Potassa has a stronger affinity for the acids, than either the earths or metals; hence it decomposes earthy and metallic salts, the earth or metallic oxide being precipitated, while it unites with the acid of the salt. It is on the same principle, that earthy and metallic salts decompose soap; and waters which are hard, and owe that property to the presence of earthy salts, will curdle, or, in other words, decompose soap. Such waters, for this reason, are called hard. Acids have the same effect in decomposing soap. The use of potassa is very apparent in the manufacture of saltpetre. When the nitric acid is combined with an earthy base, as in the calcareous nitre of the nitre caves of the western country, potassa from wood-ashes will decompose it, on the principle already stated; and, by combining with the nitric acid, form nitrate of potassa. It is used also in refining saltpetre, where earthy salts are present, besides common salt. The effect of this alkali, for that purpose, will be more obvious, by referring to the processes for the extraction and refining of saltpetre, in the article on that subject. Potassa acts as a flux for siliceous substances and forms glass. These are its prominent characters. _Sect. XXXVIII. Of Wood-Ashes._ Wood-ashes, the product of the combustion of wood, contain potassa, some foreign salts, and earthy and sometimes metallic substances, insoluble in water. The quantity of alkali, which ashes, obtained from different woods, furnish, is greater or less, according to the nature of the wood. The ashes of the oak are generally used in pyrotechny; but it seems to us, that ashes in common will have the same effect. The ashes, for this purpose, should be dry, and passed through a fine sieve. They enter into the composition of blind fuse. In some instances, the _leached_, or lixiviated ashes might be used. The residue, after the separation of alkali and saline matter by the action of water, is nothing more than the insoluble part of the ashes. Caustic ley is always obtained from wood-ashes, by mixing them with about a fiftieth part of quicklime, and putting them into a barrel or tub, and adding water. The lime takes up the carbonic acid, and the ley comes off in a caustic state. If the solution should not contain a sufficient quantity of potassa, or not bear an egg, as that is the usual criterion of its strength, (which depends on its specific gravity,) its strength may be increased by evaporation; and, if too strong, simple dilution with water, is all that is necessary.