isomeric

isomeric (ī`so*mer"ik) , adjective

[Iso- + Greek me`ros part: compare French isomérique.]

(Chemistry) 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.

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.”