252 SCIENCE PROGRESS 



The changes effected by light in carbon compounds may 

 be classified as follows : 



(i) Oxidation and reduction (reciprocal), 



(2) Autoxidation, 



(3) Polymerisation, 



(4) Condensation and synthesis, 



(5) Isomeric and stereoisomeric change, 



(6) Ring scission and hydrolysis, 



(7) Two or more of these changes simultaneously. 



All the transformations dealt with in this article occur only 

 under the influence of light : that this is true has been ascer- 

 tained in every case by a control experiment in which the 

 materials that were found to interact in light were left in 

 darkness during a period equal to that of the exposure to 

 the sun's rays without producing any positive result. 



I. Oxidation and Reduction 



The largest proportion of the changes studied are in this 

 class ; reciprocal oxidation and reduction of two substances, 

 one of which is oxidised at the expense of the other, appears, 

 in fact, to be the type of photochemical action most easily 

 brought about. In many cases the change effected consists 

 simply in the transference of one or more hydrogen atoms from 

 the one compound to the other. Thus in the first case studied 

 by Ciamician and Silber, in 1886, 1 when a solution of quinone 

 in aqueous alcohol was exposed to light in sealed glass vessels 

 the yellow-coloured quinone disappeared, giving place to colour- 

 less quinol, an equivalent of aldehyde being produced at the same 

 time ; some quinhydrone was also formed by the interaction of 

 quinol and quinone. 



QH,(0 2 ) + CH 3 . CH 2 . OH -> C 6 H,(OH) 2 + CH, . CHO 



This work was not carried further at the time but in 1901 2 it 

 was found that under similar conditions quinone was capable 

 of oxidising isopropylic alcohol to acetone, being itself, as before, 

 reduced to quinol. Tertiary butylic alcohol (trimethylcarbinol) 

 is also oxidised by quinone, quinol and quinhydrone being 

 formed as secondary products but the nature of the substances 

 into which the alcohol is converted is uncertain ; the action 



1 Gazzetta Chimica Italiana, 1886, 16, in. 



2 Atti R. Accad. Lincei, 1901, 10, i. 92. 



