OF ACIDS.

WHAT is an acid ?*

Most of the acids are substances which produce that sensation on the tongue which we call sour† but some substances are classed with the acids which have not this characteristic--though they possess some of the other properties of acids.

What are the properties of acids?

Acids change the blue, green, and purple

*The acids differ from each other in their appearance and properties as much as any class of bodies we are acquainted with; it is therefore difficult to give a definition of an acid. In general they are liquids, but some of them take a solid, and others a gaseous form; some are mild, others corrosive; some are pungent and volatile, others are mixed and inodorous.

† According to Fourcroy, the stronger the attraction of oxygen for the acidifiable radical, the weaker is the taste of the acid: this shows that the corrosive quality of the stronger acids is owing to the easy separation of this principle, and its more or less rapid transmission to animal substances.

It is desirable as soon as possible to give the chemical student correct ideas of the properties of the acids and alkalies. To this end let him be early instructed in the use of chemical tests. If he be accustomed to carry a few test papers in his pocket-book, it will be a very rational amusement to try the succulent vegetables which he will meet with in his walks, many of which will be found to contain acids of different kinds. The hope of making an important discovery will furnish an additional zest to this employment. Litmus paper is a good test for acids; and the same paper when reddened by vinegar, and afterwards dried, is a proper test for alkalies. Should litmus not be at hand, common writing-paper rubbed over with the rind of the radish will answer every purpose.

juices of vegetables to red;* and combine with alkalies,† earths, or metallic oxides,‡ so as to form those compounds called salts.

What is the origin of acids?

Most of the acids owe their origin to the combination of certain substances with oxygen, which has been called the acidifying principle.

*This is so general, that we know of only one exception. Indigo may be dissolved in concentrated sulphuric acid, and yet its original blue colour will be preserved.

† All those substances which can saturate the alkalies, and cause their properties to disappear, ought to be classed among acids.

From some late galvanic experiments of Sir Humphrey Davy, it appears that acids combine with alkalies and metallic oxides in consequence of their being in opposite states of electricity, that is, that acids are naturally negative, and metals and alkalies positive,-and that hence an union would take place between them, independently of any other power than electricity. He has also demonstrated, that, if an acid be put in a state of positive electricity, it refuses to combine with an alkali, and that, when an alkali is made negative, it refuses to combine with acids, as may be seen in the Philosophical Transactions for 1807, page 1. A brief abstract of the experiments will be found in the Additional Notes to this volume, No. 58. However, since the publication of this paper, Sir Humphrey Davy has said that "he does not suppose that chemical changes are OCCASIONED BY electrical changes, but that these are distinct phenomena, produced by the same power, acting in one case on masses, in the other case on particles." Davy's Chemical Philosophy, vol. i. page 165.

The substances which are combined with oxygen to form acids are (in all the decomposable acids) combustible substances. Indeed, several of the acids are the product of combustion: witness the sulphuric, the phosphoric, &c. Four of the mes tals, and all the other simple combustibles, except hydrogen, are convertible into acids. All bodies, to which the properties of an acid have been ascribed, are either combustibles, supporters of combustion, or may be produced by the process of combustion.

It is proper to remark that some of the acids are the productions of art, and are not known to exist in nature. This is the case with the mucous, the suberic, &c.

How is it known that oxygen imparts acidity?

This is built upon analogy; for it is found that most of the acids contain oxygen, and that they lose their acidity exactly in proportion to the quantity of oxygen which is taken from them.*

Are there any other means of ascertaining this? Yes: some acids may be decomposed, and deprived of their oxygen, and others may be formed by a direct combination of oxygen with certain radicals.t

Do the same radicals always combine with an equal portion of oxygen?

No: some of these acidifiable radicals com

Many of the acids may be decomposed and deprived of their oxygen, by combustible bodies. Any combustible body, that has a greater affinity for oxygen than oxygen has for the radical of the acid, will decompose that acid. Charcoal, when made red hot, will in this way decompose sulphuric acid.

