Further, the lead burners and plumbers are exposed to the danger of chronic lead poisoning from insufficient observance of the personal precautionary measures necessary to guard against it (see Part III). Those who are constantly engaged in burning the lead sheets and pipes of the chambers suffer not infrequently from severe symptoms. Unfortunately, the work requires skill and experience, and hence alternation of employment is hardly possible. Finally, mention should be made of poisoning by _arseniuretted hydrogen gas_ from vessels filled with sulphuric acid containing arsenic as an impurity, and by sulphuretted hydrogen gas in purifying the acid itself. In the manufacture of liquid _sulphur dioxide_, injury to health can arise from inhalation of the acid escaping from the apparatus. The most frequent cause for such escape of sulphur dioxide is erosion of the walls of the compressor pumps and of the transport vessels, in consequence of the gas being insufficiently dried, as, when moist, it attacks iron. Sulphur dioxide will come up for further consideration when describing the industrial processes giving rise to it, or in which it is used. HYDROCHLORIC ACID, SALTCAKE, AND SODA MANUFACTURE.—The production of hydrochloric acid (HCl), sodium sulphate (Na₂SO₄), and sodium sulphide (Na₂S) forms part of the manufacture of soda (Na₂CO₃) by the Leblanc process. The products first named increase in importance, while the Leblanc soda process is being replaced more and more by the manufacture of soda by the Solvay ammonia process, so much so that on the Continent the latter method predominates and only in England does the Leblanc process hold its ground. Health interests have exercised an important bearing on the development of the industries in question. At first, in the Leblanc process the hydrochloric acid gas was allowed to escape into the atmosphere, being regarded as a useless bye-product. Its destructive action on plant life and the inconvenience caused to the neighbourhood, in spite of erection of high chimneys, demanded intervention. In England the evils led to the enactment of the Alkali Acts—the oldest classical legislative measures bearing on factory hygiene—by which the Leblanc factories were required to condense the vapour by means of its absorption in water, and this solution of the acid is now a highly valued product. And, again, production of nuisance—inconvenience to the neighbourhood through the soda waste—was the main cause of ousting one of the oldest and most generally used methods of chemical industrial production. Although every effort was made to overcome the difficulties, the old classical Leblanc process is gradually but surely yielding place to the modern Solvay process, which has no drawback on grounds of health. We outline next the main features of the _Leblanc soda process_, which includes, as has been mentioned, also the manufacture of hydrochloric acid, sodium sulphate and sulphide.