~Preparation of acids.~ The principle of reversible reactions finds practical application in the preparation of most of the common acids. An acid is usually prepared by treating the most common of its salts with some other acid of high boiling point. The mixture is then heated until the lower boiling acid desired distills out. Owing to its high boiling point (338 deg.), sulphuric acid is usually employed for this purpose, most other acids boiling below that temperature. EXERCISES 1. What would take place when solutions of silver nitrate and sodium chloride are brought together? What other chlorides would act in the same way? 2. Is the reaction expressed by the equation NH_{3} + H_{2}O = NH_{4}OH reversible? If so, state the conditions under which it will go in each direction. 3. Is the reaction expressed by the equation 2H + O = H_{2}O reversible? If so, state the conditions under which it will go in each direction. 4. Suggest a method for the preparation of hydrochloric acid. CHAPTER XIV SULPHUR AND ITS COMPOUNDS ~Occurrence.~ The element sulphur has been known from the earliest times, since it is widely distributed in nature and occurs in large quantities in the uncombined form, especially in the neighborhood of volcanoes. Sicily has long been famous for its sulphur mines, and smaller deposits are found in Italy, Iceland, Mexico, and especially in Louisiana, where it is mined extensively. In combination, sulphur occurs abundantly in the form of sulphides and sulphates. In smaller amounts it is found in a great variety of minerals, and it is a constituent of many animal and vegetable substances. ~Extraction of sulphur.~ Sulphur is prepared from the native substance, the separation of crude sulphur from the rock and earthy materials with which it is mixed being a very simple process. The ore from the mines is merely heated until the sulphur melts and drains away from the earthy impurities. The crude sulphur obtained in this way is distilled in a retort-shaped vessel made of iron, the exit tube of which opens into a cooling chamber of brickwork. When the sulphur vapor first enters the cooling chamber it condenses as a fine crystalline powder called _flowers of sulphur_. As the condensing chamber becomes warm, the sulphur collects as a liquid in it, and is drawn off into cylindrical molds, the product being called _roll sulphur_ or _brimstone_.