Magic with insulating rods

We took a polythene rod and rubbed one end with a cloth. Electrons were taken from the cloth onto the rod so the rod gained a negative charge overall. We balanced it on a watch glass.

 We took a clear cellulose acetate rod and rubbed it. We were able to use it to pull the white polythene rod round without touching - magic!

 We used another polythene rod to push the first one round without touching.

The conclusions are: negative and negative repel. The clear rod attracts so we conclude that it is an opposite charge and we call it positive. It must have lost electrons to the cloth on being rubbed. Thisis what I teach. It all sounds so definite. But where did the original proof of the electron transfer theory come from? And what makes polythene gain electrons when cellulose acetate loses them?