POTASSIUM. 391 



a very pretty experiment may easily be performed by throwing a piece 

 upon a basin of water. The fragment combines with the oxygen of the 

 \vater, the hydrogen is evolved, and burns, and the potassium vapour gives 

 the gas its purple or violet colour. The metal can be procured by pulveriz- 

 ing carbonate of potassium and charcoal, and heating them in an iron 

 retort. The vapour condenses into globules in the receiver, which is sur- 

 rounded by ice in a wire basket. It must be collected and kept in naphtha, 

 or it would be oxidised. Potassium was first obtained by Sir Humphrey 

 Davy in 1807. Potash is the oxide of potassium, and comes from the 

 " ashes " of wood. 



The compounds of potassium are numerous, and exist in nature, and 

 by burning plants we can obtain potash (" pearlash "). Nitrate of potassium, 

 or nitre (saltpetre), (KNO 3 ), is a very important salt. It is found in the East 

 Indies. It is a constituent of gunpowder, which consists of seventy-five 



Fig. 397. Preparation of potassium. 



parts of nitre, fifteen of charcoal, and ten of sulphur. The hydrated oxide 

 of potassium, or " caustic potash " (obtained from the carbonate), is much 

 used in soap manufactories. It is called " caustic " from its property of cau- 

 terizing the tissues. Iodide, bromide, and cyanide of potassium, are used 

 in medicine and photography. 



Soap is made by combining soda (for hard soap), or potash (for soft 

 soap), with oil or tallow. Yellow soap has turpentine, and occasionally palm 

 oil, added. Oils and fats combine with metallic oxides, and oxide of lead 

 with olive oil and resin forms the adhesive plaister with which we are all 

 familiar when the mixture is spread upon linen. Fats boiled with potash 

 or soda make soaps ; the glycerine is sometimes set free and purified as 

 we have it. Sometimes it is retained for glycerine soap. Fancy soap is 

 only common soap coloured. White and brown Windsor are the same 

 soap in the latter case browned to imitate age ! Soap is quite soluble in 



