"The distinction would only come to Mendeleev halfway through writing his Principles of Chemistry. ...chemical practice and not chemical theory had provided his initial organizing principle... Up to this point [Chapter 20], Mendeleev had only treated four elements in any detail: oxygen, carbon, nitrogen, and hydrogen—the so-called "organogens." Mendeleev began this chapter as usual by purifying the central substance, sodium chloride, from sources such as seawater. A discussion of sodium and chlorine followed in the next few chapters, and finally the halogens appeared... that were closely related to chlorine... and the alkali metals (the sodium family) form the first chapter of volume 2. ...he had dealt with only 8 elements, relegating 55... to the second volume. ...Mendeleev's earlier system of pedalogically useful organization—using laboratory practices... could no longer sustain the burden of exposition. He needed a new system... and he hit upon the idea of using a numerical marker for each element. Atomic weight seemed the most likely candidate for a system that would (a) account for all remaining elements; (b) do so in limited space; and (c) maintain some pedagogical merit. His solution, the periodic system, remains one of the most useful tools in chemistry."