444 POTASSIUM. 



and becomes completely liquid at 150. It may be distilled at 

 a low red heat, and forms a vapour of a green colour. Potas- 

 sium is considerably lighter than water, its density being 0.865 

 at 60o. 



Potassium oxidates gradually without combustion when ex- 

 posed to air, but heated till it begins to vaporise, it takes fire 

 and burns with a violet flame. The avidity of this metal for 

 oxygen is strikingly exhibited when a fragment of it is thrown 

 upon water. It instantly decomposes the water, and so much 

 heat is evolved as to kindle the potassium, which moves about 

 upon the surface of the water, burning with a strong flame, of 

 which the vivacity is increased by the combustion of the hy- 

 drogen gas disengaged at the same time. A globule of fused 

 potash remains, which continues to swim about upon the sur- 

 face of the water for a few seconds, but finally produces an ex- 

 plosive burst of steam, when its temperature falls to a certain 

 point, illustrating the phenomenon of a drop of water on a hot 

 metallic plate (page 47.) 



Potassium appears to have the greatest affinity of all bodies 

 for oxygen, at temperatures which are not exceedingly elevated. 

 It decomposes nitrous and nitric oxides, and also carbonic 

 oxide gas at a red heat, although potash is reduced to the 

 metallic state by charcoal at a white heat. It has already been 

 stated that the oxides and fluorides of boron and silicon are de- 

 composed by potassium, and besides these elements, several of 

 the metallic bases of the earths are obtained by means of this 

 metal. It is indeed a reducing agent of the greatest value. 



COMPOUNDS OF POTASSIUM. 



Potash or potassa ; KO ; 590 or 47.26. Potassium exposed 

 in thin slices to dry air becomes a white matter, which is the 

 protoxide of potassium or potash. This compound is fusible 

 at a red heat, and rises in vapour at a strong white heat. It 

 unites with water, with ignition, and forms a fusible hydrate, 

 which is the ordinary condition of caustic potash. 



The hydrate of potash is obtained in quantity from the car- 

 bonate of potash. Equal weights of that salt and of quick- 

 lime are taken, the latter of which is slaked with water, and 

 falls into a powder consisting of hydrate of lime ; the former 

 is dissolved in from 6 to 10 times its weight of water, and both 



