effective in experimentation. The method of spectral analysis, introduced by Bunsen and Kirchhoff, was received by Crookes with great enthusiasm and to great effect. His first important discovery was that of the element thallium, made with the help of flame spectroscopy. Crookes discovered a previously unknown element with a bright green emission line in its spectrum. He named the element thallium, from Greek , , meaning "a green shoot or twig". Crookes's findings were published on 30 March 1861. Thallium was also independently discovered by Frenchman Claude Auguste Lamy, who had the advantage of access to large amounts of materials via his brother-in-law, Charles Frédéric Kuhlmann. Both Crookes and Lamy isolated the element in 1862. Crookes was elected a fellow of the Royal Society in 1863. Crookes wrote a standard treatise on Select Methods in Chemical Analysis in 1871. In 1866, Adolf Erik Nordenskiöld identified a rare mineral from Skrikerum as a selenide of copper, thallium, and silver (), and named the mineral crookesite in honor of Sir William Crookes. Crookes developed the Crookes tube, investigating cathode rays. He published numerous papers on spectroscopy and conducted research on a variety of minor subjects. In his investigations of the conduction of electricity in low pressure gases, he discovered that as the pressure was lowered, the negative electrode (cathode) appeared to emit rays (the so-called "cathode rays", now known to be a stream of free electrons, and used in cathode ray display devices). As these examples indicate, he was a pioneer in the construction and use of vacuum tubes for the study of physical phenomena. He was, as a consequence, one of the first scientists to investigate what is now called a plasma and identified it as the fourth state of matter in 1879. He also devised one of the first instruments for studying nuclear radioactivity, the spinthariscope. Crookes investigated the properties of cathode rays, showing that they travel in straight lines, cause fluorescence when they fall on some substances, and that their impact can produce great heat. He believed that he had discovered a fourth state of matter, which he called "radiant matter", but his theoretical views on the nature of "radiant matter" were to be superseded. He believed the rays to consist of streams of particles of ordinary molecular magnitude. It remained for Sir J. J. Thomson to expound on the subatomic nature of cathode rays (consisting of streams of