Iridium
3 of the 6 encyclopedias on this shelf carry an entry for Iridium. All 3 are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
i-rid i-um, a metallic element much like platinum. It was identified in 1802. With the exception of osmium, also a newly discovered metal, it is the heaviest substance known. It is very difficult to melt, requiring a temperature of at least 3600 Deg. F. It is insoluble in acid and does not rust. It is used frequently for the tips of gold and stylographic pens. Also in the construction of electric instruments, as the resistance coil. Small quantities are obtained in California, Borneo, Brazil, and in the Ural Mountains . It forms three oxids. The solutions made from the salts of the metal take on various tints; hence the name, from Iris, the rainbow.
Collier's New Encyclopedia (1921)
a tetrad metallic element of the platinum group; symbol Ir.; at. wt. 193.1; discovered by Tennant in 1804, in the black powder which remains when crude platinum is dissolved in nitro-hydrochloric acid. This powder is an alloy of iridium and osmium, called iridosmine or osmiridium. Iridium is a white, brittle, very hard metal, fusible with great difficulty, in the blow flame of the oxy-hydrogen blowpipe. It is insoluble in all acids, but when reduced by hydrogen at a red heat it oxidizes slowly and dissolves in nitro-hydrochloric acid.
The Encyclopedia of Founding (1892)
A white brittle metal, which may be fused by means of a powerful oxyhydrogen blast-furnace. In its isolated form it is unacted upon by any acid or by aqua regia, but as an alloy it is dissolved in the latter fluid. Its specific gravity is 21.15. An alloy of iridium and osmium is very hard, and is used for pointing gold pens. See METALS.
What Has Changed in the Last Century
The chemistry in these entries mostly holds: iridium is still the platinum-group metal Tennant separated in 1803-04, still barely soluble in acids, still alloyed with osmium in natural osmiridium. The numbers have been sharpened -- its melting point is about 2,446 degrees C (roughly 4,430 F), well above Aiton's 1910 figure of 3,600 F, and careful density measurements settled the old race with osmium in osmium's favour by a whisker, about 22.59 against 22.56 grams per cubic centimetre. The great change is one no one on this shelf could have guessed: in 1980 Luis and Walter Alvarez found a thin worldwide clay layer freakishly rich in iridium at the Cretaceous-Paleogene boundary, and argued the metal had arrived with an asteroid, an idea largely confirmed when the buried Chicxulub crater in Mexico was identified around 1991. Gold pen tips have mostly gone, but iridium now hardens spark-plug electrodes and crucibles, and the platinum-iridium prototype kilogram guarded the unit of mass until it was replaced by a definition based on the Planck constant in 2019.
Written for this edition, 2026.