440 POTASSIUM. 



plied to the new metallic bases are formed from the names of 

 their oxides, as potassium from potash, and calcium from calx, 

 a name sometimes given to lime ; while the original names of 

 the oxides are still retained, as those of ordinary objects, and 

 not superseded by appellations indicating their relation to the 

 metals, such as oxide of potassium for potash, or oxide of cal- 

 cium for lime. 



Preparation. In 1807, Sir H. Davy made the memorable 

 discovery that potash is resolved by a powerful voltaic battery 

 into potassium and oxygen. He placed a moistened fragment 

 of hydrate of potash upon mercury, introducing the terminal 

 wire from the zinc extremity of an active battery (the chlo- 

 roid) into the fluid metal, and touching the potash with the 

 other terminal wire (the zincoid) ; bubbles of oxygen gas ap- 

 peared at the latter wire, and potassium was liberated at the 

 former, and dissolving in the mercury, was protected from 

 oxidation by the air. To effect this decomposition, Davy em- 

 ployed a battery of 200 pairs of four-inch plates ; but an amalgam 

 of potassium may be as readily obtained by a more simple voltaic 

 apparatus, in the manner described at page 239. These pro- 

 cesses, however, afford potassium only in minute quantity. 

 Soon after the existence of this metal was known, Gay-Lussac 

 and Thenard discovered that potash is decomposed by iron at a 

 white heat, and they contrived a process by which a more abun- 

 dant supply of the metal was obtained. It was afterwards noticed 

 by Curaudau that potash, like the oxides of common metals, is 

 decomposed by charcoal as well as by iron, which is the basis 

 of the process for potassium now always followed. 



This interesting and useful process is described by Mitscher- 

 lich, as it is successfully pursued in Germany. Whenever 

 charcoal is used to deprive a metallic oxide of its oxygen, the 

 former must be in a state of minute division, and be intimately 

 mixed with the latter. Carbonate of potash requires this pre- 

 caution the more, that it fuses at a red heat, and is thus apt to 

 separate from the charcoal, and sink below it. It is found that 

 the best means to obtain a proper mixture of these substances 

 is to calcine a salt of potash containing a vegetable acid, which 

 leaves a large quantity of charcoal, when decomposed. Crude 

 tartar (bitartrate of potash) is preferred, and for one operation 

 six pounds of that salt are ignited in a large crucible or melting- 

 pot provided with a lid, so long as combustible gases are dis- 



