William Crookes

How to use this course

William Crookes never held a university chair. He worked for most of his life in a private laboratory in his London house, paid for by consulting, journalism and a family fortune. From there he discovered an element, invented two instruments that every physics department once owned, and pushed vacuum technique to the point where the electron, X-rays and the first counts of single atomic events became possible. In the same decades he investigated spirit mediums and declared some of their phenomena genuine.

This course treats both sides as history. Part II follows his experimental programme in the order he pursued it and supplies the physics that explains what he saw, much of which he himself interpreted differently. Lesson 11 sets out his psychical research factually, with the evidence and the criticism.

Each technical lesson opens with a paragraph for the general reader, then gives precise statements, derivations and exercises with folded solutions. Numerical exercises use SI units and rounded constants.

Timeline

A tailor’s son and the Royal College of Chemistry, 1832–1854

William Crookes was born in London on 17 June 1832. His father, Joseph Crookes, was a tailor who had built up a successful business and invested in property. William was the eldest son of Joseph’s second marriage, to Mary Scott, in a large household. The family’s money would later let him live as a scientist without an institutional salary.

In 1848 he entered the Royal College of Chemistry in Oxford Street, a new institution founded on the German model and led by August Wilhelm von Hofmann, a student of Justus von Liebig. Hofmann ran the College as a research school in organic chemistry, and he quickly picked Crookes out. After two years Crookes became a junior assistant, and in 1851 a senior assistant, a post he held until 1854.

Crookes admired Hofmann but did not share his devotion to organic synthesis. His first papers, published in 1851, were on selenocyanides, and his lasting interests were in inorganic chemistry, optics and photography.

Oxford, Chester, and a laboratory at home, 1854–1860

In 1854 Crookes became superintendent of the meteorological department at the Radcliffe Observatory in Oxford, under the astronomer Manuel Johnson. The job was to record instrument readings, and Crookes adapted the waxed-paper photographic process so that barometers and thermometers could record themselves continuously. Photography remained a lifelong skill; it shaped his later spectroscopy and, less happily, his photographs of séances.

In 1855 he moved to Chester as lecturer in chemistry at the diocesan training college. The post required staff to be unmarried. In April 1856 he married Ellen Humphrey of Darlington and resigned. The couple settled in London, where Crookes set up a private laboratory and lived by analytical consulting, editing and writing.

The editor

In 1859 he founded Chemical News, a weekly that he owned and edited until 1906. It was less formal than the society journals, printed news, abstracts, trade matters and short research notes, and gave Crookes a voice in British chemistry long before he held any office. He also edited photographic journals at various times and was associated with the Quarterly Journal of Science, where several of his most controversial papers would appear. Editorial independence suited him: he could publish quickly, and he could publish what society referees might reject.

Thallium and the Royal Society, 1861–1873

In 1859 Gustav Kirchhoff and Robert Bunsen showed that every element heated in a flame gives a characteristic set of bright lines, and within two years they had found caesium and rubidium this way. Crookes took up the method at once. He had a residue from a sulphuric acid works, rich in selenium, left over from his student research. Examining it in the spectroscope, he saw a brilliant green line belonging to no known element. He announced the discovery on 30 March 1861 and named the element thallium, from the Greek thallos, a green shoot.

Crookes at first thought thallium might be a non-metal related to sulphur and selenium. The French chemist Claude-Auguste Lamy independently found the element in 1862, isolated a substantial ingot, and showed it was a heavy metal. A priority dispute followed, conducted partly in the pages of journals Crookes controlled. Both men are now credited. The discovery made Crookes’s reputation, and he was elected a Fellow of the Royal Society in 1863, aged 30.

Precision weighing

He spent the next decade characterising thallium, culminating in a determination of its atomic weight published in 1873. To weigh with the precision he wanted, he worked in a balance case pumped to low pressure, so that air buoyancy and convection would not disturb the beam. He noticed that the balance responded to warm or illuminated objects nearby, even in a good vacuum. The anomaly led to the radiometer (Lesson 06), and his interest in low-pressure gases led in turn to the discharge tubes of Lesson 07.

The same years saw the start of his investigations of spirit mediums, which began after the death of his younger brother Philip in 1867. Lesson 11 treats them separately.

