Final answer:
Cathode rays are made up of electrons, which are particles with a negative charge and mass. This was determined through historical experiments that demonstrated cathode rays' deflection by magnetic and electric fields and measured their charge-to-mass ratio.
Step-by-step explanation:
Cathode rays are composed of particles known as electrons. Historical experiments conducted by scientists like J. J. Thomson and William Crookes revealed the nature of these rays. Using a cathode ray tube, Crookes observed that a paddle wheel placed between the electrodes moved from the cathode towards the anode upon the commencement of the tube, suggesting that the rays had mass. Similarly, Thomson's experiments, which involved deflecting the cathode ray with magnetic and electric fields, established that these rays are indeed made up of charged particles with a negative charge and have a specific mass-to-charge ratio.
The discovery that cathode rays are deflected away from a negatively charged electrical field and toward a positively charged field, along with their response to magnetic fields, was crucial in determining their particle nature. Thomson's calculation of the electron's charge-to-mass ratio further cemented the understanding of cathode rays, leading to the realization that electrons are fundamental components of all atoms, not just specific types of gases or metals used in the experiments.