THE CHEMICAL ACTION OF LIGHT 



271 



minutes ; 80 per cent, is converted in this time. In darkness, 

 even when iodine is present, no change occurs in six days. 

 Artificial light, such as that of a Welsbach burner, brings about 

 the transformation but more slowly. 



In view of these facts, Ciamician and Silber studied the 

 behaviour towards light of some of the oximes which exist in 

 two stereoisomeric forms ; «////-benzaldoxime and anti-piperonal- 

 doxime are not affected by light but the three nitrobenzaldoximes, 

 w-nitroanisaldoxime and chlorobenzaldoxime are each trans- 

 formed into the stable isomeride of higher melting-point. 1 The 

 case of 0-nitrobenzaldoxime is particularly interesting, as in view 

 of the transformation of o-nitrobenzylideneaniline into o-nitroso- 

 benzanilide (observed by Sachs, see p. 261) and of o-nitrobenz- 

 aldehyde into nitrosobenzoic acid, it was to be anticipated that 

 o-nitrosobenzhydroxamic acid would be formed, thus : 



NO., NO 



^>CH:NOH -> r^ 



CO.NH.OH 



Actually this change does not occur. 



If a solution of sy/w-tribromodiazobenzene-syw-cyanide in 

 benzene be exposed to light, after three days the corresponding 

 tftt//-compound 2 is formed. 



An important isomeric change brought about by light is that 

 which occurs in carvone, which is converted into a well-defined 

 new substance whose exact nature is still uncertain. The 

 change may be analogous with that undergone by the ethenoid 

 compounds considered above but an alternative possibility 

 is the following: : 3 



CH> CH CH9 



CH,:C.CH 3 



I 

 CH = C 



CH., CH CHo 



I 

 C.CH 3 



CO 



/' 



CH., 



CH — C 



CO 



CH 3 



Carvone. 



CH 3 

 New form. 



1 Atti R. Accad. Lincei, 1903, 12, ii. 528 ; Ber. 1903, 36, 4266. Ciusa, Atti 

 R. Accad. Lincei, 1906, 15, ii. 721. 



2 Ciusa, Atti R. Accad. Lincei, 1906, 15, ii. 136. 



3 Ciamician and Silber, Atti R. Accad. Li?icei, 1908, 17, i. 576; Ber. 1908, 

 41, 1928. 



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