Elements of Chemistry Part 19

This is a concrete acid, extracted from a salt procured from India called _borax_ or _tincall_. Although borax has been very long employed in the arts, we have as yet very imperfect knowledge of its origin, and of the methods by which it is extracted and purified; there is reason to believe it to be a native salt, found in the earth in certain parts of the east, and in the water of some lakes. The whole trade of borax is in the hands of the Dutch, who have been exclusively possessed of the art of purifying it till very lately, that Messrs L'Eguillier of Paris have rivalled them in the manufacture; but the process still remains a secret to the world.

By chemical a.n.a.lysis we learn that borax is a neutral salt with excess of base, consisting of soda, partly saturated with a peculiar acid long called _Homberg's sedative salt_, now _the boracic acid_. This acid is found in an uncombined state in the waters of certain lakes. That of Cherchiais in Italy contains 94-1/2 grains in each pint of water.

To obtain boracic acid, dissolve some borax in boiling water, filtrate the solution, and add sulphuric acid, or any other having greater affinity to soda than the boracic acid; this latter acid is separated, and is procured in a crystalline form by cooling. This acid was long considered as being formed during the process by which it is obtained, and was consequently supposed to differ according to the nature of the acid employed in separating it from the soda; but it is now universally acknowledged that it is identically the same acid, in whatever way procured, provided it be properly purified from mixture of other acids, by warning, and by repeated solution and cristallization. It is soluble both in water and alkohol, and has the property of communicating a green colour to the flame of that spirit. This circ.u.mstance led to a suspicion of its containing copper, which is not confirmed by any decisive experiment. On the contrary, if it contain any of that metal, it must only be considered as an accidental mixture. It combines with the salifiable bases in the humid way; and though, in this manner, it is incapable of dissolving any of the metals directly, this combination is readily affected by compound affinity.

The Table presents its combinations in the order of affinity in the humid way; but there is a considerable change in the order when we operate via sicca; for, in that case, argill, though the last in our list, must be placed immediately after soda.

The boracic radical is. .h.i.therto unknown; no experiments having as yet been able to decompose the acid; We conclude, from a.n.a.logy with the other acids, that oxygen exists in its composition as the acidifying principle.

TABLE _of the Combinations of a.r.s.eniac Acid, with the Salifiable Bases, in the Order of Affinity._

_Bases._ _Neutral Salts._

Lime a.r.s.eniat of lime.

Barytes barytes.

Magnesia magnesia.

Potash potash.

Soda soda.

Ammoniac ammoniac.

Oxyd of zinc zinc.

manganese manganese.

iron iron.

lead lead.

tin tin.

cobalt cobalt.

copper copper.

nickel nickel.

bis.m.u.th bis.m.u.th.

mercury mercury.

antimony antimony.

silver silver.

gold gold.

platina platina.

Argill argill.

_Note._--This order of salts was entirely unknown to the antient chemists. Mr Macquer, in 1746, discovered the combinations of a.r.s.eniac acid with potash and soda, to which he gave the name of _a.r.s.enical neutral salts_.--A.

SECT. XXIII.--_Observations upon a.r.s.eniac Acid, and its Combinations._

In the Collections of the Academy for 1746, Mr Macquer shows that, when a mixture of white oxyd of a.r.s.enic and nitre are subjected to the action of a strong fire, a neutral salt is obtained, which he calls _neutral salt of a.r.s.enic_. At that time, the cause of this singular phenomenon, in which a metal acts the part of an acid, was quite unknown; but more modern experiments teach that, during this process, the a.r.s.enic becomes oxygenated, by carrying off the oxygen of the nitric acid; it is thus converted into a real acid, and combines with the potash. There are other methods now known for oxygenating a.r.s.enic, and obtaining its acid free from combination. The most simple and most effectual of these is as follows: Dissolve white oxyd of a.r.s.enic in three parts, by weight, of muriatic acid; to this solution, in a boiling state, add two parts of nitric acid, and evaporate to dryness. In this process the nitric acid is decomposed, its oxygen unites with the oxyd of a.r.s.enic, and converts it into an acid, and the nitrous radical flies off in the state of nitrous gas; whilst the muriatic acid is converted by the heat into muriatic acid gas, and may be collected in proper vessels. The a.r.s.eniac acid is entirely freed from the other acids employed during the process by heating it in a crucible till it begins to grow red; what remains is pure concrete a.r.s.eniac acid.

