John Dalton proposed the Billiard Ball Model in 1802. His claim was that atoms were the smallest constituents of matter and that they were entirely solid, like billiard balls.
Dalton was a chemist whose work followed on from the writings of the ancient Greek philosopher Democritus. Democritus suggested that all matter was composed of very tiny indivisible pieces that he named atoms. Most of his contemporaries found his theory insupportable and it was largely disregarded until Dalton's work was published in the early 19th century.
After years of research into the properties of matter Dalton developed his Billiard Ball Model of atoms. It offered a foundation for later research into atoms and the behavior of elements. Dalton's model was flawed in that it supposed atoms were entirely solid when they are not and that atoms are the smallest particles in existence, which is untrue.