ACID, Acidum, anything which affects the Tongue with a Sense of Sharpness, and Sourness. See TASTE.Acids are usually divided into manifest and dubious.The Manifest Acids are those above defined, which impress the Idea sensibly. — Such are Vinegar, and its Spirit;the Juices of Citrons, Oranges; Spirit of Nitre, Spirit of Alum, Spirit of Vitriol, Spirit of Sulphur for Campanam, Spirit of Sea Salt, etc. See VINEGAR, NITRE, VITRIOL, ALUM, SULPHUR, etc.

Dubious Acids, are those which do not retain enough of the Acid Nature to give sensible Marks thereof on the Taste, but agree with the Manifest Acids in some other Properties, sufficient to refer them to the same Class. — Hence it appears that there are some Characters of Acidity more general than that of the sharp Taste; though it is that Taste which is chiefly regarded in the Denomination.

The great and general Criterion, then, of Acids, is, that they make a violent Effervescence, when mixed with another sort of Bodies, called Alkalies. See EFFERVESCENCE.

Yet, this Property alone is not universally to be depended on, to determine a Body an Acid, without the joint Consideration of the Taste, and the Changes of Colour producible in other Bodies thereby. — To distinguish dubious Acids from Alkalies, mix them with a blue Tincture of Violets: if they turn it red, they are of the Acid Tribe; if green, Alkaline. See ALKALI.

Acids are all of the Tribe of Salts; and compose a particular Species thereof, called Acid Salts. See SALT.

Add, that the Acid Salts are all found to be volatile; by which they are distinguished from the fixed, which are either fixed, or at least have a urinous, instead of an acid Taste.See VOLATILE, FIXED, and URINOUS.

Some late Chemical Philosophers have even made it very probable, that it is the Acid that is the Saline Part or Principle in all Salts. — They consider it as a subtle, penetrating Substance, diffused through the several Parts of the Globe; which, according to the different Matters it happens to be united with, produces different Kinds of Bodies: if it meets a fossil Oil, it converts it into Sulphur; if it is received into the Lapis Calaminaris, it coagulates with it, and becomes Alum; with Iron it grows into green Vitriol; with Copper, into blue Vitriol, etc.

Of this Sentiment is Sir I. Newton. — ' In decompounding ' Sulphur, says that Author, we get an Acid Salt, of the ' same Nature with Oil of Sulphur per Campanam; which ( same Acid abounding in the Bowels of the Earth, unites ' sometimes with Earth, and thus makes Alum; sometimes ' with Earth and Metal, and makes Vitriol; and sometimes ' with Earth and Bitumen, and thus compounds Sulphur.'Opticks.

In essence, all our native Salts, though without any Mixture from Art, are yet found to be real Mixtures; and their Composition and Decomposition are easily made. — 'As many as they are, they may be all reduced, according to M. Homberg, to three Kinds, to Salt-petre, Sea-Salt, and Vitriol;each whereof has its several Species. Of the Combination of these with different oily Matters, are all the other Salts produced. By the Analyses we have made of them, they all appear to be composed of an aqueous, an earthy, a sulphurous, and an acid Part; but the Acid we hold the pure Salt: This makes our Chemical Principle Salt, the common Basis of all Salts; and which, antecedent to its Determination to any particular Species, appears to be one Similar, uniform Matter, though never found alone, but always accompanied with some sulphurous Mixture or other; which determines it to some one of the three sorts of Fossil Salts abovementioned.' Mem. de l'Acad. R. des Sciences. An. 1708. See PRINCIPLE.

The Acid, accompanied with its determining Sulphur, never becomes sensible to us, except when lodged either naturally in some earthy Matter, or artificially in an aqueous one. — In the first Case, it appears under the Form of a crystallized Salt; as Salt-petre, Sea Salt, etc. In the second, it appears in form of an Acid Spirit; which, according to the Determination of the Sulphur accompanying it, is either Spirit of Nitre, or Spirit of Salt, or Spirit of Vitriol.

What is here spoken of the three simple fossil Salts may be equally applied to all the compound Salts of Vegetables and Animals, with this difference, that the latter have always a larger Proportion of the earthy Matter than the simple ones, when in form of a concrete Salt; and a larger Proportion of the aqueous Matter, when in form of an acid Spirit. — And hence we account for two important Phenomena; 1°, That the acid Spirits of Animal and Fossil Salts are always weaker, and less penetrating, as well as lighter in Weight, than those of the Fossil Salts: 2°, That after a vehement Distillation, they leave a larger quantity of earthy Matter behind them than the Fossil do.

