Thallium
2 of the 6 encyclopedias on this shelf carry an entry for Thallium. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
in chemistry, a triad metallic element discovered by Crookes in 1861, and widely distributed as a constituent in iron and copper pyrites in blende, native sulphur, and in many kinds of ores; symbol Tl., at. wt. 203.64. It can be distilled with the sulphur by heating pyrites to a bright-red heat, then dissolving out the excess of sulphur by boiling with caustic soda, collecting and washing the sulphide of thallium, converting it into sulphate, and precipitating the thallium in the metallic state by the action of pure metallic zinc. The Biskra. Here are in the solitude extenspongy metal is compressed, dried, and fused into a bright metallic button by heating under cyanide of potassium. It is a perfect metal, with high luster, not quite so white as silver, but free from the blue tinge of lead. It has a sp. gr. of 11.80-11.91, melts at 293°, is a very soft metal with less tenacity than lead, and almost devoid of elasticity. It communicates an intense green hue to a colorless flame, and its spectrum consists of one intensely brilliant and sharp green line, coinciding with the number 1442.6 on Kirchhoff's chart.
Aiton's Encyclopedia (1910)
a metallic element akin to lead and zinc. It was discovered in 1861 by Sir William Crookes in the lead chambers of sulphuric acid works in the Harz Mountains. He detected its presence by a brilliant green line shown in the spectroscope. The name is Greek, signifying a green twig. Before dross is formed on the surface, thallium presents a silvery white luster. It is softer than lead and, like it, is malleable. It melts readily. Its chief use in the arts is in the making of glass. A trace of thallium in the lens of a microscope increases its magnifying power. Thallium is also used to add luster in the manufacture of glass jewelry.
What Has Changed in the Last Century
Both entries treat thallium as a curiosity of the spectroscope, and Aiton's in 1910 even recommends it for optical glass and "glass jewelry" without a word of warning; neither mentions that the metal and its salts are severely toxic, causing hair loss, nerve damage and death. Thallium sulphate became a common rat poison and insecticide in the 1920s-1950s, was used in a string of notorious murders (Graham Young poisoned workmates in Hertfordshire in 1971), and was banned for domestic use in the United States in 1972. The element found a legitimate 20th-century career in medicine instead: radioactive thallium-201, introduced in the 1970s, was for decades the standard tracer for heart-perfusion scans, though technetium-99m has largely replaced it. Collier's figures have only been tightened rather than overturned — the accepted atomic weight is now about 204.38 and the melting point 304 °C rather than 293 °C.
Written for this edition, 2026.