In some cases the liberation of ammonia is accomplished by blowing superheated steam into the liquor, which sets free the ammonia which is combined as carbonate, sulphide, and sulpho-cyanide, but not that which is present as chloride. In other works, the gas liquor is mixed with milk of lime, which liberates all the combined ammonia. The ammonia is then expelled from the mixture by a current of steam or air and steam. In both cases, the gas which is given off is passed into sulphuric acid, whereby ammonium sulphate is formed in solution and afterwards obtained as a solid by evaporation. Ammonium Salts Ammonium Chloride. Like all other alkalis, ammonia solution neutralizes acids, forming salts. With hydrochloric acid, it produces the white solid known as _sal ammoniac_ or ammonium chloride. This compound is familiar as the one required to make the liquid used in a Leclanché cell, which is generally used as the current generator for electric bells. Ammonium Carbonate, which is also called stone ammonia and salt of hartshorn, is made by subliming a mixture containing two parts chalk and one part ammonium sulphate. It is a white solid which gives off ammonia slowly and is, therefore, used as the basis for smelling salts. Ammonium Nitrate is obtained by passing ammonia gas into nitric acid until it is neutralized. It is a white solid, which melts easily on being heated, and breaks up into water and nitrous oxide (laughing gas), which is the “gas” administered by dentists. Ammonium nitrate is also used in the composition of some explosives: for example, “ammonite” is said to contain 80 per cent. of this substance. Ammonium Sulphate is used chiefly as an artificial manure; the amount required for this purpose throughout the world is over 1,500,000 tons every year. Synthetic Ammonia. Though the soluble compounds of nitrogen are fairly abundant, the supply is by no means equal to the demand, because such enormous quantities are required for agricultural purposes. It has been already said that ammonia is obtained as a by-product in the distillation of coal, and it has been repeatedly pointed out that our coal supplies are far from inexhaustible; moreover, coal gas may not always be used for lighting and heating. It, therefore, becomes a very important question as to how the future supply of ammonium salts is to be maintained. Ammonia is a very simple compound formed from the elements nitrogen and hydrogen, and, as before mentioned, the supply of free nitrogen in the air is literally inexhaustible. In recent years, the efforts of chemists have been directed towards finding a method of converting the free nitrogen of the air into some simple soluble compound. This problem is usually spoken of as the “fixation of nitrogen.”