Potassium
3 of the 7 encyclopedias on this shelf carry an entry for Potassium. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
a bright, silvery metal lighter than water. At ordinary temperature it may be cut with a knife, but at the freezing temperature of water it becomes brittle. Its affinity for moisture is so strong that if a bit be laid on water it decomposes the water; part of the potassium unites with the oxygen, part unites with the hydrogen, burning with a characteristic blue flame. Potassium cannot be kept in the open air, but is protected usually in bottles and is covered with oil. Potassium forms many compounds. One of the best known is potash, a solution of which, obtained usually from wood ashes, is called lye. Another well known compound is saltpeter, the potassium nitrate used in making gunpowder. Potassium has too great an affinity for other elements to be found free in nature. It is widely distributed. It forms a part of many rocks and, by their disintegration, enters into the composition of most soils. It is stored up by plants and becomes a part of the bodies of animals. The eke or oil exuded by the body of the sheep and found on wool is strong with potassium. Metallic potassium is prepared by a process of distillation. Pearlash is heated with charcoal in a red-hot iron retort--a process not without danger. The pearlash, or carbonate of potassium, was obtained formerly from wood ashes, but the greasy products of wool and a residuum obtained in the manufacture of beet sugar now yield the greater part of the pearlash from which the potassium of the arts is prepared. In a compound form potassium enters into the makeup of saltpeter, gunpowder, potash, soap, alum, cream of tartar , and "caustic."
The Encyclopedia of Founding (1892)
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.
Collier's New Encyclopedia (1921)
symbol, K; at. wt., 39, a monad metallic element, discovered by Davy in 1807, and very widely diffused through the vegetable, mineral and animal kingdoms. It may be obtained by electrolysis, but is now produced in large quantity by distilling in an iron retort an intimate mixture of charcoal and carbonate of potassium, a condition readily obtained by igniting crude tartar in a covered crucible. It can only be preserved in the metallic state by immersing it in rock oil.