THE CHEMICAL ACTION OF LIGHT 273 



in the case of a mixture of derivatives of triphenylmethane with 

 the corresponding triphenylchloromethane, which in sunlight 

 becomes coloured owing to the formation of a triphenylmethyl 

 compound ; this in darkness loses its colour owing to the 

 occurrence of the reverse change ; for example 



(C - Hi Hch + ac< (CA) ' f (QHs) ' >c...< (C ' Hi> ' 



C 6 H 5 . C 6 H/ \C 6 H 4 . C 6 H 5 Darkness C 6 H5.C 6 H 4 / nCuH, . C 6 H 5 



+ HC1 



VI. Hydrolysis and Ring-Scission 



One of the best-known cases of decomposition effected by 

 light is that of the paraffinoid carboxylic acids into carbon 

 dioxide and the corresponding hydrocarbon ; this change takes 

 place in presence of an uranium salt, which acts as a catalyst. 

 In this way, acetic, propionic and butyric acids give respect- 

 ively methane, ethane and propane. 1 Succinic acid gives 

 propionic acid : 



CH 2 .C0 2 H CH 3 



I -> I + CO, 



CH 2 .C0 2 H CH 2 .CCvH 



and pyrotartaric acid butyric acid 2 : 



CH 3 .CH.CO,H CH 3 .CH„ 



I -> I 



CH 2 .C0 2 H CH 2 .C0 2 H 



In the absence of such catalysts, light alone brings about 

 striking hydrolytic effects. One of the most simple is the 

 transformation of wet acetone by sunlight into methane and 

 acetic acid 3 : 



CH 3 . CO . CH 3 + HOH = CH, . CO . OH + CH 4 



and of methylethylketone into acetic acid and ethane 4 : 



CH 3 . CO . C 2 H 5 + H . OH = CH 3 . CO . OH + C,H. 



In these experiments, the air must be displaced by carbon 



1 Seekamp, Annalen, 1862, 122, 115 ; Fay, Amer. Chem.J. 1896, 18, 269. 



J Wisbar, Annalen, 1891, 262, 232. For the analyses of the mixed gases 

 (CO, C0 2 , CH 4 and H>) obtained in the decomposition by ultra-violet rays of many 

 organic substances such as alcohols, sugars, etc., see D. Berthelot and Gaudechon 

 (Compt. Rend. 1910, 151, 395 and 478). 



3 Ciamician and Silber, Atti R. Accad. Lincei, 1903, 12, i. 235 ; Ber. 1903, 

 36, 1575. 



4 Atti R. Accad. Lincei, 1907, 16, i. 835 ; Ber. 1907, 40, 2415. 



