Lucretius (ca. 96-55 B.c.). Their atomic theory was primitive, but the principle in volved was no different from modern theories: the unobserved w invoked to explain the observed. More refined atomic theories today have explained countless phenomena undreamed of by the ancients why element A combines with elements B and C but not with D and E(and some with none at all), why certain elements and compounds have the properties that they do, why they evaporate or ignite at the temperatures they do, freeze at other temperatures, and so on. Virtually all the facts of modern chemistry have been explained in terms of atomic theory. But observed fact. have now been seen through microscopes, and thus they no longer belong to theory. But atoms and electrons, together with the other and more minute"particles" that physics now deals in, remain unobservable.)