ABSORPTION AND EMISSION CENTERS. 19 



light emission and absorption. Tschugaeff 1 has examined some five hundred 

 compounds for triboluminescence (luminescence due to the crushing of the 

 compound) and has found that 25 per cent of the organic compounds inves- 

 tigated showed a more or less intense flash when crushed. Most of the lumin- 

 escent compounds could have existed in several isodynamic forms. Phos- 

 phorescence and fluorescence may be explained as being due to a dynamic 

 isomerism existing between two or more forms. For instance, fluorescein 

 may exist in two or more forms: 



O O 



V \oh ho \>: 



c c 



c 6 h/\o 



C 6 H 4 COOH 



CO 



We shall now consider some recent work by Baly and Desch. They first 

 took up acetylacetone, CH 3 CO CH 2 CO CH 3 , and ethyl acetacetate, 

 CH 3 CO CH 2 C0 2 C 2 H 5 , and their metallic derivatives. Acetylacetone 

 and its metal derivatives were found to give similar spectra. Ethyl acetace- 

 tate was found to give only a very slight general absorption, whereas its alu- 

 minium derivative gave a spectrum almost exactly like that of acetylacetone. 

 The work of Perkin and others has shown that free acetylacetone is enolic 

 and ethyl acetacetate ketonic. 



H OH 

 CH, CO C = C CH, 



Acetylacetone (enolic). 



These results would then indicate that the metallic derivatives of ethyl 

 acetacetate have the enolic structure, the metal taking the place of hydrogen 

 in the OH group. But by using various compounds it was shown that neither 



H ' 



the ketonic group, C C , nor the enolic group, CH = C , gives rise to a 



I II I 



HO OH 



trace of banded absorption. This would also result from Hartley's work. 



It would seem probable, then, that the absorption was not to be attrib- 

 uted to any definite molecular structure, but to a dynamic isomerism exist- 

 ing between the two modifications of the compound in solution. It was stated 

 before that alkalis and acids change the velocity of the transformation from 



1 Ber. d. chem. Ges., 34, 1820 (1901). 



