There are various grades of caustic soda on the market differing one from another in purity. The soap manufacturer uses caustic liquor or lye containing about 40 per cent. of caustic soda. For other purposes, the solid containing from 60 to 78 per cent. is used. Sometimes the product is whitened by blowing air through the strong caustic liquor or by the addition of a little potassium nitrate. Finally, for analytical purposes, caustic soda is purified by dissolving it in alcohol and subsequently evaporating the clear liquid. Caustic Potash. The methods for the preparation of the corresponding potassium compound are precisely the same as those described for caustic soda; in fact, wherever the words sodium and soda occur in this chapter, the reader can always substitute potassium and potash respectively. Caustic Lime. Apart from its use in making mortar and cement, lime is very often employed to neutralize acids. For this purpose, a suspension in water, called milk of lime, is generally used, for lime itself is not very soluble. Probably it is only the soluble part which reacts; nevertheless, as soon as this is used up, more of the solid dissolves, and in this way the action goes on as if all the lime were in solution. Lime is also a very valuable substance in agriculture, especially on damp, boggy land, where there is much decaying vegetable matter, and on land which has been liberally manured. The soil in these cases is very likely to become acid and is then unproductive. Lime is added to “sweeten” the soil; in other words, to neutralize the acid. Ammonia. The pungent smelling liquid popularly known as “spirits of hartshorn” is a solution of ammonia gas in water. It is a caustic alkali and, as such, is sometimes used to remove grease spots. Here, however, we shall consider ammonia only in connection with ammonium salts, some of which are used in very large quantity as fertilizers. The principal source of ammonia at the present time is the ammoniacal liquor obtained as a by-product in the manufacture of gas for heating and lighting. Coal contains about 1 per cent. of nitrogen, and when it is distilled, some of this nitrogen is given off as ammonia, which dissolves in the water produced at the same time. This liquid is condensed in the hydraulic main and in other parts of the plant where the gas is cooled down. Gas liquor contains chiefly the carbonate, sulphide, sulpho-cyanide, and chloride of ammonia, together with many other substances, some of which are of a tarry nature. It would not be practicable to evaporate this liquid with a view to obtaining the ammonium salts, because it is only a very dilute solution. Hence, after the removal of tar, the liquor is treated in such a way that ammonia is set free.