SULPHURIC acid, in chemistry. The name of sulphuric acid is given to the combination of sulphur and of oxygen, with the greatest proportion of the latter. It was formerly called vitriolic acid, be cause it was obtained from vitriol by dis tillation, which is a compound of sulphu ric acid and an oxide of iron. When it is strongly concentrated, it is called oil of vitriol. If a quantity of flowers of sul phur be exposed to a degree of heat suf ficient to inflame it, and if, when it is in a state of ignition, it be introduced into a jar filled with oxygen gas, it burns with great splendour, and emits a great quan tity of white fumes. These fumes may be condensed, by pouring a small quantity of water into the jar, and when this is ex amined, it is found to possess acid proper ties. This .is the sulphuric acid. It is procured, as appears by this experiment, by burning sulphur in oxygen gas.
The process for obtaining sulphuric acid, in the large way, is the following. A mixture of sulphur and nitre is burnt in leaden chambers. The use of the ni tre is, to supply a quantity of oxygen for the combustion of the sulphur. There is a little water in the bottom of the vessel, which serves to condense the vapours given out during combustion.
The acid which is obtained in this way is very weak, for it is diluted with the water in which it was condensed, which water may be separated by distillation. Even after this it is usually contaminated With a little lead from the vessels, some potash, and sometimes nitric and sulphur ous acids. To obtain it perfectly pure, the sulphuric acid of commerce must be distilled. This process is conducted by putting a quantity of the acid into a re tort, and exposing it to a degree of heat sufficient to make it boil. The beak of the retort is put into a receiver, in which the acid, as it come over, is condensed. The acid thus purified, is a transparent colourless liquid, of an oily consistence. It has no smell, but a strong acid taste. It destroys all animal and vegetable sub stances. It reddens all vegetable blues. It always contains water. When this is driven off by a moderate beat, the acid is said to be concentrated. When as much concentrated as possible, the specific gra vity is 2, or double that of iwater ; but it can rarely be obtained of greater density than 1.8. The sulphuric acid suffers-no change from being exposed to the light. It bolls at the temperature of 546°, or, according to Bergman, 540°. When this acid is deprived of its caloric, it is suscep tible of congelation, and even of crystalli zation, in fiat, six-sided prisms, terminat ing in a six-sided pyramid It crystallizes most readily, when it is neither too much concentrated, nor diluted with water. Of the specific gravity of 1.6, it crystallizes at the temperature of a few degrees be low the freezing point of water. Of the specific gravity of 1.8, it resists the greatest degree of cold. Chaptel observ ed it crystallize at the temperature of48°, and Mr. Reir found that it froze at 45° of the specific gravity of 1.78. Sulphuric acid has a strong attraction for water.
In some experiments that have bees made, sulphuric acid, when exposed to the atmosphere, attracted above six times its weight of water. When four parts of concentrated sulphuric acid, and one part of ice at the temperature of 32°, are mix ed together, the moment they come is contact the ice melts, and the tempera ture rises to 212°. A greater quantity
of caloric is given out, when the two bo dies are mixed together in the liquid state. If four parts of the acid and one of water are suddenly mixed together, the temperature of the mixture rises to about 300°. This extrication of caloric, it is obvious, arises from the sudden conden sation of the two liquids, the medium bulk of which is considerably less than the two taken together. So great is the attraction of this acid for water, that the strongest that can be prepared can scarce ly be supposed to be entirely free from it. Attempts have been made to determine the proportion of oxygen and sulphur, which enter into the composition of sul phuric acid. According to the experi ments of Lavoisier, in which he measur ed the quantity of oxygen absorbed by a *yen weight of sulphur during combus tion, the proportions are, Sulphur 71 Oxygen 29 100 mom But other methods have been adopted. These are, by decomposing other sub stances which contain oxygen, by means of sulphur. According to the experiments of M. Chevenix, conducted in this way, the sulphuric acid consists of, Sulphur 61 5 Oxygen 38.5 100.0 Sulphuric acid does not combine with oxygen, nor has it any action with azotic gas. It appears that hydrogen has a great er affinity for oxygen than the sulphur has, and therefore the sulphuric acid is decomposed by means of hydrogen gas. In the cold there is no action between hydrogen gas and sulphuric acid; but if they are made to pass through a red hot porcelain tube, the acid is decomposed, water is formed, and sulphur is precipitat ed. When hydrogen gas is employed in a greater proportion than the half of the acid, the superabundant gas dissolves the sulphur, and is disengaged in the form of sulphurated hydrogen gas. Charcoal has no action on sulphuric acid in the cold; but at the boiling temperature it decom poses it, and converts it into sulphurous acid. If a piece of red hot charcoal be immersed in a quantity of concentrated sulphuric acid, part of the acid is sudden ly disengaged under the form of thick white fumes, accompanied with sulphur ous acid gas. The sulphuric acid is de composed ; part of its oxygen is attracted by the charcoal, forming carbonic acid, and thus it is reduced to the lowest pro. portion of oxygen, in the state of sulphur ous acid. A similar effect is produced by phosphorus. Phosphorus, with the assistance of heat, partially decomposes the sulphuric acid, by abstracting part of its oxygen. Phosphoric acid is formed, and sulphurous acid driven off'. In the cold, sulphur has no action on sulphuric acid ; but when they are boiled together, the sulphur is partly dissolved in the acid, and converts it into sulphurous acid. The sulphur which has been added com bines with the oxygen, which is necessary for the constitution of sulphuric acid, and thus the whole is converted into sulphur ous acid. Sulphuric acid combines with alkalies, the earths, and the metals form ing salts ; which in the present language of chemistry are denominated sulphates. This acid is employed in great quantity in many arts and manufactures. It is em. ployed also in medicine and pharmacy ; the preparation of it, therefore, has long been an object ofconsiderable importance.