The Salt naturally contained in Plants may be considered as a Mixture of Earth, Oil, a little Water, and an acid Salt: This last Ingredient being separated from the Plant with a vehement Fire, shoots into a new Salt, which sometimes retains an acid Taste, as in the Tartar of Wine;sometimes it assumes a Sweetness, as in Sugar; sometimes is bitter, as in Quinquina; and sometimes almost insipid, as in Sage. This, M. Homberg calls the essential Salt of the Plant; which, by a gentle Distillation, resolves into an insipid Water, an acid Liquor, and a ruddy fetid one; containing part of the acid Salt, and part of the fetid Oil of the Plant: of the Combination of which, is composed a particular Kind of fetid Salt, smelling like Urine, called the Volatile Salt or Volatile Alkali Salt of the Plant: And the Caput Mortuum remaining, being reduced into Ashes, is separated by Lixiviation into one Part of fixed Alcaly Salt, and another of insipid Alcaline Earth. Add that the essential salt always dissolves entirely in water, even the earthy part joined with it. But if the same salt has been robbed, by means of fire, of a great part of its acid, the earthy part will not wholly dissolve, but a sediment of insipid earth, indissoluble in water, will be found at the bottom. If an acid spirit is added to it, then it becomes entirely dissoluble in water. Hence it may be safely concluded that the other part of the ashes, before dissolved in the water, and which after evaporation appears in the form of a fixed lixivium salt, was only dissolved by virtue of the acid it contained, or as having retained enough of the acid to effect a dissolution.

Again, when the earth of the plant, saturated with its acid, becomes a crystallized salt, no more of the same acid can be introduced into it. Whereas the lixivious salt drawn from the ashes does not crystallize but still greedily imbibes the acid spirits.

Hence it may be probably concluded that the lixivious or fixed alkali salt is no other than the earth of the plant which, notwithstanding the violence of the fire, has retained a little portion of its acid salt, sufficient to dissolve it in water, still reserving a sufficient number of vacuoles or pores to lodge the first acid that shall offer itself, in lieu of that driven out of it by the fire. And as the name alkali is only given to a salt in respect of its imbibing and retaining an acid presented to it, in order to the producing a crystallized salt, the lixivious salts of plants may be said to be more or less alkali, as they absorb more or less of the acid, or which amounts to the same, as they contain more or fewer vacuoles to be filled with acids.

An alkali, after it has been fully satiated with one sort of acid, will yet sometimes admit and retain part of another acid. This is chiefly observed where a vegetable acid has been received first, and a fossil one is offered after. And it turns owing to this, that the vegetable acid having undergone a greater degree of fermentation in the body of the plant, has become rare and pervious, in respect of the more solid and weighty particles of the mineral acid, which therefore force their way in.

The same is always the case where an acid appears an alkali with respect to another acid; that is, where of two acid spirits, one whereof has a mixture of some alkali, the rarer of the two having possessed the pores of the alkali is compressed by the other denser acid. Thus, a pin cushion, though ever so full of cotton, will admit a good number of pins.

Now, urinous Salts are Alkalies as well as the Lixivious kind, i.e. they greedily imbibe Acids, retain them, and together with them compose Salts which crystallize. But their volatility seems to make it plain that they are not, like the former, a composition of a mere earthy matter with a little Acid, in regard a mere Earth can never become Volatile by such Admixture. Yet there is a great deal of reason to imagine that their composition is no other than a part of the same matter, which would have produced the Lixivious Salt, intimately mixed with a deal of the fetid oil of the plant, and that the oil is the sole cause of the volatility of these Salts.

M. Homberg, in his Essai de Sel Rincé, makes three classes of Acid Salts, corresponding to the three species of sulfurs wherewith the primitive Acids may be combined.

The first class consists of such as contain an animal or a vegetable sulfur, which amount nearly to the same. To this class belong all the distilled acids of plants, fruits, woods, etc., which must necessarily retain part of the oil of the plant, which is their sulfur. To this class also belongs Spirit of Nitre; as being a substance procured from the excrements of animals, etc.

