Democritus proposed atom as a solid tiny particle and Dalton considered it a solid sphere. Protons were, not discovered, at that time.
J.J.Thomson, in 1898, gave a visual picture of an atom. Having proved that electrons as a fundamental negatively charged particles of an atom, he proposed the necessity of an equal number of positive charges, to make the atom neutral in nature. Combining the known facts he proposed atom as a solid sphere of positive charge containing electrons on it.
He compared the atom to a jelly having the red plums embedded on it (plum model) or to a red watermelon with black seeds on it.