Crookes' tube showing the phosphorescence of rubies, 1879-1888.
© Science Museum / Science & Society Picture Library
Sir William Crookes (1832-1919), an experimental chemist and physicist, used this tube to demonstrate one of the properties of cathode rays; that they cause substances such as ruby to phosphoresce (to emit light without combustion or perceptible heat). Crookes also discovered that cathode rays travelled in straight lines, could cast shadows, heat obstacles, and were deflected by a magnet. This led him to conclude, correctly, that the rays consisted of negatively charged particles. This theory had little support, however, until J J Thomson identified the particles as electrons in 1897.