The second class is of those which contain a bituminous sulfur. Such are Vitriol, common Sulfur, and Alum, which are all usually procured from a mineral stone, wherein bitumen is the prevailing ingredient.

The third is of such as contain a more fixed mineral sulfur, approaching the nature of a metalline one. Such are the acids drawn from Sea Salts and Sal Gemma's; the latter of which is chiefly found in places near mines of metals, and the former probably arises from rocks or veins of Sal Gemma running into the sea and there dissolved.

From the peculiar nature and properties of the sulfur thus accompanying the several kinds of Acid Salts, their different phenomena and effects are to be accounted for. See the article SALT.

Acids are doubtless chiefly derived from two sources, namely, those of vegetables and animals, and those of minerals. So that there should seem to be but one spring of acidity: the diversities arise from what happens to them in passing through the organized bodies of plants and animals. Hence it is that plants and animals, especially, yield a very volatile alkaline salt; whereas the salts of minerals are found altogether acid and much more fixed and concrete; though it is the same matter in both cases, under different assumed forms. Thus, the younger Lenicry argues that as animals feed on plants and reciprocally, in the instance of Salt-petre, etc., plants feed on animals; inasmuch as their vegetation is excited by manure; it happens that what was real Salt-petre in plants becomes only a nitrous Sal Ammoniac in animals, and vice versa. The same author accounts for this double metamorphosis by supposing that the nitrous principle remains the same in both cases, and in both cases is attached to the same matrix, with this only difference, that the matrix becomes more earthy in plants and by that means, fixed, and in animals loses its earthy parts and assumes other oily ones, which render it volatile. Mem. de l'Acad. An. 1717.

As to the manner in which acids act on alkalis, the great number of little bubbles produced during their action, and the heat arising thereupon, M. Homberg explains it thus: the matter of light, which he supposes to be the chymical principle, sulphur, and to possess the whole extent of the universe, is kept in perpetual motion by the continual impulses which the sun and fixed stars give it. But this motion, happening on some occasions to be slackened, may be renewed again, and augmented by the near approach of flame, which that author supposes the only matter capable of giving motion to light. This motion of light cannot proceed without continually striking against the solid bodies and even passing through all the porous ones it meets in its way. See SULPHUR and FIRE.

Suppose now, acids to be little, solid, pointed bodies, swimming at liberty in an aqueous fluid, and kept in continual motion by the repeated impulses of the matter of light; and alkalis to be spongy bodies whose pores have formerly been filled with the points of acids and which still retain the dents or impressions thereof, and are ready to receive the like points when driven within them. It is easy to conceive that if some of those porous alkalis float in the same liquor wherein the solid acids float, these latter, being impelled by the matter of light, will enter the cavities of the former, which are framed as it were on purpose for their reception. And that they will do it the more readily, if the motion of the matter of light wherewith they are impelled have been accelerated by external heat.

This introduction of acids into the body of alkalis is, in all appearance, effected with a great velocity and a deal of friction; inasmuch as it produces so considerable a degree of heat. And as the pores of the alkalis were before filled with an aerial matter, which is now expelled by the points of the acids, that air is put in motion and produces the bubbles, which are so much the more sensible, as the heat accompanying the action is the greater. See AIR and HEAT.

Sir I. Newton accounts for the effects of acids in a different manner, viz. from the great principle of attraction. See ATTRACTION.

"The particles of acids," he observes, "are of a size grosser than those of water and therefore less volatile; but much smaller than those of earth, and therefore much less fixed than they. They are endowed with a very great attractive force, wherein their activity consists; it being by this that they affect and stimulate the organ of taste; and by this also, that they get about the particles of bodies, either of a metalline or stony nature, and adhere closely to them on all sides; so as scarce to be separable from them by distillation or sublimation: and when thus gathered about the particles of bodies, by the same power, they raise, disjoin, and shake them one from another, that is, dissolve them." See DISSOLUTION.

"By their attractive force, also, wherewith they rush towards the particles of bodies, they move fluid ones, and excite heat; shaking asunder some particles, so as to turn them into air and generate bubbles; and hence all violent fomentation; there being in all fermentation a latent acid, which coagulates in precipitation." See FERMENTATION.

