In this MIT lecture, at 7:22, the professor says that when J.J. Thomson added a positively charged plate on one side of the cathode ray and a negatively charged plate on the other side, he observed a large deflection towards the positive plate, and a small deflection towards the negative plate (see image below).
This observation implied that hydrogen gas consists of positively charged particles with lots of mass, and negatively charged particles with a small amount of mass.
However, according to Wikipedia, cathode rays are "streams of electrons observed in vacuum tubes." Thus, my question is: in J.J. Thomson's experiment, where are the protons (positively charged particles) coming from?