A good example of this will soon be met with in the case of the iron and carbon alloys. The alloys of copper and tin also furnish examples of the great changes which may take place in the alloy between the temperature at which it separates out from the fused mass and the ordinary temperature. Thus, for example, one of the alloys of copper and tin which separates out from the liquid as a solid solution breaks up, on cooling, into the compound Cu3Sn and liquid:[[278]] a striking example of a solid substance partially liquefying on being cooled.
CHAPTER XI
EQUILIBRIUM BETWEEN DYNAMIC ISOMERIDES
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—
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—
CH3.CO.CH2.CO2C2H5