Strontium
3 of the 6 encyclopedias on this shelf carry an entry for Strontium. All 3 are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
one of the metals of the alkaline earths, occupying intermediate position as regards rega an many of its properties to calcium and barium, the other members of the group. It is less abundant in nature than barium, and occurs as a constituent in the minerals strontianite and celestine, which are respectively the carbonate (SrCO3) and su sulphate (SrSO4). In the metallic state it is usually white, heavy, oxidizable in air, and decomposing water at ordinary temperatures. The most important compound is is strontia (SrO), from which Davy first obtained the metal in 1808 by electrolysis. It resembles baryta, forming a white soluble hydrate (SrO,H₂O), which is the compound formed when the metal decomposes water. Strontia is best prepared by decomposition of the nitrate by heat; while the nitrate (Sr(NO3)2) and chloride I ( SrCl₂) are easily obtained by treating the mineral carbonate with nitric and hydrochloric acid respectively. It also forms a dioxide ( SrO2), which is prepared as the corresponding barium com- In pound is by exposing the monoxide to a current of oxygen. in Strontium and its compounds burn with very characteristic crimson flame, which gives a well-marked line or banded spectrum.
Aiton's Encyclopedia (1910)
a metal closely allied to barium. It was first noticed in combination with carbon in the lead mines of Strontian in Argyllshire, Scotland, whence the name. It was separated from other elements by Davy in 1808. Strontium is a yellowish metal. It is malleable and ductile. It burns with a brilliant crimson flame in heated air, and for that reason strontium is employed widely in the preparation of fireworks. Strontium carbonate is used in the refinement of sugar.
The Encyclopedia of Founding (1892)
This metal resembles barium in its appearance and properties. The nitrate of strontia is principally in the composition of the well-known red-fire of the pyrotechnist. These metals occur abundantly as sulphate and carbonate, forming the vein-stone in many lead-mines. They are malleable, melt below a red heat, decompose water with evolution of hydrogen, and gradually oxidize in the air. See METALS.
What Has Changed in the Last Century
The chemistry in these entries still holds: strontium is an alkaline-earth metal from strontianite and celestine, isolated by Davy in 1808, and it still gives fireworks their crimson. What none of the writers could foresee is the isotope strontium-90, a fission product of the atomic bomb tests that began in 1945; because strontium follows calcium into milk and bone, fallout in children's teeth became a public scandal and a driving argument for the 1963 Partial Test Ban Treaty. The element has also become the most precise timekeeper humanity has: strontium optical lattice clocks, developed since the 2000s, keep time to better than a second over the age of the universe. Meanwhile the sugar-refining use Aiton's cites in 1910 has essentially disappeared, and archaeologists now read strontium isotope ratios in ancient teeth to work out where a person grew up.
Written for this edition, 2026.