§ 52. =Sulphur Trioxide, or Sulphuric Anhydride= (SO₃), itself may be met with in scientific laboratories, but is not in commerce. Sulphur trioxide forms thin needle-shaped crystals, arranged in feathery groups. Seen in mass, it is white, and has something the appearance of asbestos. It fuses to a liquid at about 18°, boils at 35°, but, after this operation has been performed, the substance assumes an allotropic condition, and then remains solid up to 100°; above 100° it melts, volatilises, and returns to its normal condition. Sulphuric anhydride hisses when it is thrown into water, chemical combination taking place and sulphuric acid being formed. Sulphur trioxide is excessively corrosive and poisonous.
Besides the above forms of acid, there is an officinal preparation called “Aromatic Sulphuric Acid,” made by digesting sulphuric acid, rectified spirit, ginger, and cinnamon together. It contains 10·19 per cent. of SO₃, alcohol, and principles extracted from cinnamon and ginger.
§ 53. Sulphuric acid, in the free state, may not unfrequently be found in nature. The author has had under examination an effluent water from a Devonshire mine, which contained more than one grain of free sulphuric acid per gallon, and was accused, with justice, of destroying the fish in a river. It also exists in large quantities in volcanic springs. In a torrent flowing from the volcano of Parcé, in the Andes, Boussingault calculated that 15,000 tons of sulphuric acid and 11,000 tons of hydrochloric acid were yearly carried down. In the animal and vegetable kingdom, sulphuric acid exists, as a rule, in combination with bases, but there is an exception in the saliva of the _Dolium galea_, a Sicilian mollusc.
§ 54. =Statistics.=--When something like 900,000 tons of sulphuric acid are produced annually in England alone, and when it is considered that sulphuric acid is used in the manufacture of most other acids, in the alkali trade, in the manufacture of indigo, in the soap trade, in the manufacture of artificial manure, and in a number of technical processes, there is no cause for surprise that it should be the annual cause of many deaths.
The number of deaths from sulphuric acid will vary, other things being equal, in each country, according to the manufactures in that country employing sulphuric acid. The number of cases of poisoning in England and Wales for ten years is given in the following table:--