ACIDS, (Encycl.)

ACIDS, (Encycl.) Acids form one of the general classes into which Salts are usually divided, the other two being called alkalies and neutrals. The most common and easy method of distinguishing acids is by their peculiar taste; but there are many other properties which equally distinguish them, the principal of which are: 1. With water they combine into a fluid, whose specific gravity is not a medium betwixt the weight of the water and acid separately taken. 2. With spirit of wine they unite into a substance very different from either of the component parts, called ether. 3. They unite with absorbent earths, alkalies, animals, metallic, and sometimes with pure argillaceous earths, from which they cannot afterwards be separated without considerable difficulty. 4. They change the blue colour of vegetables to red, and heighten the red colour of those which are so already. 5. If mixed with a fermentable liquor, they will prevent that process from taking place; or if the fermentation has already begun, the acid will put a stop to it. 6. They cannot be frozen but in a degree of cold considerably below

Typograss Vulgo. Beetle-Tower Grass. Sheep-Tower Grass.

Bullens' Tootal Grass.

Great Meadow Grass.

Creping soft Grass.

Small Grass
lower Mountain

Mountain Hair Grass.

Annual Meadow Grass.

New Tootal Grass.

Meadow

Tootal Grass.

Crested

Tootal Grass.

Grass.

Small Tootal

Acid. low 32°, the freezing point of water.

To two of these rules, however, there is an exception; viz. the volatile sulphureous acid, which will neither unite with absorbent earths, nor change the colour of vegetable infusions. But this is not to be attributed to the nature of the acid itself. It arises from the phlogiston with which the volatile sulphureous acid is still combined; for the pure sulphureous acid is not different from that called the vitriolic, and is exceedingly strong.

By some philosophers acids have been reckoned simple chemical elements, and by others they have been thought to be formed of water and earth. The former opinion, however, is refuted by some experiments which prove the destructibility of certain acids; and the latter seems not at all probable. We have no instance of an acid being formed out of these two ingredients; nor can we conceive how two substances, seemingly so insipid and inactive, can produce a third with properties so exceedingly different from both. The vegetable acids can all of them be destroyed by means of fire; in which case they yield a great quantity of fixed air, thereby showing that it is properly their element, or that from which they are formed. What are the constituent parts of fixed air, we know not. Dr Priestley supposes it to be a modification of the nitrous acid; but from the great quantities of dephlogisticated air yielded by the latter, this opinion seems also improbable; nor indeed can any thing concerning the origin of fixed air be deduced with certainty from such experiments as have yet been made.

The vitriolic and marine acids are resolved into a kind of air peculiar to themselves; so that we have few or no data to guess at their composition. In their action upon bodies, acids greatly resemble cold, as alkalies do heat; the former always tending to preserve, or even increase, the cohesion of such bodies as they do not chemically dissolve, and consequently to keep them from putrefaction; while alkalies, on the other hand, dissolve the cohesion, and thus sooner bring on the effects of putrefaction.

But although acids thus produce the effects of cold, this is only to be understood of them in a very diluted state; for when concentrated, they are so far from producing cold, that with some bodies a most violent heat, and even flame itself, is ingendered. Thus a mixture of the vitriolic and nitrous acid will set on fire oil of turpentine; the nitrous acid alone will kindle oil of clovers, sassafras, and some others. In this concentrated state they prove a violent and corrosive poison if taken into the body; though when diluted, they not only may be taken with safety, but in many cases prove medicines of great efficacy; particularly in fevers, where they are useful by their property of allaying thirst, as well as by their tonic and antiseptic qualities.

The acids are divided into three classes expressive of their origin, viz. the mineral, vegetable, and animal. The mineral are, the vitriolic, the nitrous, and marine acids; to which we may now add that of arsenic. The vegetable are, the acetic acid, that of tartar, the acid juices of fruits, the acids obtained from woods by distillation, and the acid of sugar. The animal acids are, that of phosphorus, the acid of ants, wasps, bees, &c.

All or most of the acids, as already mentioned, will unite themselves with alkaline salts, earths, and metals; but some of them have these properties in a much stronger degree than others. The mineral acids particularly excel in this respect; though even among these there are considerable differences. The vitriolic is reckoned the strongest; though it is probably inferior to the acid of arsenic. Of this last, however, the properties are not yet generally known.

The vitriolic, or, as it is sometimes called, the universal acid, when very pure and strong, appears like a clear oil, having twice the specific gravity of water, or even more; in which state it is called oil of vitriol, though improperly, for it has not the least oily quality, but on the contrary mixes with water so violently that a great heat immediately takes place. In this state the smallest quantity of an inflammable substance, such as a bit of cork or straw, falling into it, gives a brownish or black colour; and as this frequently happens to those who sell the acid in small quantities, we very often buy it thus discoloured.

This acid was originally named the vitriolic, from its being distilled from vitriol or copperas; and the name is still retained, though this method of distillation is universally superseded by that of obtaining it from the fumes of burning sulphur. It is found almost universally. It exists in vast quantity in sulphur; so that it can be extracted from it with profit even when great part of the acid fumes are lost. In the waters of the ocean it is found combined with calcareous earth, and with the earth called magnesia. In the former case, it subsides to the bottom of the salt-pans, forming what is called pan-seratch; and in the latter, it produces the bitter purging salt called Epson salt.—In the earth it is found in vast quantities united with calcareous matter, and crystallized in the large masses called gypsum, alabaster, Paris plaster, or stucco. It is also found in many kinds of hard water united with the same earth, and sometimes with iron; in which case it forms what are called chalybeate mineral waters. In clay of all kinds it exists, most frequently combined with calcareous earth, but sometimes with the argillaceous or substance of the clay itself. In this last case the compound formed by the union is alum. This acid exists also in great plenty in the pyrites, or hard heavy lumps called brasses, found in coal-mines. Here it is combined with inflammable matter, and the compound is sulphur; but by exposure to the air the inflammable part flies off, and the acid partly saturates itself with iron, which these substances also contain in plenty, and partly appears in its proper form. Hence the water which washes pyrites thus exposed contains partly a green vitriol formed by the union of the acid with the metal; and partly a loose acid, which if it could be concentrated would become pure oil of vitriol.—Hence probably the origin of vitriolic and acid waters.—It is also frequently found in or near copper mines; and then makes its appearance in a very acrid poisonous water of a bluish colour, from which the blue copperas or vitriol is extracted.

