That branch of science which treats of the composition
   of substances, and of the changes which they undergo in consequence of
   alterations in the constitution of the molecules, which depend upon
   variations of the number, kind, or mode of arrangement, of the
   constituent atoms. These atoms are not assumed to be indivisible, but
   merely the finest grade of subdivision hitherto attained. Chemistry
   deals with the changes in the composition and constitution of
   molecules. See Atom, Molecule.