The first person to say that matter was made of atoms, and then wrote it down, was a Greek philosopher named Democritus. But he had no proof of his idea. Then a number of scientists proposed a atomic model. But it wasn't until the early 1800s that John Dalton proposed that all matter was made of atoms and actually used it to explain a bunch of experiments that had been done on gases, and to calculate atomic weights of elements. He still hadn't PROVED that atoms existed...he just showed that the atomic concept was useful and helped explain a lot of data.