Heads of Lectures on a Course of Experimental Philosophy: Particularly Including Chemistry Part 6

The marine acid has a strong affinity to dephlogisticated air, and will take it from manganese and other substances; and with this union it becomes a different acid, called _dephlogisticated marine acid_, being water impregnated with dephlogisticated marine acid air, described above.

LECTURE XIV.

_Of the Vegetable Acids, and others of a less perfect nature._

The princ.i.p.al of the vegetable acids are the _acetous_ and the _tartareous_. The acetous acid is the produce of a peculiar fermentation of vegetable substances, succeeding the _vinous_, in which ardent spirit it is procured, and succeeded by the _putrefactive_, in which volatile alkali is generated.

Thus wine is converted into vinegar. Crude vinegar, however, contains some ingredient from the vegetable substances from which it was procured: but distillation separates them, and makes the vinegar colourless; though some of the acid is lost in the process.

The acetous acid is concentrated by frost, which does not affect the proper acid, but only the water with which it is united. It may likewise be concentrated by being first combined with alkalies, earths, or metals, and then dislodged by a stronger acid, or by mere heat. Thus the acetous acid, combined with vegetable alkali, forms a substance that is called the _foliated earth of tartar_; and it may be expelled from it by the vitriolic acid. When combined with copper it makes _verdigris_; and from this union heat alone will expel it in a concentrated state. The acetous acid thus concentrated is called _radical vinegar_. Still, however, it is weaker than any of the preceding mineral acids.

Several vegetables, as lemons, sorrel, and unripe fruit, contain acids, ready formed by nature, mixed with some of the essential oil of the plants, which gives them their peculiar flavours. All these acids have peculiar properties; but it is not necessary to note them in this very general view of the subject. Like vinegar, these acids may be concentrated by frost, and also by a combination with other substances, and then expelled by a stronger acid.

The _acid of tartar_ is very similar to that of vinegar. Tartar, from which it is procured, is a substance deposited on the inside of wine-casks, though it is also found ready formed in several vegetables.

It consists of the vegetable alkali and this peculiar acid. When refined from its impurities, it is called _crystals_, or _cream of tartar_. The acid is procured by mixing the tartar with chalk, or lime, which imbibes the superfluous acid, and this is expelled by the acid of vitriol. Or it may be procured by boiling the tartar with five or six times its weight of water, and then putting the acid of vitriol to it. This unites with the vegetable alkali, and forms vitriolated tartar; and the pure acid of tartar may be procured in crystals, by evaporation and filtration, equal in weight to half the cream of tartar. This acid of tartar is more soluble in water than the cream of tartar.

This acid, united to the mineral alkali, makes _Roch.e.l.le salt_.

Every kind of wood, when distilled, or burned, yields a peculiar acid; and it is the vapour of this acid that is so offensive to the eyes in the smoke of wood.

A peculiar acid is obtained from most vegetable substances, especially the farinaceous ones, and from sugar, by distillation with the nitrous acid. This seizes upon the substance with which the acid was united, and especially the phlogiston adhering to it, and then the peculiar _acid of sugar_ crystallizes. Thus with three parts of sugar, and thirty of nitrous acid, one part of the proper acid of sugar may be obtained. By the same process an acid may be procured from camphor.

The _bark of oak_, and some other vegetable substances, especially nut-galls, contain a substance which has obtained the name of _the astringent principle_; the peculiar property of which is, that it precipitates solutions of iron in the form of a black powder, and in this manner _ink_ is made. But by solution in water and evaporation, crystals, which are a proper _acid of galls_, may be obtained.

_Amber_ is a hard semitransparent substance, chiefly found in Prussia, either dug out of the earth, or thrown up by the sea. It is chiefly remarkable for its electrical property; but by distillation in close vessels there sublimes from it a concreted acid, soluble in 24 times its weight of cold water. Amber seems to be of vegetable origin, and to consist of an oil united to this peculiar acid.

The acids I shall mention next are of a mineral origin; but being of a less perfect nature as acids, I shall only just note them here.

_Borax_ is a substance chiefly found in a crystallized state in some lakes in the East Indies. It consists of the mineral alkali and a peculiar acid, which may be separated, and exhibited in white flakes, by putting acid of vitriol to a solution of it in water. This acid has been called _sedative salt_, from its supposed uses in medicine. It is an acid that requires fifty times its weight of water to dissolve it.

Several other mineral substances, as _a.r.s.enic_, _molybdena_, _tungsten_, and _wolfram_, in consequence of being treated as the preceding vegetables ones, have been lately found to yield peculiar acids. They are also produced in a concrete state, and require a considerable proportion of water to make them liquid; but as the water in which they are dissolved turns the juice of litmus red, and as they also unite with alkalis, they have all the necessary characteristics of acids.

LECTURE XV.

_Of the Phosphoric Acid._

The most important acid of _animal_ origin, though it has lately been found in some mineral substances, is the _phosphoric_.

