What evidence led thomson to propose that the mysterious negative particles he was studying were smaller than atoms?

Thomson calculated a charge-to-mass ratio for the electron that was almost 2000 times greater than that of a positive hydrogen ion – the smallest particle known at the time. It made sense to Thomson that the magnitude of the two opposite charges should be equal which meant that the mass of the electron must be approximately 2000 less than the smallest atom.

The gas would begin to expand, and eventually the can would burst or explode.