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The liquor is not thrown away, but preserved to wash the impure borax; the solid matters held in suspension being first separated by subsidence and decantation. The washing is continued with the same liquid, clarified by subsidence as often as applied, until it is no longer rendered turbid. In this way a great portion of the fatty matter may be washed away as an insoluble soap of lime. of lime. The salt thus purified is dissolved in two and a half parts of boiling water, and mixed with a solution of chloride of calcium containing two parts of that salt to one hundred parts of tincal. A precipitate is thereby produced, consisting chiefly of the insoluble soap of lime; the liquor is separated from the precipitate by filtration, and evaporated down to the density 1.14 or 1.16. It is then run off into crystallizing vessels, and cooled very gradually in order to obtain large crystals.

The preceding process has received various modifications; but as tincal is now no longer the European source of borax, an account of these is unnecessary.

Borax has a sweetish and alkaline taste, and changes certain vegetable colours in the same manner as an alkali. Notwithstanding its alkaline reaction, it is believed to contain two proportions of acid* to one of soda; but the acid characters of boracic acid are too feeble to neutralize completely the alkaline properties of the base. In this respect borax may be assimilated to the bicarbonates of the alkalies. The specific gravity of borax is 1.705. It is soluble in twelve parts of cold water, and in twice its weight of boiling The crystals of borax which are formed at

water.

* It is doubtful whether the equivalent of boracic acid should be taken as thirty-five or twice that number.

common temperatures have the following composition:

One equivalent of soda

31

16.2

36.6

90

47.2

Two equivalents of (or one eq. ?) boracic acid 70

Ten equivalents of water

One equivalent of borax

.... 191 100.0

When exposed to a dry atmosphere at the ordinary temperature, this salt commonly effervesces slightly, and loses a portion of its water.* If moderately heated, it readily fuses, swells considerably from the escape of steam, and at a red heat becomes anhydrous.

If the heat has been insufficient to fuse the borax, it remains as a light white porous mass known as calcined borax.

When borax is crystallized from a solution the temperature of which is above 133° Fahr., it assumes. a form different from the above crystals, and combines with a different proportion of water of crystallization, namely, five equivalents. The shape of these crystals is the regular octohedron. They contain

Soda
Baracic acid
Water

21.3

47.9

30.8

100.0

The specific gravity of this compound is 1.815. From containing a smaller proportion of water than ordinary borax, this variety is preferred by braziers in soldering. It is usually distinguished as octohedral borax. The possibility of obtaining another borax with a still smaller proportion of water of crystallization, is a subject which well deserves the attention of the borax manufacturer.

* According to Mr. O. Sims, this salt does not effloresce if entirely free from carbonate of soda.

.Anhydrous borax fuses when heated strongly, and forms a limpid and colourless glass, which is still capable of being dissolved in water. Like many

other vitreous substances in a state of fusion, borax is capable of dissolving most metallic oxides, forming with several glasses of particular colours, from the tint of which the nature of the metallic oxide may often be determined.*

* The use of borax as a re-agent in analysis by the blow-pipe depends on this property. If a minute quantity of either of the oxides mentioned in the following table is added to a bead of borax fused on the end of a platinum wire bent in the form of a hook, and heated in the exterior blow-pipe flame, the bead acquires a well-marked colour, according to the nature of the oxide. In the exterior blow-pipe flame these colours are as follow:

[blocks in formation]

If the bead is brought into the interior flame of the blow-pipe, or the reducing flame, the colour produced by some metallic oxides wholly disappears; but more frequently the bead passes from one colour to another. This is the case with most of the oxides mentioned in the preceding table. The colours of the beads in the interior flame are as follow :

Cobalt
Manganese
Chromium
Copper
Uranium
Iron

Intense blue.

Colourless.

Emerald green.
Ruby red.

Dark green.
Brownish-green.

Nickel

Antimony

Grey from metallic nickel.
Grey from metallic antimony.

On this property of dissolving metallic oxides. is founded the use of borax to braziers, silversmiths, and others, in the soldering of metals; for which purpose, merely, about two tons of borax are consumed in Birmingham weekly.* When heated, with exposure to the air, most metals acquire a coating of oxide, which would, if allowed to remain, effectually prevent the perfect contact of two pieces. But if a little borax is sprinkled on the metal, the oxide is dissolved, and the access of air prevented by the fused salt; the surface is thus cleaned, and the liquid salt offers no impediment to the combination of the metal with the solder.

The most considerable consumption of borax at present is at the potteries, chiefly in the formation of a porcelain and earthenware glaze, an application dependent on the vitreous character of most combinations of this salt with metallic oxides and earthy bases. The particular composition of the borax glaze is varied at different potteries, but the essential constituents, besides borax, are felspar and soda. Nitre is sometimes added in small quantity, and a mixture of powdered flint-glass and flints is occasionally substituted for felspar. At a single pottery there is a consumption of from eighty to a hundred

tons of borax per annum.

Borax also enters into the composition of some varieties of glass, and would be extensively used for that purpose, did its price permit. A glass recommended for optical purposes has been made with seven parts of red lead, two parts of calcined borax,

* Information supplied by Mr. Kurtz of Liverpool.

and three parts of ground quartz. The addition of a little borax to the materials for plate-glass and crownglass has been recommended; but a large quantity seems to communicate an exfoliating property to the glass. The vitreous body of artificial gems called strass (from the name of its German inventor) is a kind of flint-glass, in most receipts for which borax is mentioned as an essential constituent. According to Dr. Shaw (Chem. Lectures), by fusing together four parts of borax and one part of fine white sand, a pure glass is formed, sufficiently hard to cut common glass like the diamond.

Borax is used to a large extent in assaying metallic ores to dissolve the mineral, and facilitate the reduction of the oxide by carbonaceous matters. It is eminently adapted for this purpose, as it forms fusible combinations, not only with bases, but also with silica.* An aqueous solution of borax has the remarkable property of dissolving shell-lac, affording a solution which may be employed as a kind of varnish. The proper proportions of the materials are five parts of borax and one of shell-lac.

II. BORACIC ACID.

When a solution of borax in water, nearly saturated at its boiling point, is mixed with a quantity of oil of vitriol equal to one fourth the weight of the borax, thin crystalline plates of boracic acid are

* See an account of some interesting experiments on the solvent power of fused borax by M. Berthier in his Traité des Essais par la voie sèche, t. i. p. 475, et seq.

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