To indicate the number of atoms in a molecule, Berzelius employed a superscript notation, as in H2O. Later, for no special reason, the fashion became to render the number as subscript: H2O.
This doesn't come across in the Goodreads note, but H2O was apparently literally just written like that. With the 2 in the middle. For some reason, people just decided to start making it subscript.