Flame spectroscopy and the green line

For the general reader. Every kind of atom, when heated, gives off light at a few sharp colours of its own. Spread the light with a prism or grating and the colours become a pattern of lines, a fingerprint. In 1861 Crookes saw a green line in no catalogue, and concluded that the material held an unknown element. Today we know each line comes from an electron jumping between two energy levels.

Kirchhoff and Bunsen’s rules

Rule 2 is what made spectroscopy a tool of discovery: a line matching nothing in the tables signals a new element. The sensitivity is extreme, because a strong line is visible from a tiny mass of material in a flame.

The grating equation

Light of wavelength \(\lambda\) falling normally on a grating with line spacing \(d\) is diffracted into angles \(\theta_m\) satisfying

Two wavelengths \(\lambda\) and \(\lambda+\Delta\lambda\) can be separated in order \(m\) when \(N\) lines are illuminated provided

The Rayleigh criterion behind this bound puts the maximum of one wavelength on the first zero of the other.

Lines as level differences

In the quantum picture, a line at wavelength \(\lambda\) is a photon of energy \(E=hc/\lambda\), equal to the difference between two atomic levels. A useful conversion is \(hc\approx1239.8\ \text{eV nm}\). Thallium has one valence electron outside closed shells, and its ground configuration \(6p\) is split by spin–orbit coupling into \(6p\,{}^2P_{1/2}\) (ground) and \(6p\,{}^2P_{3/2}\). The excited level \(7s\,{}^2S_{1/2}\) decays to both, giving the ultraviolet line at 377.6 nm and the green line at 535.0 nm that Crookes saw.

Rarefied gases and the mean free path

For the general reader. Air at ordinary pressure is crowded: a molecule travels a fraction of a micrometre before hitting another. Pump most of the air out and the distance between collisions grows until it is larger than the container itself. Almost everything Crookes discovered with vacuum apparatus happens near that crossover, where gas stops behaving like a fluid and starts behaving like a hail of separate particles.

Mean free path

Treat molecules as hard spheres of diameter \(d\) in a gas of number density \(n=p/(k_BT)\). A molecule sweeps a collision cylinder of cross-section \(\sigma=\pi d^2\). Accounting for the relative motion of the target molecules in a Maxwell–Boltzmann gas gives a factor \(\sqrt2\).

The mean free path is inversely proportional to pressure. The ratio to the size \(L\) of the apparatus is the Knudsen number \(\mathrm{Kn}=\lambda_{\text{mfp}}/L\). For \(\mathrm{Kn}\ll1\) the gas is a continuum (viscous flow); for \(\mathrm{Kn}\gg1\) molecules hit walls far more often than each other (free molecular flow); the transition regime \(\mathrm{Kn}\sim 0.1\)–\(10\) is the hardest to calculate and the one where radiometers turn fastest.

Effusion and thermal transpiration

In a Maxwell–Boltzmann gas the mean speed is \(\bar v=\sqrt{8k_BT/(\pi m)}\), and the number of molecules striking unit area of wall per unit time is

Now connect two vessels at temperatures \(T_1\) and \(T_2\) through a hole much smaller than the mean free path. In the steady state the molecular fluxes through the hole must balance, \(\Phi_1=\Phi_2\). This gives a result that would be false for a continuum gas, whose pressures would simply equalise.

Osborne Reynolds introduced thermal transpiration in 1879 precisely to explain Crookes’s radiometer, and James Clerk Maxwell’s last major paper, on stresses in rarefied gases, came out of refereeing Reynolds’s work.

The radiometer: light pressure or gas?

For the general reader. The Crookes radiometer is a glass bulb holding four vanes, black on one side and silvered on the other, on a needle. Put it in sunlight and it spins. Crookes thought light itself was pushing the vanes. It is not: light does push, but far too weakly, and in the wrong direction. The real driver is the thin gas left in the bulb, warmed more on the black side.

The history in brief

Crookes found the effect in 1873 in his vacuum balance and published his interpretation, that light exerts a repulsive force, in 1874. He built the four-vane “light mill” and patented it in 1876. That year Arthur Schuster suspended a whole radiometer and showed that the bulb experiences a torque opposite to the vanes, so the force is internal to the instrument. Reynolds proposed thermal transpiration in 1879, and Maxwell published his analysis of rarefied-gas stresses the same year. In 1901 Pyotr Lebedev, with better pumps, showed that the vanes do not turn in a hard vacuum. In 1924 Einstein analysed the forces at the vane edges, where the temperature difference between the faces causes a net push.

