148 GIBBS. 



spectrum."^ Bancroft ^* in expounding the second law of Grotthuss ' 



"If the active light is light which is absorbed by the substance to be oxidized 

 and not by the oxygen, then the substance to be oxidized has been made active 

 by the light and the oxygen is the depolarizer. If the active light is absorbed 

 by oxygen and not by the substance to be oxidized, then this latter is the depo- 

 larizer and the oxygen is made active by light." 



Eder ^° with regard to this phase of the question states that a com- 

 pound which is acted upon by light absorbs only those vibrations of the 

 light with which its atoms vibrate, or are capable of vibrating, in syn- 

 chrony. In this way the absorption of the light is connected with the 

 intra-molecular constitution of the compound. The amplitude of the 

 vibrating atoms composing a molecule of a substance sensitive to light 

 may be increased by the light waves vibrating in coincidence, and when a 

 certain limit is exceeded, a breaking of the molecular bonds ensues; that 

 is, photo-chemical decomposition takes place. This effect is well exempli- 

 fied by certain color substances employed in photography to increase the 

 optical sensibility of the silver salts. °' Bancroft says/' concerning the 

 action of optical sensitizers on photographic plates. 



"The theory of Grotthuss enables 'us to make a definite statement in regard 

 to sensitizers, and one that differs to a certain extent from any of the previous 

 ones. A sensitizer must be a depolarizer, directly or indirectly. It must be a 

 reducing agent in the broad sense of the term or it must be changed into one 

 by the action of light. In either case, the sensitizer is decomposed by the 

 action of light on the sensitized plate;" and "Eder's theory of molecular vibra- 

 tion can not account for Abney's experiment with cyanine plates to which the 

 silver bromide was added after the e.\posure to light." "" 



The possibility that ozone is formed during the combination of 

 phenol with oxygen is not excluded. From the fact that small amounts 

 of carbon dioxide have been found in the tubes of pure phenol and 

 purified atmospheric air after exposure in the sunlight until the major 

 portion of the oxygen had disappeared, it seems to be quite possible that 

 the sunlight reaction produces in a measure the same products as the 

 reaction between ozone and phenol in the absence of light. It is also 

 possible that a peroxide, other than hydrogen peroxide, is formed during 

 the reaction. 



" In the case of moist phenol, where there is hydrogen peroxide formation, other 

 considerations enter. 



",/oMrn. Phys. Chem. (1908), 12, 258. 



^^ Ibid., 212. "The action of a ray of light is analogous to that of a voltaic cell 

 (Grotthuss)". 



"■ Photochemie, Halle a/S (1906), 43. 



"Ibid., 45. 



''Loc. cit. 360. 



"Ibid., 376. 



