CH3CO.CHX.CO2C2H5

ch3co.chx.co2c2h5, derivatives of acetoacetic ester. these compounds generally behave as ketones; but at the same time they may act as alcohols, i.e. as if containing the oh group; this leads to the formula h3c.c(oh):cx.co2c2h5. in reality such tautomeric compounds are apparently a mixture of two isomers in equilibrium, and indeed in some cases both forms have been isolated; then one speaks of _desmotropy_ (gr. [greek: desmos], a bond or link, and [greek: trope], a turn or change). nevertheless, the relations obtained in reversible cases such as sulphur have not yet found application in the highly interesting cases of ordinary irreversible isomerism. a further step in this direction has been effected by the introduction of reversibility into a non-reversible case by means of a catalytic agent. the substance investigated was acetaldehyde, c2h4o, in its relation to paraldehyde, a polymeric modification. the phenomena were first observed without mutual transformation, aldehyde melting at -118 deg., paraldehyde at 13 deg., the only mutual influence being a lowering of melting-point, with a minimum at -120 deg. in the eutectic point. when a catalytic agent, such as sulphurous acid, is added, which produces a mutual change, the whole behaviour is different; only one melting-point, viz. 7 deg., is observed for all mixtures; this has been called the "natural melting-point." it corresponds to one of the melting-points in the series without catalytic agents, viz. in that mixture which contains 88% of paraldehyde and 12% of acetaldehyde, which the catalytic agent leaves unaffected. such an introduction of reversibility is also possible by allowing sufficient time to permit the transformation to be produced by itself. by r. rothe and alexander smith's interesting observations on sulphur, results have been obtained which tend to prove that the melting-point, as well as the appearance of two layers in the liquid state, correspond to unstable conditions. (j. h. van't h.) isotherm (gr. [greek: isos], equal, and [greek: therme], heat), a line upon a map connecting places where the temperature is the same at sea-level on the earth's surface. these isothermal lines will be found to vary from month to month over the two hemispheres, or over local areas, during summer and winter, and their position is modified by continental or oceanic conditions.