Phosphorus itself is a remarkable substance, much resembling sulphur, but much more inflammable. It has been procured chiefly, till of late, from urine, but now more generally from _bones_, by means of the vitriolic acid, which unites with the calcareous earth of which bones consist, and sets at liberty the phosphoric acid, or the base of that acid, with which it was naturally combined. The acid thus procured, mixed with charcoal, and exposed to a strong heat, makes phosphorus.

This substance burns with a lambent flame in the common temperature of our atmosphere, but with a strong and vivid flame if it be exposed to the open air when moderately warm. In burning it unites with the dephlogisticated air of the atmosphere, and in this manner the purest phosphoric acid is produced.

This acid is also procured in great purity by means of the nitrous or vitriolic acids, especially the former, which readily combines with the phlogiston of the phosphorus, and thus leaves the acid pure. In this process phlogisticated air is produced.

This acid is perfectly colourless, and when exposed to heat loses all its water, and becomes a gla.s.sy substance, not liable to be dissipated by fire, and readily uniting with earths.

United to the mineral alkali, it forms a neutral salt, lately introduced into medicine. United to the mineral and vegetable alkalis naturally contained in urine, it has obtained the name of _microcosmic salt_, frequently used as a flux for mineral substances with a blow-pipe.

Besides the phosphoric, there are other acids of an animal origin; as that of _milk_, that of _sugar of milk_, that of the _animal calculus_, and that of _fat_.

The acid of milk is the sour whey contained in b.u.t.ter-milk, which, by a tedious chemical process, may be obtained pure from any foreign substance.

The sugar of milk is procured by evaporating the whey to dryness, then dissolving it in water, clarifying it with whites of eggs, and evaporating it to the consistence of honey. In this state white crystals of the acid of sugar of milk will be obtained.

By distilling these crystals with nitrous acid, other crystals of the proper _acid of sugar of milk_ will be obtained, similar to those of the acid of sugar.

If the human calculus be distilled, it yields a volatile alkali, and something sublimes from it which has a sourish taste, and therefore called the _acid of the calculus_. It is probably some modification of the phosphoric acid.

Animal fat yields an acid by distillation, or by first combining it with quick-lime, and then separating it by the vitriolic acid. Siliceous earth is corroded by this acid.

LECTURE XVI.

_Of Alkalis._

The cla.s.s of substances that seems particularly formed by nature to unite with acids, and thereby form _neutral salts_, are the _alkalis_.

They have all a peculiar acrid taste, not easily defined. They change the blue juices of vegetables green, or purple, and in common with acids have an affinity with water, so as to be capable of being exhibited in a liquid form; though when this water is expelled by heat, some of them will a.s.sume a solid form.

Alkalis are of two kinds; the _fixed_, which have no smell, and the _volatile_ which have a pungent one.

The fixed alkalis are of _vegetable_ or _mineral_ origin. When in a solid form, they both melt with a moderate heat, and uniting with earthy substances, make _gla.s.s_. With an intense heat they are volatilized.

Vegetable alkali is procured by burning plants, and lixiviating the ashes; a purer kind by the burning of tartar, hence called _salt of tartar_; but the purest of all is got by the deflagration of nitre; the charcoal uniting with the acid as it a.s.sumes the form of dephlogisticated air, and the alkali being left behind.

Mineral alkali is found in ashes of sea-weed. It is likewise the basis of sea-salt; from which it is separated by several processes, but especially by the calx of lead, which has a stronger affinity with the marine acid with which it is found combined.

Alkalis united with fixed air are said to be _mild_, and when deprived of it _caustic_, from their readiness to unite with, and thereby _corrode_, vegetable and animal substances. To render them caustic, they are deprived of their fixed air by quick-lime; and in this state they unite with oils, and make _soap_.

Alkalis have a stronger affinity with acids than metals have with them; so that they will precipitate them from their solutions in acid menstruums.

The vegetable fixed alkali has a strong attraction to water, with which it will become saturated in the common state of our atmosphere, when it is said to _deliquesce_; and having the appearance of _oil_, the salt of tartar is thus said to become _oil of tartar per deliquium_. On the other hand, the mineral, or fossil alkali, is apt to lose its water in a dry atmosphere, and then it is said to _effloresce_. In this state it is often found on old walls.

Volatile alkali is procured by burning animal substances; in Egypt (from whence, as contained in _sal ammoniac_, we till of late imported it) from camel's dung; but now from bones, by distillation. To the liquor thus procured they add vitriolic acid, or substances which contain it.

This acid unites with the alkali, and common salt being put to it, a double affinity takes place. The vitriolic acid uniting with the mineral alkali of the salt, makes _Glauber salt_, and the marine acid uniting with the volatile alkali, makes _sal ammoniac_. Slaked lime added to this, unites with the marine acid of the ammoniac, and sets loose the volatile alkali in the form of _alkaline air_, which combining with water, makes the liquid caustic volatile alkali. If chalk (containing calcareous earth united with fixed air) be mixed with the sal ammoniac, heat will make the calcareous earth unite with the marine acid, while the fixed air of the chalk will unite with the volatile alkali, and a.s.sume a solid form, being the _sal volatile_ of the apothecaries.