5. Soda is sometimes called the _mineral alkali_; because it is found, in some parts of the world, in the earth. It was known to the ancients, at a very early period, under the denomination of _natron_. It received this appellation from the lakes of Natron, in Egypt, from the waters of which it was produced by evaporation, during the summer season.
6. The soda of commerce is now chiefly obtained from the _salsola_, a genus of plants which grows on the sea-shore. In Spain, the plant from which soda is obtained is denominated _barilla_; hence, the substance produced from it by incineration has received the same appellation. The ashes of a sea-weed which grows on the coasts of Scotland and Ireland, is called _kelp_. In Europe, barilla and kelp are more extensively employed in the manufacture of soap than any other alkaline substances; but, in this country, where wood is so much used for fuel, common ashes are generally preferred.
7. The process of making the ordinary brown or yellow soap, from wood-ashes, is conducted in the following manner. The alkali is first obtained in a state of solution in water, by _leeching_ the ashes as described in page 26, and then poured, in a weak state, into a copper or iron caldron, having a large wooden tub carefully affixed to the top of it.
8. When the ley has been properly heated, the tallow, either in a _tried_ state or in the suet, is gradually added. More ley, of greater concentration, is poured in; and the ingredients are moderately boiled for several hours; while a person, as represented in the preceding cut, aids their chemical union by agitating them with a wooden spatula.
9. After a quantity of rosin has been added, and properly incorporated with the other materials, the fire is withdrawn until the next morning, when it is again raised; then, with the view of forming the _paste_ into hard soap, a quantity of muriate of soda (common salt) is added. The muriatic acid of this substance, uniting with the potash, forms with it muriate of potash, which dissolves in the water, while the soda combines with the tallow and rosin. Hard soap, therefore, contains no potash; although this alkali is generally employed during the early part of the process of making it.