"Acids, also, by attracting water as much as they do the particles of other bodies, occasion the dissolved particles readily to mingle with water, or swim or float in it after the manner of salts. And as this globe of earth, by the force of gravity, attracting water more strongly than it does lighter bodies, causes those bodies to ascend in water and go upwards from the earth; so the particles of salts, by attracting the water, mutually avoid and recede from one another as far as they can, and are thus diffused throughout the whole water.

The particles of alkalies consist of earthy and acid parts united together; but these acids have so great an attractive force that they can't be separated therefrom by fire; and that they even precipitate the particles of dissolved metals by attracting from them the acid particles which before had dissolved and kept them in solution." See PRECIPITATION.

"If these acid particles be joined with earthy ones in a small quantity, they are so closely retained by the latter as to be quite suppressed and lost, as it were, in them. So that they neither stimulate the organ of sense nor attract water; but compose bodies which are not acid, i.e., fatty and sweet bodies; as Mercurius Dulcis, Brimstone, Luna Cornea, etc. From the same attractive force in these acid particles thus suppressed arises that property of fatty bodies, that they stick or adhere to almost all bodies and are easily inflammable. Thus, the acid that lies suppressed in sulphurous bodies, by more strongly attracting the particles of other bodies (earthy ones, for instance) than its own, promotes a gentle fermentation, produces and cherishes natural heat, and carries it on so far sometimes, as to the putrefaction of the compound. Putrefaction arising hence that the acid particles which have long kept up the fermentation, at length insinuate into the little interstices that lie between the particles of the first composition; and so intimately uniting with those particles, produce a new mixture or compound, which cannot be returned into its original form." See PUTREFACTION.

"Water has no great dissolving force because there is but a small quantity of acid in it; for whatever strongly attracts and is strongly attached may be reputed an acid. But in such things as are dissolved in water, the dissolution is slowly performed and without any effervescence."See WATER and MENSTRUUM.

"When these acids are applied to the insensible, excoriated part of the body, leaving the insensible earth wherewith they were before united, they rush into the sensory, as there as menstruums, and disjoin its parts, thus causing a painful sensation."The illustrious author, it must be owned, here carries the Notion of Acidity a great length: Dissolution, according to him, is only effected by Attraction and is proportional to the degree of attractive Power in the Dissolvent; but all Bodies which attract much are Acids, on his Principle, and consequently all powerful Menstruums must belong to that class. And yet, Spirit of Urine, which readily dissolves Iron or Copper, even in the cold, is allowed an Alkali; and accordingly makes a vehement clash with Aqua fortis.Boyle's Interests of Chym. Dobson's ed.

Some chemical Philosophers, in the last Century, endeavoured to derive the Qualities of Bodies and the other Phenomena of Nature from the Consideration of Alkali and Acid. See ALKALI.

It has been a point much controverted among the physicians, whether or not there be any sincere Acid in human Blood? The generality stand for the negative; and all Mr Boyle's experiments, in his History of Blood, seem to give the thing on that side. But the accurate M. Homberg has at last turned the scale the other way, and shown, by repeated experiments, that an Acid, or what is commonly called so, and judged such by the change of colour it causes in a tincture of violets, may be drawn from the Blood of all Animals in general, and human Blood in particular. Mem. de l'Acad. Roy. des Sciences. An. 1712.

Hence, and from the careful analyses that author has made of the Flesh and Excrements of divers Animals, particularly Man; he asserts that the Acid or Sea Salt of the Aliment taken into the Bodies of Animals is not destroyed therein, but passes into the Substance of them: the superfluous portion being returned unaltered along with the increments. See BLOOD, DIGESTION, etc.

Acids are prescribed in Medicine as Coolers, Antiseptics, Antiscorbutics, Diaphoretics, Alexipharmics, etc.See SCURVY, PLAGUE, etc.

"Acids," Mr. Boyle observes, "not only disturb the body while they continue sensibly acid, but in many cases create distempers, whereof they should seem the remedies.Though they be reputed to have an incisive and resolutive virtue and accordingly are prescribed to cut tough phlegm and dissolve coagulated blood, yet there are some acids which must evidently coagulate the animal fluids and produce obstructions, with all their train of consequences. Thus, it is known that milk readily curdles with spirit of sea salt, etc." See COAGULATION.