About the middle of the eighteenth century, “Dr.” Ward took out a patent for the manufacture of sulphuric acid, to be carried on at Richmond in Surrey. He used large glass bell jars of about 40-50 galls. capacity, in which he placed a little water and a flat stone to support a red-hot iron ladle. A mixture of saltpetre and sulphur was thrown into the ladle and the mouth of the vessel quickly closed. After the vigorous chemical action was over, the ladle was re-heated and the process repeated until at last fairly concentrated sulphuric acid was produced. The large glass vessels used by Ward were costly and easily broken. They were soon replaced by chambers about 6 ft. square, made of sheet lead, but otherwise the process was just the same. The next advance consisted in making the process continuous instead of intermittent. An enormously increased output was thereby rendered possible, and the main features of the modern process gradually developed. The Lead Chamber Process. We can now consider the actual working of the lead chamber process, aided by the diagrammatic plan of the works shown in Fig. 2. Sulphur dioxide is produced in a row of kilns (A-A) by burning iron pyrites in a carefully regulated current of air. The mixture of gases which leaves the pyrites burners contains sulphur dioxide, excess of oxygen, and a very large quantity of nitrogen. To this is added the vapour of nitric acid, generated from sodium nitrate and concentrated sulphuric acid contained in the “nitre pots,” which are placed at B. The mixture of gases then passes up the Glover tower (C) and through the three chambers in succession, into the first two of which steam is also introduced. Sulphuric acid is actually produced in the chambers, and collects on the floors, from which it is drawn off from time to time. The residual gas from the last chamber is passed up the Gay Lussac tower (D), and after that is discharged into the air by way of the tall chimney (J). [Illustration: Fig. 3. GENERAL VIEW OF SULPHURIC ACID WORKS] The Oxygen Carrier. We have seen that sulphur dioxide, oxygen, and water are the only substances required to produce sulphuric acid. Why, then, is the nitric acid vapour added to the mixture? As described in a former paragraph, the combining of these gases was represented as being a very simple operation. So indeed it is, for it even takes place spontaneously. Yet, as a commercial process, it would be quite impracticable without the nitric acid vapour, for although the gases combine spontaneously, they do so very slowly, and it is the nitric acid vapour which accelerates the rate of combination.