The behavior of cathode rays in a glass tube containing gas at low pressure led scientists to conclude that they were composed of negatively charged particles, now known as electrons.

Here's why:

* Deflection by magnetic fields: Cathode rays were observed to bend in a predictable way when exposed to magnetic fields, indicating that they carried an electric charge. The direction of the deflection suggested a negative charge.

* Deflection by electric fields: Cathode rays also deflected when placed in an electric field, again confirming their charged nature.

* Independent of the gas: The properties of the cathode rays remained consistent regardless of the type of gas used in the tube, suggesting a fundamental constituent of matter.

These experiments, most notably those conducted by J.J. Thomson, led to the discovery of the electron and provided evidence for the subatomic structure of atoms.