strongly to the tongue, insoluble in water and alcohol, but soluble in caustic potash, and soda, and also slightly in caustic ammonia, when no ammoniacal salts are present in solution specific gravity 4.2. It unites with acids, but constitutes a weak base, The salts are generally colourless and soluble in water, having a sweet and astringent taste; are not precipitated by prussiate of potash and tincture of nutgalls, by which they are distinguished from salts of Yttria and Glucina. -Alumina is supposed in some cases to act as an acid: as when dissolved by soda. next to silica, is the principal constituent of soils.

Alumina,