Radiation pressure

A beam of intensity \(I\) carries momentum flux \(I/c\). At normal incidence a perfectly absorbing surface feels pressure \(I/c\), a perfect mirror \(2I/c\).

Light pressure therefore pushes the silvered faces harder, and a light-pressure radiometer would turn with the black faces leading. A real radiometer in sunlight turns the other way, with the black faces trailing, retreating from the light.

A thermal estimate

Consider the free-molecular limit. Gas at temperature \(T\) strikes a vane face at temperature \(T_w\) and is re-emitted diffusely at \(T_w\) (full accommodation). The incoming molecules contribute pressure \(p/2\); the outgoing ones carry momentum proportional to \(\sqrt{T_w}\) and contribute \(\tfrac p2\sqrt{T_w/T}\). For faces at \(T+\Delta T/2\) and \(T-\Delta T/2\), the net pressure, to first order, is

pointing from the hot face toward the cold face. This crude model overstates the effect in a closed bulb and fails in the transition regime, where edge effects (thermal creep around the rim, the mechanism Reynolds and Einstein emphasised) dominate. But it explains the sign and why the force vanishes both at high pressure (gas conducts heat away and the faces equalise) and in hard vacuum (\(p\to0\)).

The Crookes tube and cathode rays

For the general reader. Seal two metal plates into a glass tube, pump out most of the air and apply a high voltage. At moderate vacuum the gas glows in colourful bands. Pump further and the glow dies, but the glass opposite the negative plate shines green. Something invisible is streaming from the negative electrode, the cathode. Crookes built tubes that made these “cathode rays” throw shadows, turn paddle wheels and bend in magnets.

Background

Heinrich Geissler’s low-vacuum tubes of the 1850s showed glowing discharges. Julius Plücker and Johann Hittorf found that at lower pressure the glass near the cathode fluoresced and that obstacles cast sharp shadows. Eugen Goldstein in 1876 showed the rays start at the cathode and named them. Crookes’s contribution, from about 1875, was a much better vacuum and a set of demonstration tubes: a Maltese cross casting a shadow, a mica paddle wheel rolling along rails, tubes showing magnetic deflection, and tubes in which the rays heated metal to redness. He described the dark region in front of the cathode, which lengthens as pressure falls and is now called the Crookes dark space.

Kinematics of the beam

Electrons freed at the cathode and accelerated through potential difference \(V\) gain kinetic energy \(eV\). Relativistically,

with \(m_ec^2=511\) keV. In a magnetic field \(B\) perpendicular to the beam the Lorentz force \(evB\) bends it into a circle of radius

Crookes’s tubes ran at residual pressures of roughly \(10^{-6}\) to \(10^{-7}\) atmosphere and several kilovolts. At such pressures the mean free path (Lesson 05) is centimetres to metres, so electrons cross the tube without colliding, which is why the rays travel straight and the gas no longer glows.

From radiant matter to electrons and X-rays

For the general reader. In 1879 Crookes told the British Association that the tube revealed a fourth state of matter, neither solid, liquid nor gas, which he called radiant matter: molecules flying freely from the cathode. He was right that the rays were particles and wrong about which particles. Within twenty years his tubes had yielded X-rays (1895) and the electron (1897).

Radiant matter

In his lecture at Sheffield on 22 August 1879, Crookes argued that at very low pressure the residual molecules, charged negatively at the cathode, are repelled from it and fly in straight lines with mean free paths longer than the tube. Heinrich Hertz and Goldstein preferred an explanation in terms of waves in the ether, partly because Hertz found no deflection in an electric field in 1883. The German view held for over a decade.

Thomson’s measurement

In 1897 J. J. Thomson, with a better vacuum, obtained the electric deflection that Hertz had missed and measured the charge-to-mass ratio.

Thomson found \(q/m\) about a thousand times that of the hydrogen ion and independent of the gas and cathode metal. The rays were a universal constituent of matter, much lighter than any atom. Crookes’s particle picture survived with a new particle.

X-rays

In November 1895 Wilhelm Röntgen, working with a Crookes-type tube covered in black card, saw a fluorescent screen glow across the room. Electrons decelerating in the glass or anode emit bremsstrahlung, whose spectrum has a sharp short-wavelength cut-off: an electron can give at most all of its energy \(eV\) to one photon.

