How can you say that electron is a universal constituent all atoms?

Discharge tube experiments have revealed that electrons constituting the cathode rays are identical irrespective of material of cathode or gas used in the discharge tube. All these electrons are found to have same value of ^e/_m . Thus cathode rays consist of fundamental common particles known as electrons.

Moreover, electrons can be also emitted:

(i) by heating certain metal filaments to high temperature,
(ii) by the radioactive substances in the form of β-rays and
(iii) by exposing certain metal surfaces especially the alkali metals to certain suitable high frequency radiations such as ultraviolet or X-rays.

From the above discussion, it is clear that electrons can be emitted from all kinds of matter and value of ^e/_m is found to be the same regardless of the source or method by which electrons are obtained. Therefore, it can be concluded that electrons are universal (or common) constituents of all matter.