Having the same percentage composition; -- said of two or
   more different substances which contain the same ingredients in the
   same proportions by weight, often used with with. Specif.: (a)
   Polymeric; i. e., having the same elements united in the same
   proportion by weight, but with different molecular weights; as,
   acetylene and benzine are isomeric (polymeric) with each other in this
   sense. See Polymeric. (b) Metameric; i. e., having the same elements
   united in the same proportions by weight, and with the same molecular
   weight, but which a different structure or arrangement of the ultimate
   parts; as, ethyl alcohol and methyl ether are isomeric (metameric) with
   each other in this sense. See Metameric.