Potassium
Potassium was discovered by Sir Humphry Davy in 1807, together with sodium, barium, strontium, and calcium. Before this time the alkalies and alkaline earths had been considered as simple bodies, and it is to him we owe the discovery of their compound nature. He obtained it in very small quantity by exposing a piece of moistened potassic hydrate to the action of a powerful voltaic battery; the positive pole gave off oxygen, and metallic globules of pure potassium appeared at the negative pole. Potassium is a brilliant white metal with a high degree of lustre; at the common temperature of the air it is soft, and may be easily cut with a knife, but at 32 deg.; F. it is brittle and crystalline. It melts completely at 130 deg.; F. and distils at a low-red heat. It floats on water, its specific gravity being only 0.865. It oxidizes instantly in the air; takes fire if thrown upon water. Potassium decomposes nearly all compounds containing oxygen if brought in contact with them at high temperatures; hence to preserve it pure it is kept under naphtha, a liquid containing no oxygen. See METALS; ALLOYS.