SULPHUROUS acid, was formerly call ed spirit of sulphur, and volatile sulphur ous acid. It was not till the yea; 1774 that its nature and composition dis covered by the labours of Priestley and Lavoisier. Berthollet afterwards inves tigated the formation, decomposition, combinations, and uses, of this acid. Fourcroy and Vauquelin also have exa mined many of its properties, especially the saline compounds which it forms, so that now its properties are well known. The sulphurous acid exists in nature in great abundance, and particularly in the neighbourhood of volcanoes. It is disen gaged from some lavas in a state of fu sion, and from the soil which is impreg nated with sulphur, when a sufficient de gree of heat is applied. It was by the vapours of sulphurous acid that Pliny the naturalist was suffocated in the erup tion of Mount Vesuvius, which destroyed Herculaneum, in the 79th year before the Christian era. When sulphur is burnt in the open air, the fumes that are gene rated by this slow combustion are sul phurous acid. It was in this way that this acid was formerly obtained. The method of procuring it, which is now fol lowed, is, to decompose the sulphuric acid by means of any substance which de prives it of part of its oxygen. If one part of mercury and two parts of concen trated sulphuric acid be exposed to heat in a glass retort, the mixture effervesces, and a gas is disengaged, which may be collected in jars over mercury In this process the mercury attracts part of the oxygen of the sulphuric acid, and leaves behind that portion which constitutes the sulphurout acid. Sulphurous acid thus obtained is in the state of gas, and it is an elastic, invisible, and colourless fluid, like Common air. It is rather more than dou ble the weight of atmospheric air. It reddens vegetable blues, and then de stroys the greater number of them. It is on account of this property that the fumes of sulphur are employed to remove the stains of fruit from linen, and that the sulphurous acid is often used in bleach ing. When the sulphurous acid is in the form of gas, it does not readily combine with oxygen. In its fluid form it unites more readily, and is converted into sul phuric acid. In making a mixture of sulphurous acid gas and oxygen gas pass through a red hot tube, they combine to and are converted into sulphuric acid. There seems to be no action be tween sulphurous acid and azotic gas.
Water has a strong attraction for sulphur ous acid gas. A piece of ice brought in contact with it is immediately melted, without any perceptible change of tempe rature. When water is saturated with this gas, it is known by the name of sul phurous acid, or liquid sulphurous acid. The specific gravity is 1.04. At the tem perature of 43° water combines with one third of its weight of sulphurous acid gas ; but as the temperature increases, it absorbs it in smaller proportion. It freezes at a temperature a few degrees below 32°, and it passes into the solid state with out parting with any of its acid. The li quid sulphurous acid has the smell, taste, and other properties of the gas, and par ticularly that of destroying vegetable co lours. When exposed to the atmosphere, it gradually absorbs oxygen, and passes into the state of sulphuric acid. This change goes on more rapidly when it is diluted with water, and agitated in con tact with the air. The sulphuric acid separates the sulphurous acid in the ga seous form from its combinations, and even frem water. Concentrated sulphu ric acid absorbs this has, which imparts to it a yellowish brown colour, and ren ders it pungent and fuming. The two acids strongly attract each other, so that when they are exposed to the action of heat, the first vapour which rises crystal lizes in long, white This is a compound of the two acids. It smokes in the air, dissolves with effer vescence in it, and when thrown into wa ter produces a hissing noise, like a red hot iron. Sulphurous acid is very much employed in the arts, and sometimes in medicine. In the state of gas it is used for the bleaching of silk and wool, by ex tracting the colouring matter. It re moves also the stains arising from vegeta ble juices, and spots of iron from linen. According to Fourcroy, 100 parts of this acid are composed of Sulphur 85 Oxygen 15 100 But accotding to the analysis of Dr. Thomson, Sulphur 68 Oxygen 32 100 The compound salts formed by this acid are denominated sulphites.
SUM, in mathematics, signifies the quantity that arises from the addition of two or more magnitudes, numbers, or quantities together. The sum of an equa tion is, when the absolute number being brought over to the other side of the equation, with a contrary sign, the whole becomes equal to 0: thus, the sum of the equation r3 — + = 42, is ri — 12 2" + 41 — 42 = 0.