Subject: biography, physics
English physicist and chemist who, in a scientific career lasting more than 50 years, made many fundamental contributions to both sciences. He is best known in physics for his experiments with high-voltage discharge tubes and for inventing the Crookes radioscope or radiometer. A major achievement in chemistry was the discovery of the element thallium. Crookes never held a senior academic post, and carried out most of his researches in his own laboratory.
Crookes was born in London on 17 June 1832, the eldest of the 16 children of a tailor and businessman. Little is known of his early education but in 1848 he began a chemistry course at the new Royal College of Chemistry under August von Hofmann, who later made him a junior assistant, promoting him to senior assistant in 1851. Crookes's only academic posts were as superintendent of the Meteorological Department at the Radcliffe Observatory, Oxford 1854-55, and as lecturer in chemistry at Chester Training College 1855-56. He left Chester after only one year, dissatisfied at being unable to carry out original research and after having inherited enough money from his father to make him financially independent for life.
He returned to London and became secretary of the London Photographic Society and editor of its Journal in 1858. The following year he set up a chemical laboratory at his London home and founded as sole manager and editor the weekly Chemical News, which dealt with all aspects of theoretical and industrial chemistry and which he edited until 1906. Among his greatest achievements in chemistry was the discovery in 1861 of the element thallium using the newly developed spectroscope. He also applied this instrument to the study of physics. He was knighted in 1897 and made a Member of the Order of Merit in 1910. He died in London on 4 April 1919.
One of Crookes's earliest pieces of chemical research concerned the preparation and properties of potassium selenocyanate (potassium cyanoselenate (VI)), which he made using selenium isolated from 5 kg/11 lb of waste from a German sulphuric-acid works. Then in 1861, while examining selenium samples by means of a spectroscope - the method newly developed by Robert Bunsen and Gustav Kirchhoff - Crookes observed a transitory green line in the spectrum and attributed it to a new element, which he called thallium (from the Greek thallos, meaning a budding shoot). Over the next few years he determined the properties of thallium and its compounds; using thallium nitrate and a specially constructed sensitive balance he measured thallium's atomic mass as 203.715±0.0365 (modern value: 204.39).
This work involved weighing various samples of material in a vacuum, and he observed that when the delicate balance was counterpoised it occasionally made unexpected swings. He began to study the effects of light radiation on objects in a vacuum and in 1875 devised the radioscope (or radiometer). The instrument consists of a four-bladed paddlewheel mounted horizontally on a pinpoint bearing inside an evacuated glass globe. Each vane of the wheel is black on one side (making it a good absorber of heat) and silvered on the other side (making it a good reflector). When the radioscope is put in strong sunlight, the paddlewheel spins round. Although little more than a scientific toy, it defied attempts to explain how it works until James Clerk Maxwell correctly showed that it is a demonstration of the kinetic theory of gases. The few air molecules in the imperfect vacuum in the radioscope bounce more strongly (with more momentum) off the heated, black sides of the vanes (than off the cooler, silvered sides), creating a greater reaction that ‘pushes’ the paddlewheel around. Crookes own observations were described in his paper ‘Attraction and repulsion resulting from radiation’ (1874).
During the 1870s Crookes's studies concerned the passage of an electric current through glass ‘vacuum’ tubes containing rarified gases; such discharge tubes became known as Crookes tubes. The ionized gas in a Crookes tube gives out light - as in a neon sign - and Crookes observed near the cathode a light-free gap in the discharge, now called the Crookes dark space. He named the ion stream ‘molecular rays’ and demonstrated how they are deflected in a magnetic field and how they can cast shadows, proving that they travel in straight lines. He made similar observations about cathode rays, but it was left to J J Thomson to understand the true significance of such experiments and to discover the electron (in cathode rays) in 1897. Crookes also noted that wrapped and unexposed photographic plates left near his discharge tubes became fogged, but he did not follow up the observation, which was later the basis of Wilhelm Röntgen's discovery of X-rays in 1895.
In the 1880s Crookes studied the phosphorescent spectra of rare-earth minerals, principally substances containing yttrium and samarium. He made the first references to what Frederick Soddy was to call isotopes. While experimenting with radium, he devised the spinthariscope. The instrument consists of a screen coated with zinc sulphide at the end of a tube fitted with a low-powered lens. When alpha particles emitted from a radioactive source (such as radium) hit the screen they produce a small flash of light.
Crookes's interests were very wide. Topics covered by his publications included chemical analysis; the manufacture of sugar from sugarbeet; dyeing and printing of textiles; oxidation of platinum, iridium and rhodium; use of carbolic acid (phenol) as an antiseptic in the treatment of diseases in cattle; the origin and formation of diamonds in South Africa; and the use of artificial fertilizers and their manufacture from atmospheric nitrogen. Crookes was never afraid of pursuing an idea counter to the trend of contemporary opinion. For several years, for example, he was very interested in spiritualism and published several papers that described experiments undertaken by a medium. To many of his scientific colleagues this was akin to heresy.
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