18 



THE ABSORPTION SPECTRA OF SOLUTIONS. 



disappearance of the bands in the lutidines and trimethylpyridine. The dis- 

 appearance of the bands as the number of methyl groups in the pyridine ring 

 increases is analogous to the lessened persistency of the absorption band as 

 the number of methyl groups is increased when the substances are examined 

 in alcoholic solution. No absorption bands are found for alcoholic solutions of 

 piperidine. 



Alcoholic solutions of furane, thiophene, and pyrrol show no bands 

 according to Purvis. 1 Neither do the liquids furfuraldehyde, thiophene, or 

 pj'rrol have any absorption bands. 



Theory of Dynamic Isomerism. 



Baly, Stewart, Desch, and others have recently supported the view that 

 the absorption of light by organic compounds does not take place under ordi- 

 nary conditions, but only when there is a change in the union of the atoms. In 

 some cases this change of union takes place when a chemical compound 

 changes into an isomeric form. Dynamic isomerism exists when there is 

 some third substance to act as an intermediary. Many substances are iso- 

 dynamic only at high temperatures or in the presence of a catalytic agent. 

 Sometimes the solvent may promote isomeric change. The point of equilib- 

 rium is determined by the velocities of the isomeric change, and these veloc- 

 ities are affected by the solvent, concentration, temperature, catalytic reagent, 

 or the presence of free alkalis or acids. 



An example of the above change is the transfer of a labile hydrogen atom 

 from an oxygen or sulphur atom to a carbon or nitrogen atom. Sometimes 

 an OH or CN group is transferred in the same way. The atom or radical 

 transferred assumes a neutral condition compared with its condition as a 

 powerful negative radical in the inorganic acids. 



The theory of dynamic isomerism is useful in explaining a large number 

 of phenomena in organic chemistry, and especially those connected with 



1 Journ. ('hem. Soc, 98, 1648 (1910). 



