The alkalies potash and soda, the alkaline earths, and several metallic oxides, may be combined with sulphurous acid, and they form sulphites ; but these compounds are not of much importance, except perhaps that of soda. [Sown!, sulphite of.] In contact with air they are slowly converted into sulphates.
Sulphuric Acid (HO, SO,).—This acid has been long known, and is very extensively employed, 100,000 tons being annually consumed in Great Britain alone. When combined with water, in which state it is best known, it was originally, and still is frequently, called oil of vitriol: first, because it is an oily looking liquid ; and secondly, on account of its being obtained from green vitriol, or copperas, now called sulphate of iron.
The process just alluded to was first followed at Nordhausen in Germany, and the peculiar compound of sulphuric acid and water pro duced by it is still called Nordhausen oil of vitriol, to distinguish it from common oil of vitriol, a different compound as regards the water which they contain, and obtained by a different process. We shall first describe the original process and product.
Iron pyrites is a well known and very abundant natural substance : it is correctly termed bisulphide of iron, and consists of two equivalents of sulphur and one equivalent of iron. When certain varieties of this compound arc exposed to air and moisture, both the sulphur and iron are oxidised at the expense of the oxygen of the air ; and though sulphur by itself is incapable of undergoing this change, yet, when combined with iron, it acquires from the air sufficient oxygen to become sulphuric acid, and the iron attracts enough to become protoxide ; and these combining together, and with water, constitute the well-known crystalline body, hydiated sulphate of protoxide of iron, usually called, for brevity's sake, merely sulphate of iron, and originally green vitriol. This consists of one equivalent of sulphuric acid, one of protoxide of iron, and seven equivalents of water. Sul phuric acid consists of three equivalents of oxygen and one equivalent of sulphur. In order to procure the acid from it, the salt is mode rately heated, so as to expel the greater part of the water : in this state it is put into earthen retorts, and subjected to a. very high tempe
rature, during which there conies over and condenses in the receiver a dark-coloured dense fluid, which is the Nordhausen oil of vitriol ; the cause of the colour has not been ascertained, but it appears to be an accidental and not a necessary accompaniment. This liquid emits a white vapour when exposed to the air, and hence it is called fuming sulphuric acid. It is composed of two equivalents of sulphuric acid and one equivalent of water (HO, 2S0,).
Now it happens that anhydrous sulphuric acid is more volatile than that combined with water, so that when the above acid is heated in a retort, there first comes over and condenses in the receiver anhydrous sulphuric acid, and there remains in the retort hydrated sulphuric acid.
We shall first and briefly state the properties of the anhydrous acid. It is a colourless crystalline solid, is tough and elastic, liquefies at 66°, and boils at a temperature between 104' and 122°, forming a trans parent vapour, provided no water is present ; it unites with moisture when exposed to the air, and forms with it dense white fumes. It is sometimes prepared as a matter of curiosity, but is hardly applied to any use. Though called an acid, it in reality possesses no acid pro perties, and may be moulded in the fingers, liko wax, without injuring the skin. When thrown into water it hisses as red-hot iron would do, and then has the usual powerful properties of common hydrated !sulphuric acid.
The hydrated acid, commonly called oil of vitriol, or simply sulphuric acid, is the compound which is so largely employed in nume rous chemical operations and manufactures. It is, however, and has indeed for many years been, prepared in a much preferable mods to that described by the decomposition of !sulphate of iron. The process consists in oxidising sulphurous acid through the agency of nitrous acid.
First, nitrate of potash or soda is decomposed by the addition of sulphuric acid, in the same mode as that employed for preparing nitric acid.