ABSORPTION AND EMISSION CENTERS. 



21 



ing a close connection between the anion and cation in such solutions. Baly 

 and Desch conclude that the Faraday tubes may be lengthened out on dilu- 

 tion, and that the force necessary for the separation is furnished by the attrac- 

 tion of the solvent. Solvents which thus exert a strong attracting force are 

 ionizing agents; and this attraction is exerted both on molecules and ions. 

 We have thus hydrated molecules and ions. When the Faraday tubes are 

 lengthened beyond a certain critical value, an interchange of ions between 

 the molecules becomes possible. A completely dissociated solution of a salt 

 is not one in which the ions are moving independently of one another, but 

 one in which the length of the Faraday tubes is greater than the critical 

 value. In tautomeric compounds the Faraday tubes connect the labile atom 

 with the rest of the molecule, being lengthened out to such an extent as to 

 allow these atoms to change their positions in the molecule, and there is a 

 sort of internal ionization within the molecule. To this making and breaking 

 of Faraday tubes may be attributed the absorption of the light. 



In the tautomeric aliphatic compounds the substitution of an alkyl group 

 for the labile atom destroys the tautomerism. This would be expected, since 

 alkyl ions are unknown. Water and other solvents do not have sufficient 

 attractive force to lengthen out the Faraday tubes in this case. Whether 

 there is a banded absorption in these cases is not stated. One consequence of 

 the theory can be tested. Since the persistence of the absorption band is a 

 measure of the number of molecules undergoing transformation at any mo- 

 ment, this persistence should reach a limit for each tautomeric compound when 

 the length of the Faraday tubes has reached their critical length, so that free 

 interchange takes place. By the successive addition of an accelerating com- 

 pound a maximum should be found. Experiment shows this to be true. Take 

 the case of ethyl benzoylsuccinate, to which 1, 10, 20, and 100 equivalents of 

 sodium hydroxide have been added. The limits of persistence referred to a 

 0.0001 normal solution of the ester are as follows: 



Absorption band begins at 



Absorption band ends at 



Change of dilution over which the 

 band persists 



At this point it may be well to refer to the work of Stewart and Baly. 

 Quite a number of chemical facts have been explained by the theory of steric 

 hindrance, although this theory also fails to explain a great many things. 

 For instance, acetic acid, CH :i COOH, is esterified with ease. The methyl, 

 ethyl, etc., derivatives are much more difficult to esterify. This is explained 

 as due to the larger volumes occupied by these radicals, and the consequent 

 hindrance to the approach of the alcohol to the carboxyl radical. If, as is 

 probable, however, the intra-molecular mean free path is large compared with 

 the size of these radicals, this explanation in terms of steric hindrance breaks 

 down. The theory of isodynamic change will explain all these facts and also 



