The art of _soap─boiling_ may easily be illustrated by the following experiment. Take a piece of quick─lime, slake it gradually by sprinkling on it a sufficient quantity of water. When it is completely slaked, add to it about twenty times its weight of water. To this mixture add two parts, by weight, of common subcarbonat of soda, previously dissolved in a sufficient quantity of water. Boil the whole for about half an hour, strain it through a cloth, and boil it till so much of the water is evaporated that a phial that will contain an ounce of water will hold one ounce, seven pennyweights and a half, of this ley. Then mix in an earthenware pipkin or basin, one part of the ley, with two parts of olive oil. Place the mixture in a gentle heat, capable only of making the liquor simmer, and allow it to simmer, stirring the liquor continually, with a wooden stick, till, by letting a few drops of it fall on a plate, the soap will be found to coagulate, and the water become speedily separated from it. After which, pour out the contents into a cup, and suffer it to cool.—Soap may also be prepared without heat. If one part of the ley be mixed with two parts of olive oil, in a glass or stone ware vessel, and the mixture be stirred, from time to time, with a wooden spoon or spatula, it will become thick, and white; in seven or eight days afterwards the combination will be completed, and a white and firm soap will be obtained. _White soap_ is formed of ingredients similar to those that have just been mentioned. _Yellow soap_ is made with tallow, resin, and soda. Soap may be formed by boiling shreds of woollen cloth with ley till the whole has acquired a certain consistence. This kind of soap has been made, and applied with success, in several manufactories in France.—The combination of oil and other ingredients with potash (205), instead of soda, affords what is called _soft soap_. 201. _NATRON, or CARBONAT of SODA, is a salt which consists of soda (200) in combination with carbonic acid (26). It is massive, of greyish colour, soluble in water, and has a disagreeable alkaline taste._ This salt is found in Egypt, on the surface of the earth, and particularly near the margins of certain lakes called natron lakes. In the summer season the water of these lakes is evaporated by the heat of the sun, leaving a bed of natron generally about two feet in thickness. This is broken with wedges and hammers; and packed up for sale in the European markets. The waters of some of the lakes contain both common salt and natron; and these, on evaporation, crystallize in successive beds. Natron is found in considerable quantity under the form of an efflorescence, on the surface of the earth, in the plains of Debreczin in Hungary. It is likewise found in small quantity in the ashes of most vegetables, but particularly in those of _salsoda_ and _salicornia_.