Potassium
3 of the 6 encyclopedias on this shelf carry an entry for Potassium. All 3 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.
What Has Changed in the Last Century
The chemistry described in 1892, 1910 and 1921 is still sound cut with a knife, fizzing on water, kept under naphtha or oil but nearly everything around it has moved. Collier's 1921 method of distilling charcoal with potassium carbonate in an iron retort is obsolete: metallic potassium is now made by reducing molten potassium chloride with sodium vapour, and the potash of commerce comes from mined evaporite salts (Germany, and above all Saskatchewan) rather than wood ashes, wool grease or beet-sugar residues. Collier's flat "at. wt., 39" has since been resolved into three natural isotopes, one of them radioactive potassium-40, whose 1.25-billion-year half-life underpins potassium-argon dating of rocks and makes every human body faintly radioactive. And where Aiton's noted only that plants store potassium up, physiologists from the 1950s showed the potassium ion driving nerve and muscle impulses across cell membranes, while the K in NPK fertiliser became one of the pillars of modern agriculture.
Written for this edition, 2026.