Corrected by Mail

Mendeleev did not just predict that elements would exist — he predicted their numbers, including density. That precision led to one of science's great moments.

When the French chemist Lecoq de Boisbaudran isolated gallium and measured its density, his value came in lower than Mendeleev predicted. Mendeleev, who had never held a sample, wrote to insist the measurement was wrong.

Re-checking with a purer sample, Boisbaudran found the density matched Mendeleev's figure almost exactly. A pattern on paper had out-measured the man at the bench.