On November 30, 1878, English chemist and physicist Sir William Crookes (1832–1919) presented his Bakerian Lecture to the Royal Society of London titled “On the Illumination of Lines of Molecular Pressure, and the Trajectory of Molecules”.

Crookes invented the Crookes Tube (a high-vacuum discharge tube, pressure $< 10^{-6}\,\text{atm}$), conducting classic experiments demonstrating that cathode rays travel in straight lines, cast sharp shadows, turn mechanical paddle wheels, and defect in magnetic fields.

“Radiant Matter”: The Fourth State of Matter

In 1879, Crookes proposed that cathode rays represent an ultra-attenuated, ionized state of matter which he named “Radiant Matter”:

“In studying cathode rays, we have actually touched the borderland where Matter and Force seem to merge into one another… radiant matter is a Fourth State of Matter.”

Magnetic Deflection Experiment

Crookes placed a horseshoe magnet across the tube, proving that the cathode ray stream is deflected by magnetic fields:

$$\mathbf{F} = q (\mathbf{v} \times \mathbf{B})$$

Proving that cathode rays carry negative electrical charge and momentum.

Modern Legacy: Plasma Physics & The Electron

Crookes’s “Radiant Matter” was recognized in the 20th century as Plasma—the fourth state of matter.

His magnetic deflection experiments directly enabled J.J. Thomson in 1897 to measure the charge-to-mass ratio ($e/m$) of the electron.

Key Takeaways

  • Year: 1878
  • Key Figure: Sir William Crookes (English Physicist)
  • Core Discovery: Invented the Crookes Tube, proved cathode ray magnetic deflection, and proposed Radiant Matter (Plasma).
  • Experimental Triumph: Demonstrated cathode ray mechanical momentum and negative electric charge.
  • Quantum Relevance: Prefigured Plasma Physics and J.J. Thomson’s discovery of the electron.