How did thompson know that the atom was made of more than just electron?

Because of the research already gathered from scientists before him foe which he compared data with to build his own model

a  solution  is always transparent, light passes through with no scattering from solute particles which are molecule in size. the  solution  is homogeneous and does not settle out. a colloid is intermediate between asolution  and a  suspension. while asuspension  will separate out a colloid will not.