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which is volatilized in oxygenated muriatic acid gas. The salt remains colourless. An augmentation of temperature alone is sufficient for the yellow acidulous sulphate of cerium to lose its colour, by losing the excess of its oxygen. If the heat is increased still more, the surplus of the acid is carried off, and a saturated sulphate of cerium remains. By a continued calcination, it regains oxygen, becomes red, and yields a sulphate of cerium at a maximum. The sulphate of cerium, disoxygenated by the muriatic acid, is more difficult to re-oxydate by calcination.

In the humid way, the alkalies only decompose the sulphate of cerium incompletely.

If a solution of acidulous sulphate of cerium is precipitated by potash, a triple combination of cerium, sulphuric acid, and potash, is separated, before the acid is saturated. If too much potash is added, the combination is partly destroyed. The sulphate of cerium and potash, at a maximum, is of an orange colour; that which is at a minimum is white.

These characters offer a ready method of separating the sulphate of cerium from iron. It must, however, be observed, that when the solutions are saturated, a little iron is also deposited, which gives a yellow colour to the precipitate ; but by adding a little sulphuric acid in excess, the iron is re-dissolved, and leaves the precipitate entirely white. This combination is only dissolved in part by dilute sulphuric acid, and the greatest quantity of that separates afterwards.

SECTION XXIX.

OF TANTALIUM.

As the existence of this metal is questionable, although announced as such by Ekeberg, and supposed to be an oxyd of tin, we shall not trouble the reader with its history or properties.

PART XVI.

OF ACIDS.

Acids are a class of bodies, which possess a sour taste, and certain determinate properties. Substances partaking of the following characters, are considered under this head.

1. When applied to the tongue, they excite that sensation, which is called acid or sour.

2. They change the blue colour of vegetables, as the infusion of litmus and syrup of violets, to a red. If these colours have been previously converted to a green by alkalies, the acids restore them again. This effect is apparent in the following experiments.

Experiment 1. Make an infusion of red roses, violets, or mallow flowers; treat it with solution of potash, and it will become green; the addition of diluted muriatic acid will convert it immediately to a red.

Experiment 2. Pour a little tincture of litmus into a wine glass, and into another some diluted sulphate of indigo; pour these two blue fluids together, and the mixture will become perfectly red.

Experiment 3. Take a slip of blue litmus paper, dip it into acetous acid, and it will immediately become red.

Experiment 4. Take some water impregnated with carbonic acid, and add to it a little blue tincture of litmus, the whole will be changed to a red.

3. They unite with water in indefinite proportions.

4. They combine with alkalies, earths, and metallic oxyds, and form with them a class of bodies called salts.

Experiment 5. Take some of the same carbonated water, and boil it. Then add a little tincture of litmus, and the blue colour will experience no change.

Not to state the opinions, or the different hypo. theses which have been advanced, we shall observe,

that, according to the theory of Lavoisier, all acids consist of a certain base united to oxygen, which is considered to be the cause of acidity. We are acquainted with the bases of the sulphuric, nitric, &c. acids: we know that hydrogen, carbon, and oxygen, form the bases of the citric, malic, and all the rest of the acids called vegetable; and that the same substances, in combination with nitrogen, constitute those called animal acids, such as prussic, &c. But we are totally unacquainted with the radicals of some acids; they must therefore be analogically considered as simple bodies in the present state of our knowledge.

The bases which become acid by the union of oxygen are capable of being united to it in various degrees. When the oxygenation is complete they are called perfect acids. But if they are not saturated with oxygen, they mostly appear in a state of less acidity. In the new nomenclature the termination IC is given to the complete oxygenation of these bodies, and the termination OUS when they present themselves in an imperfect state of combination with oxygen. Hence by the name of sulphuric acid, we mean sulphur completely saturated with oxygen; and by the name sulphurous acid, we understand sulphur not saturated with oxygen.

All acids are either products of combustion, or supports of combustion, or combustible. We shall not, however, confine ourselves to the division,

SECTION I.

OF SULPHURIC ACID.

Experiment 1. If sulphur be burnt in a close vessel in contact with nitre, and the vapour collected in water, the product will be diluted sulphuric acid ; and,

Experiment 2. If this be concentrated by evaporation, the residue will be strong sulphuric acid.

Remark. Sulphur we observed is capable of combining with oxygen, so as to form either an oxyd, or the sulphurous or sulphuric acid. In the two last the proportion of oxygen varies. In using nitre, the object is to furnish oxygen from that salt, and to oxyge. nate, or in other words to acidify the sulphur completely: this is effected in the large way as follows:

Peculiar buildings or apartments, with their insides lined with lead, are constructed. In these apartments sulphur is burnt, in order to absorb oxygen from the air during its combustion. But as the sulphur would not burn or become fully acidified in a confined quantity of air, fth to sth of nitrate of potash is previously mixed with it. The sulphuric acid which is formed is absorbed by a stratum of water at the bottom of the chamber. The water, after being sufficiently imprego nated, is exposed to heat in large glass retorts, so as to deprive the acid of the superfluous water, the sulphurous acid, and nitrous gas, with which it is mixed.

This acid has been obtained from vitriol by distillation; hence it was called oil of vitriol, and afterwards vitriolic acid. The method of obtaining it, by burning sulphur under a glass bell gave rise to the name of spio ritus or oleum sulphurus per campanum.

Sulphuric acid, when fully concentrated, is said to be 2.000 specific gravity; but it seldom exceeds 1.8. It changes all vegetable blues except indigo. It boils at 546°. When exposed to a sufficient degree of cold, it crystallizes. It has a considerable attraction for water, and in its union with it, it emits a large quantity of free caloric, as is shown by the following experiment:

Experiment 3. Weigh one pound of water into a vessel capable of bearing a sudden heat. Pour gradually four pounds of sulphuric acid upon the water, and stir the mixture. So great will be the condensation of the water by this mixture, that the temperature of the whole will instantly rise to 300°

Sulphuric acid is not altered by oxygen, azote, or hydrogen. It combines with nitric acid, and forms

compound which dissolves silver, but scarcely any other metal. Boiled with sulphur, phosphorus, or charcoal, it is partly disoxygenized, and converted into sulphurous acid. See the Preparation of Sulphurous Acid Gas. Its action on zinc or iron, in a diluted state, is manifest in the production of hydrogen gas. See the Preparation of Hydrogen Gas. "Its union with different bases, constitutes sulphates. Its composition is 42.3 sulphur, and 57.7 oxygen in the hundred.

Sulphuric acid is detected by muriate of barytes. See Salts of Barytes.

SECTION II.

OF SULPHUROUS ACID.

Experiment 1. To prepare sulphurous acid, take one part of mercury and four of concentrated sulphuric acid, put them into a retort, furnished with a receiver, and connected with Pepy's apparatus. The sulphurous acid gas which is disengaged is absorbed by the water in the vessel, and constitutes sulphurous acid.

Experiment 2. Sulphurous acid is likewise formed during the slow combustion of sulphur. See Sulphur.

Remark. Stahl gave to this acid the name of phlogisticated sulphurous acid. Scheele obtained it by decomposing sulphite of potash by tartaric acid.

The acid properties of sulphurous acid are not considerable. It does not redden blue vegetable colours, but, on the contrary, renders them white. It exerts little action on metallic substances, and has a feeble attraction for the alkalies and earths. It is exceedingly volatile, and assumes the state of gas, if moisture be excluded.

Sulphurous acid unites with certain bases, and forms sulphites.

See Sulphurous Acid Gas.

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