PROPERTIES OF ALUM.

507

nesium; they crystallize in fine silky needles. A native sulphate of aluminum and manganese was stated by Kane and by Apjohn to contain 25 atoms of water.

met with in the native state.

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A similar salt of iron has been

These fibrous salts, according to

How, contain only 22 H2O, so that the formula of the manganese salt would be MnAl, 4 SO ̧ . 22 H2O.

Ordinary alum has a sweetish, astringent taste; it is soluble in about 18 parts of cold water, and in less than its own weight of boiling water. The solution has a strongly acid reaction, and dissolves iron and zinc with evolution of hydrogen. When heated, this salt first melts in its water of crystallization, which amounts to 45'53 per cent. of its weight; as it loses water it froths up, and forms a tough, tenacious paste, which is ultimately converted into a voluminous, white, infusible, porous mass of anhydrous or burnt alum. If crystallized alum be submitted to a regularly increasing heat, a certain proportion of the water contained in it is readily driven off: thus by a temperature of 2120, 5 atoms out of the 12 are expelled, and 5 more at 248°. If the salt be now heated to 3920, it is rendered anhydrous and insoluble in water (Gerhardt). By ignition, alum loses a great part of its acid.

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Alum is largely employed in dyeing: when used in this process its solution is gradually mixed with carbonate of sodium, so long as the precipitate is redissolved on agitation, which happens till two-thirds of the acid have been neutralized. The solution employed, therefore, contains a mixture of 3 salts, viz., Al¿Ð¿SO3+ K2SO4+2 Na2SO, Cloths dipped into this liquid remove the alumina thus redissolved, and contract an intimate mechanical combination with it, by which they are enabled, as already mentioned, to retain the colours of the dye-stuffs employed. Upon evaporation, cubic crystals of alum are deposited from this solution, and the excess of alumina separates. A hydrated basic sulphate of aluminum (Al,O,, SO, . 9 H,O), containing the same proportion of sulphuric anhydride and alumina as that formed in the mordanting liquid just described, is obtained by precipitating the normal sulphate of aluminum incompletely by caustic ammonia ; it is a white insoluble powder. A white earthy-looking mineral, termed aluminite, said to have the same composition as this basic sulphate, is found near Newhaven.

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(668) PHOSPHATES OF ALUMINUM.-Several minerals occur, into the composition of which phosphate of aluminum enters. The blue turquoise is a hydrated native phosphate, Al P1⁄2Ð11 · 5 H ̧Ð, or 2 Al2O3, PO. 5 Aq, coloured by copper and iron. Gibbsite which was formerly considered to be a hydrate of alumina, was

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SILICATES OF ALUMINUM.

found by Hermann to consist of a hydrated phosphate of the metal, mixed with variable proportions of hydrate of alumina. Phosphate of aluminum (AlPO) may be prepared artificially by mixing a solution of phosphate of sodium with one of alum; the precipitate must be well washed. If this precipitate be redissolved in an acid, and ammonia be added, the precipitate thus occasioned has, according to Rammelsberg, the composition (4 Al03, 3 P0.18 H2O). Wavellite is a mineral which crystallizes in radiating tufts of needles; according to Berzelius, it is a combination of fluoride of aluminum with the last-mentioned basic phosphate of aluminum, 3 (4 Al ̧Ð ̧, 3Р ̧Ð ̧ . 18 H2O),AlF.... The mineral amblygonite is a combination of fluoride and basic phosphate of aluminum with phosphate of lithium. Lazulite, 2 (3 [CaMgFe]O,P,5),(4 Al‚Ð ̧, 3 P20),6 H,, is a blue mineral composed of another double phosphate which contains the same phosphate of aluminum, coloured by basic phosphate of iron (Rammelsberg).

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Phosphate of aluminum, in its hydrated form, is readily soluble in hydrochloric acid. Its solution may be precipitated by hydrate of potash, but the precipitate is redissolved by an excess of the alkali. In the operations of analysis it is often necessary to separate phosphoric acid from alumina: this is most readily effected by Chancel's method, in which the solution in nitric or acetic acid, perfectly freed both from hydrochloric and sulphuric acid, is mixed with an acid solution of nitrate of bismuth (448) Phosphoric acid is thus precipitated as the phosphate of bismuth (Bi””’P→ ̧), and the whole of the aluminum remains in solution.

(669) SILICATES OF ALUMINUM.-The compounds of silica with aluminum are numerous and important. All the varieties of clay consist of hydrated silicate of aluminum more or less mixed with other matters derived from the rocks which, by their disintegration, have formed the clay. Clay is, in fact, the result of the combined action of air and water upon felspathic and siliceous rocks, and therefore necessarily varies considerably in composition. The fundamental constituent of the more important varieties of clay, according to the researches of Brongniart, Malaguti, and others, is represented by the formula (Al¿Ð ̧, 2 Si,O ̧ . 2 H ̧Ð), or (Al,O,, 2 SiO2. 2 HO). This appears to be the composition of the fire-clay of the Staffordshire coal-measures. The ordinary varieties of clay, however, contain fragments of undecomposed rock, a certain proportion of potash, and variable amounts of silica in the hydrated condition, mixed with oxide of iron, lime, and magnesia; the cha

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racter of the clay is materially modified according as one or other of these ingredients predominates.

