Selenium
2 of the 6 encyclopedias on this shelf carry an entry for Selenium. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
a non-metallic hexad element occupying an intermediate place between sulphur and tellurium: Symbol Se; at. wt. 79.5. It was discovered by Berzelius in 1817. Though not very abundant in nature, it enters into the composition of many minerals, and has been found in the free state in certain parts of Mexico. It is prepared from cuproplumbic selenide by heating the pulverized ore with hydrochloric acid, igniting the insoluble residue with an equal weight of black flux and dissolving out the selenide of potassium with boiling water. By exposing this solution to the air selenium is deposited as a gray powder. Like sulphur, it occurs in the amorphus and crystalline states. In the former it may be drawn out into rubycolored threads, and when melted and quickly cooled becomes vitreous with a sp. gr. of 4:3, and nearly insoluble in bisulphide of carbon. In the crystalline condition it forms monoclinic prisms of sp. gr. 4.5-4.7. It boils below a red heat and gives off a deep yellow vapor which condenses in scarlet flowers, and when thoroughly heated burns with a blue flame forming selenious anhydride. It is oxidized and dissolved by nitric acid, yielding selenious acid. It is used in certain electric contrivances on account of the changes its electric resistance undergoes when it is subjected to light.
Aiton's Encyclopedia (1910)
a non-metallic element discovered by the chemist Berzelius in 1818. In the series of elements, it falls between sulphur and tellurium. It is found free in small quantities in connection with sulphur, and in compounds with copper, lead, mercury, silver, etc. In one of its forms, selenium is a red powder which melts on the application of heat. On cooling, it solidifies into a dark glassy mass. Selenium possesses a peculiar property. Its resistance to the passage of an electric current diminishes as the intensity of the light to which it is exposed increases. The metal is of no great importance in the arts. In combination with hydrogen it forms a gas quite as disagreeable as the sulphuretted hydrogen of rotten eggs.
What Has Changed in the Last Century
Aiton's judgement in 1910 that selenium is "of no great importance in the arts" did not survive the century. The light-sensitivity both entries note as a curiosity became the basis of photographic exposure meters, rectifiers and early television camera tubes, and above all of xerography: Chester Carlson's 1938 process was commercialised by Xerox using selenium-coated drums, so that for decades every office copier depended on the element. Selenium also turned out to be essential to life, shown in 1957 to prevent liver necrosis in rats and later found at the active site of glutathione peroxidase, incorporated into proteins as the amino acid selenocysteine; it is toxic in excess, as the poisoning of waterbirds at California's Kesterson Reservoir in the 1980s showed. Berzelius's discovery is now conventionally dated 1817, as Collier's has it, and the atomic weight stands at 78.97 rather than 79.5.
Written for this edition, 2026.