272 



SCIENCE PROGRESS 



The behaviour of camphor and fenchone is dealt with in 

 Section VI. 



The recent results of Stoermer 1 are of great interest in 

 showing that the ultra-violet rays, in many instances, produce 

 effects which are the opposite of those brought about by 

 ordinary light. Thus when the stable, less fusible forms of com- 

 pounds containing an ethenoid linkage are exposed in benzene 

 or alcoholic solution to ultra-violet rays, they give rise to the 

 labile stereo-isomerides of lower melting-point. The following 

 changes take place from left to right in ultra-violet rays and 

 from right to left in sunlight : 



Methylcoumaric acid ^_ Methylcoumarinic acid. 



Dimethyl 0-nitrocoumarate ^> Dimethyl 0-nitrocoumarinate. 



Methoxycinnamic acid (or amide) ^ «//<?-Methoxycinnamic acid (or amide). 



Fumaric acid 





Maleic acid. 



In some instances, as for example the change of cinnamic acid 

 to isocinnamic acid and of methylic coumarate to coumarin, the 

 action is not reversible but takes place in one direction only. 



In this place it is not possible to do more than mention the 

 so-called " phototropic " changes studied by Marckwald and 

 others. 2 Such changes occur particularly in the case of certain 

 aldehyde-phenylhydrazones, which change in colour under the 

 influence of sunlight, the products formed regaining their 

 original colour when kept in darkness. No doubt a change of 

 structure is involved in the alteration. The same is true of the 

 fulgides of^Stobbe 3 of the general structure: 



R \ c 

 R \ 



c.co 



\ 



o 



C = C . CO 



R' 



which undergo similar " phototropic " change. Schlenk and 

 Herzenstein 4 have observed a somewhat similar transformation 



1 Ber. 191 1, 44, 637. 



3 Marckwald, Zeit. physikal. Chem. 1899, 30, 140; Biltz, ibid. 30, 527 ; Padoa 

 and others, Atti R. Accad. Lincei, 1910, 19, i. 490; ii. 302 ; 191 1, 20, i. 675 ; ii. 

 712. 



• Zeit. Elektrochem. 1908, 13, 479. 



* Rn, 1 910, 43, 3545. 