Mr Scheele's process, which was repeated with great success by Mr Morveau, in the laboratory at Dijon, is as follows: Distil muriatic acid from the black oxyd of manganese, this converts it into oxygenated muriatic acid, by carrying off the oxygen from the manganese, receive this in a recipient containing white oxyd of a.r.s.enic, covered by a little distilled water; the a.r.s.enic decomposes the oxygenated muriatic acid, by carrying off its supersaturation of oxygen, the a.r.s.enic is converted into a.r.s.eniac acid, and the oxygenated muriatic acid is brought back to the state of common muriatic acid. The two acids are separated by distillation, with a gentle heat increased towards the end of the operation, the muriatic acid pa.s.ses over, and the a.r.s.eniac acid remains behind in a white concrete form.

The a.r.s.eniac acid is considerably less volatile than white oxyd of a.r.s.enic; it often contains white oxyd of a.r.s.enic in solution, owing to its not being sufficiently oxygenated; this is prevented by continuing to add nitrous acid, as in the former process, till no more nitrous gas is produced. From all these observations I would give the following definition of a.r.s.eniac acid. It is a white concrete metallic acid, formed by the combination of a.r.s.enic with oxygen, fixed in a red heat, soluble in water, and capable of combining with many of the salifiable bases.

SECT. XXIV.--_Observations upon Molybdic Acid, and its Combinations with Acidifiable Bases[43]._

Molybdena is a particular metallic body, capable of being oxygenated, so far as to become a true concrete acid[44]. For this purpose, one part ore of molybdena, which is a natural sulphuret of that metal, is put into a retort, with five or six parts nitric acid, diluted with a quarter of its weight of water, and heat is applied to the retort; the oxygen of the nitric acid acts both upon the molybdena and the sulphur, converting the one into molybdic, and the other into sulphuric acid; pour on fresh quant.i.ties of nitric acid so long as any red fumes of nitrous gas escape; the molydbena is then oxygenated as far as is possible, and is found at the bottom of the retort in a pulverulent form, resembling chalk. It must be washed in warm water, to separate any adhering particles of sulphuric acid; and, as it is hardly soluble, we lose very little of it in this operation. All its combinations with salifiable bases were unknown to the ancient chemists.

TABLE _of the Combinations of Tungstic Acid with the Salifiable Bases._

_Bases._ _Neutral Salts._

Lime Tungstat of lime.

Barytes barytes.

Magnesia magnesia.

Potash potash.

Soda soda.

Ammoniac ammoniac.

Argill argill.

Oxyd of antimony(A), &c. antimony(B), &c.

[Note A: The combinations with metallic oxyds were set down by Mr Lavoisier in alphabetical order; their order of affinity being unknown, I have omitted them, as serving no purpose.--E.]

[Note B: All these salts were unknown to the ancient chemists.--A.]

SECT. XXV.--_Observations upon Tungstic Acid, and its Combinations._

Tungstein is a particular metal, the ore of which has frequently been confounded with that of tin. The specific gravity of this ore is to water as 6 to 1; in its form of cristallization it resembles the garnet, and varies in colour from a pearl-white to yellow and reddish; it is found in several parts of Saxony and Bohemia. The mineral called _Wolfram_, which is frequent in the mines of Cornwal, is likewise an ore of this metal. In all these ores the metal is oxydated; and, in some of them, it appears even to be oxygenated to the state of acid, being combined with lime into a true tungstat of lime.

To obtain the acid free, mix one part of ore of tungstein with four parts of carbonat of potash, and melt the mixture in a crucible, then powder and pour on twelve parts of boiling water, add nitric acid, and the tungstic acid precipitates in a concrete form. Afterwards, to insure the complete oxygenation of the metal, add more nitric acid, and evaporate to dryness, repeating this operation so long as red fumes of nitrous gas are produced. To procure tungstic acid perfectly pure, the fusion of the ore with carbonat of potash must be made in a crucible of platina, otherwise the earth of the common crucibles will mix with the products, and adulterate the acid.

TABLE _of the Combinations of Tartarous Acid, with the Salifiable Bases, in the Order of Affinity._

_Bases._ _Neutral Salts._

Lime Tartarite of lime.

Barytes barytes.

Magnesia magnesia.

Potash potash.

Soda soda.

Ammoniac ammoniac.

Argill argill.

Oxyd of zinc zinc.

iron iron.

manganese manganese.

cobalt cobalt.

nickel nickel.

lead lead.

tin tin.

copper copper.

bis.m.u.th bis.m.u.th.