The great 19th-century Austrian physicist, Ludwig Boltzmann was one of the most important proponents of the idea that all matter is made of atoms.
Today no one doubts this is true but in Boltzmann's day it was a controversial idea and many of his contemporaries disagreed with him. But Boltzmann used brilliant mathematical arguments to show that many aspects of the world we observe, like the behaviour of heat, can be explained if one accepts that atoms are real.