ISOMERISM (from the Greek word isorares, composed of equal parts), a term applied to those organic compounds which are identical in their ultimate or percentage compo sition, but present differences in their chemical properties. Isomeric compounds, or isomerides, are divisible into metameric compounds, of metamerides, and polymeric com pounds, or polymerides.
In all metameric compounds, the equivalent number is the same, while in all poly meric compcun Is the equivalent numbers are simple multiples of the. equivalent of the lowest number of the group. As an illustration of metamerides, propionic acid, (HO,C,I160s), acetate of methyl and formic ether may be taken. Their rational formulae, which express their probable constitution, are per. fectly distinct, yet it will be at once seen that they all have the same empirical formula (0.11404), and consequently the same percentage composition, and the same equivalent number, viz. 74.
As an illustration of polymerides, the hydrocarbons homologous with olefiant gas may be taken Olefiant gas is represented by the formula by butylene by amyleue by These substances have the same percentage composition, but different equivalent numbers, ull the formulae being multiples of the more simple formula, which represents the composition of an alcohol-radical, methylene, which has not yet been isolated.
The carbohydrates, which are represented by the general formula C„H.O., present well-marked examples of isomerism. Thus, cellulose (CIDH1001 0, starch and gum (Ciallio0.) are metameric; while grape-sugar possesses the same percentage composition, but twice as high an equivalent number, as hydrated lactic acid and the same percentage composition, but three times as high an equivalent number, as hydrated acetic acid (0411404); hence the three last-named substances are polymeric.