It figured out a bunch of details about electrons and specially that the atom is not the smallest particle in matter.
In 1878 William Crooks observed that a beam of light travels from cathode to the anode when he lowered the pressure of the Crooks Tube with a gas to 1Pa.
In 1897 J.J. Thomson did experiments to figure out the characteristics of the Cathode Rays .
To read more of the experiments done click here to visit Wikipedia sources .
You can watch some animations and some youtube videos which I reccomend to you. (You are welcome to watch more if you like)
However with these experiments he figured out some important facts about Cathode Rays.
1. Cathode Rays travel in direct lines.
2. Cathode Rays are negative in charge.
3. Cathode Rays are made of particles (later Rutherford said that they are the same particle units which J.G.Stoney called Electrons).
4. Cathode Rays are emitted in ##90^0## angles from the Cathode.
5. When they hit a surface they emit X-Rays.
6. The ##e/m## value of Cathode Rays is ##1.76*10^11Ckg^-1##.
When electrons which are smaller particles than atoms were found John Dalton’s statement that the atom is the tiniest particle of mass was broken.