Production of sulphuric acid by the _contact method_ depends on the fact that a mixture of sulphur dioxide and excess of oxygen (air) combines to form sulphur trioxide at a moderate heat in presence of a contact substance such as platinised asbestos or oxide of iron. The sulphur dioxide must be carefully cleaned and dried, and with the excess of air is passed through the contact substance. If asbestos carrying a small percentage of finely divided platinum is the contact substance, it is generally used in the form of pipes; oxide of iron (the residue of pyrites), if used, is charged into a furnace. Cooling by a coil of pipes and condensation in washing towers supplied with concentrated sulphuric acid always forms a part of the process. A fan draws the gases from the roasting furnaces and drives them through the system. The end product is a fuming sulphuric acid containing 20-30 per cent. SO₃. From this by distillation a concentrated acid and a pure anhydride are obtained. From a health point of view it is of importance to know that all sulphuric acid derived from this anhydride is pure and free from arsenic. The most important _uses_ of sulphuric acid are the following: as chamber acid (110°-120° Tw.) in the superphosphate, ammonium sulphate, and alum industries; as Glover acid (140°-150° Tw.) in the Leblanc process, i.e. saltcake and manufacture of hydrochloric acid, and to etch metals; as sulphuric acid of 168° Tw. in colour and explosives manufacture (nitric acid, nitro-benzene, nitro-glycerine, gun-cotton, &c.); as concentrated sulphuric acid and anhydride for the production of organic sulphonic acids (for the alizarin and naphthol industry) and in the refining of petroleum and other oils. Completely de-arsenicated sulphuric acid is used in making starch, sugar, pharmaceutical preparations, and in electrical accumulator manufacture. EFFECTS ON HEALTH.—The health of sulphuric acid workers cannot in general be described as unfavourable.