POT 



POT 



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 Petersburg!!. 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. 



When a sufficient quantity has been 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 



POTASIUM, and SODIUM, or SODAIU>I. 

 Of these, and of the experiments which 

 led to the discoveries, we shall proceed to 

 give some account, having attended the 

 repetition of his experiments at the lec- 

 tures delivered last spring at the Royal 

 Institution. Mr. Davy, in his first at- 

 tempts to decompose the alkalies, made 

 use of the aqueous solutions, and failed. 

 He next made use of the potash in a state 

 of igneous fusion, which he brought with- 

 in the sphere of the galvanic battery : 

 with this also he was unsuccessful in the 

 main point ; but some brilliant phenomena 

 were produced. The potash appeared a 



