DISCOVERY OF SUB-ATOMIC PARTICLES The atomic theory of the Greek philosophers, Leucippus and Democretus (400 B.C.) held that continued subdivision of matter would ultimately yield atoms which would not be further divided. The word 'atom' is derived from the Greek word, aromos, which means "uncut" or indivisible. Dalton (1808) based his atomic theory on the ideas of Democretus and was able to explain the laws of chemical combination. Toward the end of nineteenth century, it began to appear that the atom itself might be composed of even smaller particles. This discovery was brought about by experiments with electricity. Attempts to pass a high voltage electric current through gases under reduced pressure led to Julius Plucker's discovery (1859) of cathode rays, Fig. 1.1. The cathode rays stream from the negative electrode, which is called the cathode. These rays consist of negatively charged particles which travel in straight lines. The cathode rays give off flashes of light, when they strike a screen coated with substances like zinc sulphide. The picture tubes in television sets and computer monitors, function on this principle.