That comparative quantity by weight of an element which
   possesses the same chemical value as other elements, as determined by
   actual experiment and reference to the same standard. Specifically: (a)
   The comparative proportions by which one element replaces another in
   any particular compound; thus, as zinc replaces hydrogen in
   hydrochloric acid, their equivalents are 32.5 and 1. (b) The combining
   proportion by weight of a substance, or the number expressing this
   proportion, in any particular compound; as, the equivalents of hydrogen
   and oxygen in water are respectively 1 and 8, and in hydric dioxide 1
   and 16.