The vitriolic acid is of very considerable use in many different mechanical arts. In tanning of leather, it is used to give consistency to the leather after it has been almost dissolved by the lime. In dyeing, it is of use for fixing and brightening of various colours, especially if

Acid. if combined with the argillaceous earth into the compound called alum. In bleaching, it is also exceedingly useful for dissolving the impurities which cannot be extracted by fixed alkali, whence the cloth acquires a clearer colour than if no acid was used. In medicine it is a good antiseptic and corroborant. It is useful in many other cases, so that the consumption of it is very great; and though several manufactories of vitriolic acid have been erected, the market cannot be overstocked.

The nitrous acid, commonly called aqua fortis, when pure, is a limpid liquor considerably lighter than oil of vitriol, its weight being to that of water as 19 to 12. A very small quantity of inflammable matter communicates to a large portion of this acid a deep orange or reddish colour, and causes it emit very dense fumes of a red colour and suffocating nature. It is obtained only by distilling the neutral salt called saltpetre with oil of vitriol. Concerning its origin there have been great disputes; some asserting that it is naturally contained in the air; others, that it is the effect of putrefaction; some, that it is only the vitriolic acid altered by having been mixed with putrefying vegetable and animal substances; and lastly, which is the most probable opinion, it has been asserted, that the nitrous acid is naturally found in vegetables, and consequently that its being procured in large quantities from putrefied vegetables is not a sign of its being produced, but only exposed to view by that process; and in confirmation of this hypothesis, it is observed, that almost all those salts called essential by the chemists, are no other than the nitrous acid combined with volatile alkali. In the earth, this acid is also found in various parts and in many different proportions; but the accounts we have of its production, notwithstanding their being so often repeated by different authors, are still so confused and uncertain, that very little confidence can be put in them.

The uses of aqua fortis in dyeing, and in other mechanic arts, is very considerable. It affords one method of separating gold from silver, being endowed with a power of dissolving the latter, but not the former. It is also of considerable use in chemistry and medicine, particularly in the making of gun-powder; for to this we must ascribe the whole power exerted by the explosion of that composition. In this case, however, it is not made use of in its pure state, but in conjunction with a fixed alkali, forming the neutral salt called nitre or saltpetre.

Though the nitrous acid is weaker than the vitriolic, in point of chemical affinity and quantity of acid power, yet it acts much more briskly upon all substances than the former; and by its means the vitriolic acid may be made to dissolve many which it cannot do by itself. Being used for so many purposes, the consumption of nitre is immense.

The marine acid, muriatic acid, or acid of sea-salt, has its name from being found in the waters of the ocean, forming our common salt in conjunction with the fossile fixed alkali. Besides this, and the fossile or rock-salt, which is of the same nature with the salt of the ocean, the marine-acid has no other origin excepting the bitter liquor which remains after the extraction of sea-salt. Here it is found in combination with the earth called magnesia, forms a liquor very pun-

gent to the taste, which cannot be crystallized; and if evaporated to dryness, soon attracts moisture from the air sufficient to reduce it again to a fluid state.

The marine acid is the weakest and the lightest of all the mineral acids. In its most concentrated state it is of a lemon-colour, continually exhaling dense white fumes which spread to a great distance, and are of a very suffocating nature. The marine acid is scarce ever concentrated equally with the other two, on account of its being much more volatile; and therefore its specific gravity cannot well be ascertained. The marine acid dissolves metals with less heat and ebullition, and fewer vapours, than the other two, but adheres to them with much more obliquity; so that almost all the metals, even gold itself, become volatile, and arise with this acid in form of a sublimate. The uses of marine acid, except what is employed in sea-salt, are very limited, and therefore there is no great demand for it. However, it is useful in the solution of some metals, particularly gold; in which case it is mixed with a proportion of aqua fortis, and then forms a compound called aqua regia.

The vegetable acids most generally used are vinegar or the acetic acid, and the acid of tartar. The former is produced by a kind of fermentation; the latter from wine which is long kept, and seems to be a kind of essential salt. The uses of vinegar as an article of food are very extensive. It is also the basis of some elegant and useful medicines. Cream of tartar makes no inconsiderable article in the materia medica, and is also used in some of the mechanic arts. The other vegetable acids are but little known or made use of, the acid juices of fruits excepted, which are almost entirely confined to culinary purposes.

With regard to the animal acids, that of phosphorus excepted, they have been very little the subject of investigation. The phosphoric acid, however, is not entirely confined to the animal kingdom. It is obtained by distillation from mustard, and some other vegetables. By means of the vitriolic acid also, it may be had in great quantity from calcined bones and other animal earths. The only substance from which it can be had without the trouble of distillation, is urine.—By simple evaporation, a kind of salt composed of phosphoric acid and volatile alkali is obtained. The phosphoric acid being used only for matters of curiosity, the consumption of it is inconsiderable.