Uranium X and the spinthariscope

For the general reader. Uranium salts darken photographic plates. In 1900 Crookes found that a simple chemical step removes a tiny fraction of the material that carries nearly all of this photographic activity, leaving uranium that seems almost inert. He called the fraction uranium X. In 1903 he made radioactivity visible atom by atom, as tiny flashes of light on a screen viewed through a lens.

Uranium X

Crookes precipitated uranium compounds in a way that left a small residue, uranium X, which carried the photographic activity. The uranium itself was then nearly inactive on a plate. Within a few years it was found that the activity of the separated fraction decays while that of the uranium recovers, one of the observations that led Ernest Rutherford and Frederick Soddy to the transformation theory of radioactivity in 1902–1903. Today “uranium X” is known to be thorium-234 (half-life 24.1 days) together with its short-lived daughter protactinium-234m, whose energetic beta rays dominate the photographic effect. The alpha rays of uranium itself are stopped in the material and its wrapping.

The spinthariscope

Crookes noticed in 1903 that a zinc sulphide screen near a speck of radium salt did not glow uniformly under a magnifier. It twinkled, each flash marking the impact of one alpha particle. He mounted a needle tip carrying a trace of radium a millimetre or so from a screen, with a lens to view it, and called the device the spinthariscope, from the Greek for spark. Scintillation counting by eye became the standard method for alpha particles. Hans Geiger and Ernest Marsden used it in the scattering experiments of 1909 that led Rutherford to the nucleus.

Rare earths and the genesis of the elements

For the general reader. The rare earths are a family of metals so chemically alike that separating them took the whole nineteenth century. Crookes studied them with a new kind of spectroscopy, bombarding samples with cathode rays and watching them glow. His results convinced him that elements might not be fixed and indivisible, and in 1886 he speculated that they had evolved from a single primal stuff. The speculation was premature, but the questions it asked were the right ones.

Radiant-matter spectroscopy

In the 1880s Crookes examined the phosphorescence that rare-earth compounds give under cathode-ray bombardment. Sulphates in particular glowed with bright, banded spectra. Fractionating yttria thousands of times, he found that different fractions gave different phosphorescent bands while their atomic weights barely differed. He proposed that an element such as yttrium was a cluster of “meta-elements”, very close in weight and chemistry. In fact the bands came largely from traces of other rare earths, whose phosphorescence is highly sensitive to small impurities.

Prout, protyle and the periodic table

In 1815–1816 William Prout suggested that atomic weights are whole multiples of hydrogen’s, so that hydrogen might be the building block of all elements. Precise weights, such as chlorine at 35.5, seemed to refute it. In his 1886 presidential address to the Chemical Section of the British Association, Crookes revived a version of the idea. He imagined a primordial matter, “protyle”, condensing as the universe cooled into the elements, in an inorganic analogue of Darwinian evolution, with Mendeleev’s periodic table as its trace. He drew the table as a spiral, swinging between electropositive and electronegative elements as it descended.

Psychical research

Crookes’s brother Philip died of yellow fever in 1867, and Crookes began attending séances soon afterwards. In July 1870 he announced in the Quarterly Journal of Science that he would examine spiritualist phenomena with the methods of physical science. Many readers expected an exposure.

Home and “psychic force”, 1871

From May 1871 he tested Daniel Dunglas Home, the most celebrated medium of the day, in his own house, with witnesses including the astronomer William Huggins and the lawyer Edward Cox. He reported that an accordion held by Home inside a wire cage played without being touched, and that a board resting on a spring balance changed its apparent weight when Home touched only its far end. He concluded that a new “psychic force” was at work. He submitted a report to the Royal Society in June 1871; it was not published there, and his accounts appeared in his own Quarterly Journal of Science. Balfour Stewart suggested in Nature that the observers had been misled. An anonymous attack in the Quarterly Review, now attributed to the physiologist William Carpenter, questioned Crookes’s standing as a scientist, and a long public quarrel followed.

Florence Cook and “Katie King”, 1873–1874

From December 1873 to May 1874 Crookes sat with the young medium Florence Cook, in whose séances a figure calling herself Katie King appeared. He reported seeing the medium and the figure at the same time, and he took many photographs, which he said showed differences between them. He collected his papers in Researches in the Phenomena of Spiritualism (1874). Cook was caught in fraud on other occasions, and historians have questioned his judgment in this case; some have suggested a personal relationship, which has never been established. In 1875 he endorsed the medium Anna Eva Fay, who later admitted trickery.