Pure clay, before it has been ignited, forms, when kneaded, a tenacious, plastic paste, which is insoluble in water, but may readily be diffused through it in particles which are in an extreme state of subdivision; the deposit, when freed from the excess of water, as it subsides, resumes its plastic character. This paste, when slowly dried, and exposed to a higher temperature, shrinks very much, and splits into masses which are extremely hard, but they do not undergo fusion in the furnace. Pure hydrated silicate of aluminum is very slowly acted upon by hydrochloric or by nitric acid; but it is decomposed when heated with concentrated sulphuric acid; and upon this fact one of the processes for preparing alum (667) is founded. A gentle roasting of the clay, previous to the addition of the acid, frequently favours its disintegration; but ignition at a high temperature renders it proof against the action of acids, except the hydrofluoric. Strong solution of caustic potash dissolves unburnt clay very slowly; but if the hydrated alkali in excess be fused with clay, the resulting mass is easily soluble in water.

The intermixture of lime, magnesia, or oxide of iron, in any considerable quantity, with the clay, greatly increases its fusibility, diminishes its plasticity, and causes it to be more readily attacked by acids whilst an excess of silica renders it less fusible.

Clay emits the peculiar odour known as argillaceous when breathed upon or slightly moistened: its presence in any soil may be roughly but readily distinguished by the absorbent quality which it exhibits when applied in a dry state to the tongue or the lips; it adheres to them strongly, and absorbs the saliva from their surface. This absorbent property of clay causes it to retain ammonia in the soil to an extent which is of great importance to growing plants, and, as Way has shown, it arrests the ammoniacal portions of the manure applied to the surface, and thus not only ministers to the growth of the crop, but exerts a very important purifying influence upon water impregnated with organic and other substances, which find their way slowly through the soil. Indeed, mere agitation of such water with finely divided clay is sufficient to remove a considerable amount of the organic and saline matter previously in solution. It was found that both sulphate and chloride of ammonium were partially decomposed by the lime of the clay, the ammonia being retained, whilst a corresponding amount of sulphate or of chloride of calcium was formed

in the solution. A similar decomposing action was also exerted by clay upon nitrate of potassium.

Varieties of Clay.-The most important varieties of clay are the following:

1. The celebrated kaolin, or porcelain clay of China, is a very pure white clay, which is furnished by the decomposition of a granitic rock, the constituents of which are quartz, felspar, and mica, the felspar having gradually mouldered into this substance. A very similar description of clay is obtained near St. Austel, in Cornwall, and at St. Yrieix, near Limoges, in France. It is in these cases chiefly produced by the disintegration of a rock known to geologists as pegmatite, which is, in fact, a species of granite in which mica is almost wanting, and quartz present in but small quantity. The Cornish stone used by the porcelain-makers is the same rock in a less advanced stage of disintegration. The plasticity of kaolin is much less than that of the clay derived from disintegration of the secondary rocks.

2.—Pipeclay is a white variety of clay, which is nearly free from iron. That of the Isle of Purbeck, in Dorsetshire, where it occurs nearly at the base of the clay deposits, is preferred: it is used in the manufacture of tobacco-pipes without any addition; before the oxyhydrogen blowpipe it melts to a transparent, nearly colourless glass.

3.-The blue clay of Devonshire and Dorsetshire is highly prized, as it is eminently plastic. The organic matter to which it owes its colour is destroyed when heated, and it yields a white paste when fired. It is employed as one of the materials in the manufacture of porcelain. The upper beds of this clay frequently contain a large proportion of sand mixed with the plastic material, and are well suited for making salt-glazed stoneware without further admixture.

4. When the proportion of carbonate of calcium in a clay is considerable, it constitutes what is known as a marl; if the aluminous constituent preponderate, it forms an aluminous marl; if the carbonate of calcium be in excess, it is a calcareous marl. The aluminous marls are extensively used in the manufacture of the coarser and more porous kinds of pottery.

5.-Loam is a still more mixed substance, belonging to the more recent alluvial formations: it is the common material of which bricks are made; its red or brown colour is derived from the large proportion of peroxide of iron which it contains.

6.-Yellow ochre and red bole are clays which derive their

ZEOLITES-ALUMINOUS MINERALS.

511

colour from oxide of iron, which is present in them in large quantity.

Halloysite is a white hydrated silicate of aluminum which greatly resembles kaolin in appearance, but it is destitute of any plastic character, and is therefore unfitted for the manufacture of porcelain. Fuller's earth is a porous silicate of aluminum which has a strong adhesion to oily matters: if made into a paste with water, and allowed to dry upon a spot of grease upon a board or a cloth, it removes most of the oil by capillary action. Amongst other localities in England, it is found abundantly near Reigate, in Surrey.

The following table exhibits the composition of some of the more important varieties of clay used in the arts. The first two are results obtained by Ebelmen and Salvetat; the others are from analyses executed in Richardson's laboratory, and are quoted in the second volume of the English translation of Knapp's Technological Chemistry :

Besides these amorphous silicates of aluminum, there are many which occur in a crystalline form. Disthene, or cyanite, is a bluecoloured soft mineral of this kind (Al ̧Ð ̧,SiO2).

(670) Other aluminous Minerals.-The zeolites are hydrated double silicates in which the principal bases are alumina and lime. They boil up when heated upon charcoal before the blowpipe, and are dissolved by acids, leaving the silica in a gelatinous state. In these minerals the lime is liable to displacement more or less complete by protoxide of iron, by magnesia, or by the alkaline bases. They are often very beautifully crystallized. Analcime (Na,O, Sie,. Ale, 3 Sie,. 2 H2O) is one of these minerals; it crystallizes in cubes. Stilbite crystallizes in radiated needles, and has the composition of hydrated labradorite (Ea✪, 3 SiO ̧ . Al ̧Ð ̧ 3 Sie,.6 H2O). Prehnite crystallizes in six-sided prisms; it may be represented by the formula, 2 (CaO,SiQ,). Al,,,SiÐ,. H2O.

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