452 



30. /3-Hydrindon. Is useless as a photocatalyst, as this substance itself is very 

 readily attacked by oxygen in alcoholic solution. 



31. Indanedion 1.2. 



32. Pentanetriketone. 



33. Diphenyltriketone. 



34. Alloxane. 



Inactive. 

 Inactive. 

 Exceedingly slight activity. 



These data allow us to draw (lie f'ollo\ving ooiiclusioiis ; 



(I.. The velocities of activation are independent of the concentration 

 1)1' the ketone (printed in bold type in the tables) witiiin compara- 

 tively wide limits, qnile corresponding to the reduction velocities 

 fonnd before '). This phenomenon does not, indeed, manifest itself 

 III ail the e.xamined cases, bnt it should not be forgotten that the 

 circumstances of the expei'iment necessitate a certain degree of 

 activity and solubility of the ketone to reach the maximum velocity 

 of activation. 



Clear examples in which the oxygen absorption remains constant 

 within wide limits, are benzophenon, diacetyl, and benzoyl acetyl 

 (compare the graphical representations in fig. 5 and 6). We some- 

 times see the activity diminish again in very high ketone concen- 

 trations (20) or in the neighbourhood of the [)oint of saturation (1), 

 which must then be attributed to mutual disturbances of the ketone 

 molecules'). The diminution of activity in lower concentrations must 

 simply be accounted for by the absence of a sufficient quantity of 

 activable ketone molecules, in which part of the available light is 

 left uiiusetl. That really in the concentration region of the maximum 

 activation all the photo-active light is absorbed by a layer only 

 1 mm. thick. I have been able to prove very clearly by means of 

 the reaction vessel according to fig. 4, which can, therefore, be 

 |)erfectly compared with the "mantle tubes" described formerly for 

 the phoco-chemical reduction. When e.g. an alcoholic (or a benzolic) 

 solution of benzophenon in a concentration necessary for the maxi- 

 mum activation is brought into the outer reaction vessel, a benzo- 

 phenon solution in the inner leaction vessel appears to absorb no 

 trace of oxygen; the absorption begins, however, to become imme- 

 diately perceptible, as soon as the ketone concentration in the outer 



-) Cohen, Rec. 39, 253 (1920). 

 2) Tbid.'p 273. 



