POTASH, in chemistry, a substance which is procured from the burnt ashes of vegetables, hence the termination ash; the prefix pot was given on account of its being prepared in iron pots. It obtain ed the name of vegetable alkali, because it was supposed to exist only in vegetable substances : and being prepared from ni tre and tartar, it was called the " alkali of nitre," and likewise " salt of tartar," a name by which it is still known in the shops. By some it is distinguished by the name of " kali," the plant from which it was originally procured. This substance, in its rough state, is prepared by burning wood, or other vegetable matter, and thus reducing them to ashes. The ashes are washed repeatedly with fresh waters, till the liquid comes off perfectly tasteless. The liquid thus obtained is evaporated, and the salt obtained is potash. If this substance is exposed to a red heat, many of the substances which are mixed with it are driven off, and what remains is much whiter, and on account of its colour it is called " pearl-ash." In this state it is deemed sufficiently pure for the ordina ry purposes of life, though by no means adapted to the purposes of the experi mental chemist. Even when apparently freed from all extraneous substances, it is found to possess very different proper ties after having been subjected to cer tain processes. In one state it is mild and inactive ; in another extremely acrid and corrosive. In the former case it is united with carbonic acid gas, and is a carbonate of potash, and not pure potash. When deprived of this acid gas, it is powerful, corrosive, and highly caustic. Different methods have been proposed by differ ent chemists to obtain this substance quite pure : we shall transcribe that given by Professor Lowitz, of Petershurgh. He boils in an iron pot for two or three hours any quantity of potash with double its weight of quicklime, and eight times the weight of the whole mixture of distilled or rain water. The liquor is to be set by to cool, and then filtered and evaporated, till a thick pellicle is formed on the sur face. It is then set by till crystals are formed on it, which are crystals of extra neous salts, that are to be removed. The evaporation is to be continued, and the several pellicles removed as fast as they are formed. When the fluid ceases to
boil, and no more pellicles arise, it is re moved from the fire, and kept stirring till it is cold. It is then dissolved in double its weight of water ; the solution is filter ed and evaporated in a glass retort, till regular crystals begin to be deposited.
AVb en a sufficient quantity has be en form ed, the liquid is decanted, and the salt is re-dissolved, after it is suffered to drain, in the same quantity of water. The de canted liquor is preserved in a well-clos ed bottle for several days, till it subside and become clear. It is then decanted, evaporated, and crystallized again, and the process repeated as long as the crys tals afford with the least quantity of water solutions that are perfectly limpid.
Potash thus obtained is a white solid substance, which is susceptible of crystal lization in long compressed, quadrangular prisms, terminating in sharp-pointed py ramids. These crystals, which are only obtained from very concentrated solu tions, are soft and deliquescent. The taste is extremely acrid ; and it is so cor rosive, that it destroys the texture of the skin the moment it touches it : hence it has derived the name of caustic, and is employed in surgery for the purpose of opening abscesses, or for destroying ex crescences. Its specific gravity is about 1.7. By a similar mode to that above de scribed, pure soda may be prepared, sub stituting the carbonate of soda for the pearl-ash. They both possess the follow ing properties :-1. They convert vege table blues into a green colour. 2. They powerfully attract moisture. 3 They readily dissolve in water, and produce heat during the solution. They are not volatilized by a moderate heat, hence they are called fixed alkalies. Fixed al kalies have till very lately been number ed among the simple substances, not, however, without exciting in the minds of chemists a suspicion that they were compounds. Professor Davy has, in the course of the present and preceding years, put the matter beyond all doubt, and has proved to the satisfaction of eve ry chemist, that they are compound of oxygen and certain metallic bases, to which he has given the names of