PHYSICAL CHEMISTRY 25 



of Perrin's that the light emitted is due to the flashes of exploding 

 molecules. Using a very intense beam of light, he obtained 

 from solutions of eosin, rhodamine, and fluorescein, a number 

 of photocompounds which were coloured but non-fluorescent. 

 Their adsorption bands had a totally different form from those 

 of the original substances. Rhodamine, which is almost non- 

 fluorescent at 100° C, decomposes under the action of light 

 almost as rapidly as at ordinary temperatures. This is contrary 

 to Perrin's theory. 



Jaeger {Trans. Chem. Soc, 1921, 119, 2070), when studying 

 the effect of light on the salts of some organic acids, noticed that 

 neutral salts first increased the velocity of the reaction up to a 

 maximum, but that still larger quantities of salt diminished the 

 velocity. The reaction could be completely stopped by 

 addition of a sufficient quantity of the salt. The greater the 

 electrical charge on the cation the greater the effect, ferric ion 

 being particularly effective. 



Electrical Phenomena in Heterogeneous Systems. — In the 

 report on the general discussion of the Physics and Chemistry 

 of Colloids held jointly by the Faradaj^ Society and the Physical 

 Society of London, contributions have been made to our know- 

 ledge of the nature of the electrical charge at the interface 

 between two phases. The complex character of this charge 

 at a solid-water interface is pointed out by Freundlich (p. 146), 

 who considers that it is composed of {a) a space charge at the 

 solid interface, and {b) a diffuse electrical double layer which 

 projects rather deeply into the liquid (after Gouy, Journ. de 

 Phys., 1 9 ID, 9, 457). Only that portion of the charge ^ which 

 is entirely within the liquid and is given by the Helmholtz 

 formula is effective in electrokinetic processes, electroendos- 

 mosis, kataphoresis, and allied phenomena, whereas the 

 Nernst potential difference e comprises the total fall in 

 potential across the interface. ^ and e have been measured 

 for a glass surface in contact with solutions of electrolytes of 

 varying concentrations, the former by measurement of the 

 stream potential in a glass capillary tube and the latter by 

 the method of Haber and Klemensiewicz. e is, of course, 

 dependent only upon the activities of the ions for which the 

 electrode is reversible, in this case H* ions. ^ may, however, 

 have either a positive or negative sign independent of e, and 

 it approaches zero at high electrolyte concentrations. Certain 

 physical properties are to some extent dependent on both e 

 and t, thus the surface tension of the water-mercury inter- 

 face is not alwa3^s at a maximum when e possesses a zero 

 value. 



Mukherjee (p. 103) has studied the neutralisation of the 

 charge on colloidal particles which occurs on coagulation by 



