John Dalton (1766–1844) was a quiet Quaker schoolteacher from the north of England who turned an ancient hunch — that everything is made of tiny indivisible bits — into real, weigh-it-on-a-scale science. He started teaching at age twelve, kept meticulous weather records nearly every day for over fifty years, and along the way gave us the modern idea of the atom.

Atoms you could actually count

Dalton's leap was to say each element is made of atoms with a definite weight, and that atoms combine in simple whole-number ratios to make compounds. That's why water is always the same recipe of hydrogen and oxygen, never a random mush. He even drew his atoms as little circles with symbols inside — the first serious attempt at a model of the atom. Later scientists found atoms aren't indivisible after all, but Dalton's framework is where chemistry as a quantitative science truly begins.