Among the experiments on electricity were some that involved sending
electric currents through glass tubes containing various kinds of gas.
the current was turned on, the negative electrode, or cathode,
glowed with a strange greenish light, and a similarly glowing greenish
spot would appear on the opposite glass wall of the tube. It was clear
that something was traveling in a straight line across the tube
from the cathode; this "something" was named a cathode ray.
Some suggested that cathode rays might be waves, like light or the recently
discovered x-rays. But waves, as far as
anyone knew then, couldn't carry electric charge; only particles could do
that. That's why Thomson's experimental results convinced him that cathode
rays had to be made up of particles.