Later years

After 1874 Crookes published little on the subject but did not recant. He joined the Hermetic Order of the Golden Dawn in 1890 and was president of the Society for Psychical Research from 1896 to 1899. In his presidential address to the British Association in 1898 he said he had “nothing to retract”. He presided over the Ghost Club from 1907 to 1912. After his wife’s death he accepted as genuine a spirit photograph of her taken by William Hope, which critics found to show signs of double exposure.

Assessments

Critics from Harry Houdini to later psychologists concluded that Crookes was deceived, whether through trust, an unscientific séance setting controlled by the mediums, or his poor eyesight; William Ramsay and others raised the last point. Supporters noted his apparatus and his reputation. The episode remains a standard case study in how a skilled experimenter can fail when the subject of the experiment is a person who may be trying to deceive.

Honours, wheat, death and legacy, 1880–1919

From 1880 Crookes lived and worked at Kensington Park Gardens in London, where he built a well-equipped private laboratory. He received the Royal Medal (1875), the Davy Medal (1888) and the Copley Medal (1904) of the Royal Society, and the Albert Medal of the Royal Society of Arts (1899). He presided over the Chemical Society, the Institution of Electrical Engineers (1891), the Society of Chemical Industry and the British Association (1898). He was knighted in 1897, received the Order of Merit in 1910, and was President of the Royal Society from 1913 to 1915. In 1895 he confirmed spectroscopically that the gas Ramsay had obtained from the mineral cleveite was helium, and he contributed spectra to the identification of argon.

The wheat problem

His 1898 presidential address to the British Association at Bristol was one of the most influential scientific speeches of its time. Crookes argued that the world’s wheat-eating population was growing faster than the land and fertiliser to feed it, that the Chilean nitrate deposits would run out, and that the solution lay in fixing the nitrogen of the air. Within two decades, arc processes and then the Haber–Bosch synthesis of ammonia did exactly that.

Last years

In 1913 he developed glass containing cerium that absorbed ultraviolet and much infrared radiation, for spectacles to protect glassworkers’ eyes. His wife Ellen died during the war. He died in London on 4 April 1919, aged 86.

Legacy

• Chemistry: the discovery of thallium, rare-earth spectroscopy and decades of editorial work in Chemical News.
• Vacuum physics: the radiometer, which drew Reynolds, Maxwell and Einstein into the theory of rarefied gases; and the Crookes tube, the instrument of Röntgen’s X-rays and Thomson’s electron.
• Radioactivity: uranium X, an early example of separating a radioactive daughter; and the spinthariscope, the first method for counting single atomic events.
• Science and society: the wheat address framed nitrogen fixation as a global problem; his psychical research remains a case study in the limits of laboratory skill when the subject can deceive.

Sources and further reading

Biography

1. William Crookes, Wikipedia.
2. Encyclopaedia Britannica, Sir William Crookes.
3. IET Archives, Sir William Crookes (1832–1919).
4. W. H. Brock, William Crookes (1832–1919) and the Commercialization of Science, Ashgate, 2008.
5. E. E. Fournier d’Albe, The Life of Sir William Crookes, London, 1923.
6. Encyclopedia of Occultism and Parapsychology, Crookes, Sir William (detailed chronology of the psychical research).

Science

1. W. Crookes, On the existence of a new element, probably of the sulphur group, Chemical News 3 (1861).
2. W. Crookes, On attraction and repulsion resulting from radiation, Phil. Trans. R. Soc. 164 (1874).
3. W. Crookes, On Radiant Matter, lecture to the British Association, Sheffield, 22 August 1879.
4. W. Crookes, Radio-activity of uranium, Proc. R. Soc. Lond. 66 (1900) 409–422.
5. W. Crookes, Researches in the Phenomena of Spiritualism, London, 1874.
6. R. K. DeKosky, Spectroscopy and the elements in the late nineteenth century: the work of Sir William Crookes, Brit. J. Hist. Sci. 6 (1973) 400–423.
7. O. Reynolds, On certain dimensional properties of matter in the gaseous state, Phil. Trans. R. Soc. 170 (1879).
8. J. C. Maxwell, On stresses in rarified gases arising from inequalities of temperature, Phil. Trans. R. Soc. 170 (1879).
9. Crookes radiometer and Crookes tube, Wikipedia.
10. J. J. Thomson, Cathode rays, Phil. Mag. 44 (1897) 293–316.