On 13 August 1894, John William Strutt, 3rd Baron Rayleigh and William Ramsay announced the detection of a new gas in the atmosphere. On 31 January 1895 they made a full report to the Royal Society on the new gas, argon. In addition, William Crookes, who had been asked to examine a sample, presented on the spectra of argon, reported that argon displayed two distinct spectra. In this way, Crookes identified the first known sample of terrestrial helium and established its correspondence to observations of solar helium. The discovery of argon and of helium led to identification of the noble gases and the reorganization of the periodic system. 
Crookes himself suggested a design for a Periodic table in the style of a space lemniscate in 1898.
Crookes was knighted in 1897.
Crookes was named president of the British Association for the Advancement of Science in 1898. In his inaugural address, he outlined in detail a coming catastrophe: The wheat-eating peoples of the world were going to start running out of food in the 1930s. The reason, he said, was a dearth of nitrogen fertilizer available from natural sources. Crookes called on chemists to develop new ways of making fertilizer from the enormous stock of nitrogen in the atmosphere (which is roughly 80 percent nitrogen). His remarks on the coming famine achieved wide distribution in the press and were turned into a popular book. Scientists addressing the problem in the first years of the twentieth century included Kristian Birkeland, whose technology helped found Norsk Hydro, and Fritz Haber and Carl Bosch, whose Haber–Bosch process forms the foundation of today's nitrogen fertilizer industry.
In 1903, Crookes turned his attention to the newly discovered phenomenon of radioactivity, achieving the separation from uranium of its active transformation product, uranium-X (later established to be protactinium). Crookes observed the gradual decay of the separated transformation product, and the simultaneous reproduction of a fresh supply in the original uranium. At about the same time as this important discovery, he observed that when "p-particles", ejected from radio-active substances, impinge upon zinc sulfide, each impact is accompanied by a minute scintillation, an observation which forms the basis of one of the most useful methods in the detection of radioactivity.
In 1913, Crookes created an ultraviolet blocking lens made from glass containing cerium, but only lightly tinted. They were an unintended by-product of Crookess research to find a lens glass formulation that would protect glass workers from cataracts. Crookes tested more than 300 formulations, each numbered and labelled. Crookes Glass 246 was the tint recommended for glassworkers. The best-known Crookes tints are A (withdrawn due to its uranium), A1, B, and B2, which absorb all ultraviolet below 350 nm while darkening visual light. Crookess samples were made by Whitefriars, London, stained glass makers, and Chance Brothers, Birmingham.