SULPHUROUS ACID 963 



imported in stoneware bottles, having a stoneware screw for a stopper. It is probably 

 only a solution of anhydrous sulphuric acid in ordinary oil of vitriol, as, after being 

 subjected to a gentle heat, nothing remains but the latter. It often contains several 

 impurities. It is principally used for dissolving indigo, which it does completely 

 without destroying the colour. 



Ordinary Sulphuric Add, or OU of Vitriol. HO.SO 3 (H 2 SO 4 ). Sp. gr. 1-845. This is, 

 when pure, a colourless, transparent, highly acrid, and most powerfully corrosive 

 liquid. It is a very strong mineral acid, one drop being sufficient to communicate the 

 power of reddening litmus-paper to a gallon of water, and produces an ulcer if placed 

 upon the skin. It chars most organic substances. This depends upon its attraction 

 for water, which is so great that, when exposed in an open saucer, it imbibes one-third 

 of its weight from the atmosphere in twenty-four hours, and fully six times its 

 weight in a few months. Hence it should be kept excluded from the air. If four 

 parts, by weight, of the strongest acid be suddenly mixed with one part of water, 

 both being at 50 Fahr., the temperature will rise to 300 Fahr. ; while, on the other 

 hand, if four parts of ice be mixed with one of sulphuric acid, they immediately liquefy 

 and sink the thermometer to 4 below zero. In this last case the heat, that would 

 otherwise have been given off, has been employed in liquefying the ice. Upon the 

 mixing the acid and water they both suffer condensation, the dilute acid, thus formed, 

 occupying less space than the two separately, and hence the evolution of heat. This 

 affinity for water, which sulphuric acid possesses, is often made use of for evaporating 

 liquids at a low temperature. The liquid is placed in a dish over another dish con- 

 taining sulphuric acid, and both are placed under the receiver of an air-pump. 

 Such is the rapidity with which the evaporation is carried on, that if a small vessel 

 of water be so placed it will speedily be frozen. Sulphuric acid is decomposed by several 

 substances when boiled with them ; such are most organic substances, sulphur, phos- 

 phorus, and several of the metals, as mercury, copper, tin, &c. 



Sulphuric acid of sp. gr. 1 -845, boils at about 620 Fahr., and may be distilled 

 unchanged. This is the best way to obtain it pure. It is a most powerful poison. 

 If swallowed in its concentrated state, even a small quantity, it acts so powerfully on 

 the throat and stomach as to cause intolerable agony and speedy death. Watery 

 diluents, mixed with chalk or magnesia, are the readiest antidotes. 



Ordinary oil of vitriol generally contains some sulphate of lead, which will be 

 precipitated, as a white powder by dilution with water ; since so much of it is made 

 from iron pyrites at the present day, it contains arsenic in variable quantities. The 

 best test for sulphuric acid, either free or combined, as soluble salts, is a salt of 

 barium. An extremely small quantity of sulphuric acid, or a soluble salt of it, is 

 thus easily detected by the greyish-white cloud of sulphate of baryta which" it 

 occasions in the solution. 100 parts of the concentrated acid are neutralised by 143 

 parts of dry pure carbonate of potash, and by 110 of dry pure carbonate of soda. 



The presence of saline impurities in sulphuric acid may be determined by evapo- 

 rating a certain quantity to dryness in a platinum capsule. If more than 2 grains of 

 residue remain out of 500 of acid it may be considered impure. 



Of all the acids, the sulphuric is most extensively used in the arts, and is, in fact, 

 the primary agent for obtaining almost all the others, by disengaging them from their 

 saline combinations. In this way nitric, hydrochloric, tartaric, acetic, and many 

 other acids, are procured. It is employed in the direct formation of alum, of the 

 sulphates of copper, zinc, potassa, soda ; in that of sulphuric ether, of sugar by the 

 saccharification of starch, and in the preparation of phosphorus, &c. It serves also 

 for opening the pores of skins in tanning, for clearing the surfaces of metals, for 

 determining the nature of several salts by the acid characters that are disengaged, &c. 



The Table on next page, which shows the quantity of concentrated and dry sulphuric 

 acid in 100 parts of dilute, at different densities, was constructed by Dr. Ure. 



SULPHUROUS ACID. (SO 2 .) Sulphur fumigations are mentioned by Homer, 

 but sulphurous acid, of which these are composed, was first accurately examined by 

 Stahl, Scheele, and Priestley, and afterwards by Gay-Lussac and Berzelius. 



It escapes from the earth, in the gaseous form, in the vicinity of volcanoes, but is 

 always prepared artificially when required for use, and for this purpose several pro- 

 cesses are employed. 



1. By heating copper-cuttings, or mercury, with concentrated sulphuric acid in a 

 glass flask, sulphate of copper, or persulphate of mercury, and sulphurous acid are 

 formed : 



Cu + 2(HO.S0 3 ) = CuO.SO 3 + SO 3 + 2HO. 



Copper. Sulphuric acid. Sulphate of copper. Sulphurous acid. Water. 

 Cu + 2H-S0 1 = CuSO 1 + SO 2 + 2H O. 



S Q 2 



