ON DYNAMIC ISOMERISM. 293 
value are afforded by a detailed study of their absorption spectra. 
Attention has already been called to the observations of Hartley and of 
Dobbie and Lauder, but further reference must be made to the recent 
work of Baly and Desch.! These authors have shown that neither of the 
ethyl- derivatives of ethyl acetoacetate give absorption bands, and con- 
clude that the absorption of light by ethyl acetoacetate depends directly on 
the occurrence of oscillatory isomeric change. They even suggest that the 
intensity of the absorption band is a direct indication of the rate at which 
the reversible isomeric change is proceeding. This theory of the origin 
of colour is in accord with the fact that nearly all coloured substances 
can be represented by two formule, and that colour is most frequent 
amongst aromatic compounds in which a migration of the linkages is of 
frequent occurrence. If this view should be confirmed by subsequent 
observations it would form a most important application of the theory of 
dynamic isomerism. 
9. Luminosity—Whilst colour may perhaps depend only on the 
selective absorption of light-energy by certain groups of atoms, many of 
the phenomena of luminosity appear to be directly due to the inter- 
* conversion of dynamic isomerides.” 
The simplest of these is the flash of light that is sometimes observed 
when a crystal is crushed or powdered, and which in organic compounds 
is usually associated with one of the structures that give rise to dynamic 
isomerism. Thus saccharine and menthylphenylformylacetate, which 
give an exceedingly brilliant flash,* are normally ketonic compounds, 
though their solutions may contain a trace of the labile enolic isomeride 
1 
o,f SN 
tty 
$0, ‘so, 7 
CHO.CHPh.CO.R < HO.CH : CPh.CO,R. 
During rapid crystallisation a small amount of the enolic form may 
be entangled in the crystals, and the flash of light appears to be due to 
the energy liberated when the labile form undergoes isomeric change at 
the moment of crushing. 
Fluorescence appears to be a modification of this phenomenon in which 
the labile isomeride is continuously reproduced by the action of ultra- 
violet light, and phosphorescence may be regarded as fluorescence 
taking place in a viscous medium which will only permit a gradual rever- 
sion to the stable form. The relationship between fluorescence and 
dynamic isomerism has been discussed by Hewitt, and the nature of 
phosphorescence may be illustrated by reference to the ketones, which 
become brilliantly phosphorescent after exposure to ultra-violet light at 
low temperatures (Dewar), probably owing to the liberation on warming 
of energy stored up at low temperatures in the labile enolic form. 
VIII. Reversible Polymeric Change. 
Reversible polymeric changes obey nearly all the Jaws that govern 
reversible isomeric change, and give rise to phenomena similar to those 
that have been described in the preceding sections. But whilst reversible 
1 Trans. 1904, 85, 1029. 
* Armstrong and Lowry, Proc. Roy. Soc. 1903, 72, 258. 
3 Pope, Trans. 1895, 67, 985; Lapworth, 77ans. 1902, 81, 1495. 
* Proc. Chem. Soe. 1900, 16, 3; Zeit. phys. Chem. 1900, 84, 1. 
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