+ This is shown by the composition of sulphuric acid, which, for experiment, may be formed thus: Mix a little sulphur coarsely powdered with one eighth of its weight of ground nitre. Put the mixture upon a small tile, and place the tile upon a low stand in the middle of a large flat glass or earthen dish. Then pour a small quantity of water into the dish, and procure a large glass jar to invert over the brimstone, so as that its edges shall dip into the water, which must be deep enough to form a water lute. Things being thus prepared, the brimstone is to be set on fire in several places, by means of a red-hot iron, and immediately covered with the jar. The sulphur will burn with great rapidity, and will be chiefly converted into sulphuric acid, which may be concentrated by evaporating the superfluous water. In this process the sulphur unites with the oxygen of the atmospheric air within the jar, at the same time that the heat and light, which are unnecessary in the new compound, are evolved, and become sensible to our feelings: the result is a new substance widely different from either, viz., sulphuric acid. The use of the water is to absorb the gas, and render it liquid.

bine with different proportions of oxygen, and consequently produce different states of acidity.* How do chemists distinguish this difference?

When two acids have the same radical, but contain different quantities of oxygen, they are distinguished by their termination. The name of that which contains most oxygen ends in Ic, the other in ous. Thus we say sulphuric acid, and sulphurous acid; phosphoric acid, and phosphorous acid.

Have acids ever any other degree of acidity?

Attempts have been frequently made to supersaturate several of the acids with oxygen, but in only one instance with success.

Which of the acids has been thus supersaturated with oxygen?

The muriatic acid, forming with two different

The first portion of oxygen converts some bodies into oxides; the second, into that class of acids of which the specific names drawn from their particular bases terminate in ous, as the sulphurous acid; the third degree of oxygenizement changes some of these into that division of acids which are distinguished by the termination in ic, as the sulphuric acid; and lastly, we can express a fourth degree of oxygenizement by adding the word oxygenized to the name of the acid, as oxygenized muriatic acid.

The term OXYGEN is derived from the Greek words oug yen, signifying that which produces or generales acids: but as Sir Humphrey Davy has discovered that this substance is necessary also to the production of the alkalies, some other term, surely, should be found that would be more applicable to this wonderful invisible agent! Had the framers of the new nomen*clature happened to have called it hydrogen, no objection, that I can perceive, could have been made to it.

I have not thought it advisable to make any alteration in

portions of oxygen, two other distinct acids, called oxygenized muriatic acid,* and hyperoxygenized muriatic acid.t

What substances are capable of being acidified by oxygen?

The mineral, the vegetable, and the animal kingdoms, all furnish bases or radicals, which become acid by their union with oxygen.‡

Do all the acids owe their acidity to the presence of oxygen?

The greater number of the acids are evidently indebted to oxygen for their acidity, but there

this part of the text, at present; but if Sir Humphrey Davy's opinion, that oxymuriatic acid is a simple substance, should be verified, we must have new terms for this acid and its combinations. On account of its yellowish green colour when combined with water, he has named it Chlorine.

* Though this compound has hitherto been classed among acids by chetnists, it possesses few properties which characterize that class of bodies. Its taste is not acid, but astringent; it does not convert vegetable blues to red, but destroys them, and combines very sparingly with water.

For an account of the properties of this acid see the chapter of Salts, article hyperoxygenized muriates.

The mineral acids are generally formed with a peculiar base and oxygen; the vegetable acids, with carbon, hydrogen, and oxygen; while the animal acids are composed of the same substances united with nitrogen.

Some of the mineral acids are decomposable by charcoal heated to redness. Some of the vegetable acids are also decomposed, and reduced into water and carbonic acid, by leaving them in an exposed situation to the action of their own principles: others may be changed into different acids, by imparting or abstracting a portion of oxygen.

The animal acids are of all others the most liable to decomposition. In an elevated temperature the carbon and oxygen unite to form carbonic acid, and the hydrogen and nitrogen produce volatile alkali.