It has long been known that certain substances, _e.g._ acetoacetic ester, are capable when in solution or in the fused state, of reacting as if they possessed two different constitutions; and in order to explain this behaviour the view was advanced (by Laar) that in such cases a hydrogen atom oscillated between two positions in the molecule, being at one time attached to oxygen, at another time to carbon, as represented by the formula-- CH_{3}.C--CH.CO_{2}C_{2}H_{5} . ^ . | O<-H When the hydrogen is in one position, the substance will act as an hydroxy-compound; with hydrogen in the other position, as a ketone. Substances possessing this double function are called _tautomeric_. Doubt, however, arose as to the validity of the above explanation, and this doubt was confirmed by the isolation of the two isomerides in the solid state, and also by the fact that the velocity of change of the one isomeride into the other could in some cases be quantitatively measured. These and other observations then led to the view, in harmony with the laws of chemical dynamics, that tautomeric substances in the dissolved or fused state represent a _mixture_ of two isomeric forms, and that equilibrium is established not by _intra_- but by _inter_-molecular change, as expressed by the equation-- CH_{3}.CO.CH_{2}.CO_{2}C_{2}H_{5} <--> CH_{3}.C(OH):CH.CO_{2}C_{2}H_{5} {196} In the solid state, the one or other of the isomerides represents the stable form; but in the liquid state (solution or fusion) the stable condition is an equilibrium between the two forms. A similar behaviour is also found in the case of other isomeric substances where the isomerism is due to difference of structure, _i.e._ structure isomerism (_e.g._ in the case of the oximes C_{6}H_{5}.C.H C_{6}H_{5}.C.H || and || ), N.OH HO.N or to difference in configuration, _i.e._ stereoisomerism (_e.g._ optically active substances), or to polymerism (_e.g._ acetaldehyde and paraldehyde). In all such cases, although the different solid forms correspond to a single definite constitution, in the liquid state a condition of equilibrium between the two modifications is established. As a general name for these different classes of substances, the term "dynamic isomerides" has been introduced; and the different kinds of isomerism are classed together under the title "dynamic isomerism."[279] By reason of the importance of these phenomena in the study more especially of Organic Chemistry, a brief account of the equilibrium relations exhibited by systems composed of dynamic isomerides may be given here.[280]