First of all it follows from this constant ratio that the ketone is 

 prominent in the photo-reaction, that this passes into a photo-active 

 condition. Further, that the diverse ketones are activated in a 

 perfectly analogous manner in such a way that either a nnmber of 

 molecules (the same for all ketones) become photo-active, which molecnles 

 then react with the alcohol with a velocity specitic for the ketone; 



or, a number of molecules specitic for each ketone becomes 

 activated which, with a definite velocity which is independent of 

 the ketone, dehydrogenises the alcohol. 



A choice from these alternatives can only be made by a further 

 study of the photo-reaction. 



The active lu/ht of the ketone reduction. 



The first attempt to ascertain the position of the active light in 

 the spectrum has been made by Ciamician and Silbek ^). They in- 

 vestigated, for instance, the reduction of benzophenone and alcohol, 

 employing two photo-filters. 



As a red photo-filter was used a cold saturated solntion of flno- 

 rescein in alcohol (thickness of layer 15 mm.) which extino-uishes 

 all light to 0.510, u; by adding gentian-violet the absorption could 

 be raised to 0.620 fi. 



As a blue filter served a i07„ solution of cobalt chloride in alcohol 

 which transmits rays of a wavelength less than 0,480 ft; a green 

 band at ±0,560 ft and a red one at ±770fi remain, however, 

 unextinguished. They arrive at the result that all the reactions with 

 which they were engaged, took place under the infiuence of blue 

 light. We have used a larger number of photo-filters and carried 

 out the research in jacketed tubes; the inner tubes were those which 

 were used by us in the other experiments; the intervening space 

 was '15 mm. Above the liquid in the jacket the outer tubes were 

 covered with black lacquer, so that none but filtered light could 

 penetrate into the inner tube. 



As photo-filters were selected : 



I. lied : aqueous solution of chrysoidin '). 



II. Green : ,, ,, ,, potassium dichromate -f- acid green 



B. extra.') 

 Blue and violet. 



III. i07„ alcoholic solution of CoCl,. 



IV. Cold saturated aqueous solution of crystal violet 5 B. 0. '). 

 V. ,, ,, ,, ,, ,, acid violet 4 B. N. '). 



VI. Solution of iodine in CCl^. 



^) B. 35, 3598 (1902) 



2} Colouring matters from the "Geselischafl f. chem. Ind. Basel". 



