Order in the Chemical Elements
The idea of atoms as forming the ultimate constituents of matter already arose in the great speculative or philosophical minds of the ancient world, although no exact chemical or physical atomic theory actually evolved from it at the time, and although, most importantly, there was no direct evidence for it. The fundamental questions were: What are the chemical elements? What are the basic units which carry the identity of a chemical element? Can matter be divided indefinitely while still preserving its identity?