Scientists spent the 19th-century caught in the cross fire between experimental evidence in favor of the atomic theory and philosophic arguments against it. From Dalton's chemistry to Maxwell's gas theory, strong evidence that matter is composed of atoms accumulated rapidly. By about 1870 the theory should have been regarded as proven. Instead, many physicists and chemists were in the grips of a post-Kantian empiricism that rejected all hypotheses not verified by direct observation. These scientists led a zealous fight against the atomic theory. This fascinating chapter in the history of science dramatically illustrates the power of philosophy.