GNU Collaborative International Dictionary of English

Found one definition

      
isomeric , a. [Iso- + Gr. μέρος part: cf. F. isomérique.] (Chem.) Having the same chemical composition and molecular weight; having the same number of atoms of each kind in the molecule; -- said of chemical compounds with known chemical composition.  This property used to be called metameric to distinguish it from other forms of isomerism.
PJC
☞  Formerly the term isomeric applied also to comopounds having the same percentage chemical composition, even if the number of atoms in a molecule differed.  In the 1913 dictionary isomeric was defined as:
“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 with 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.”
1913 Webster

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