270 SCIENCE PROGRESS 



and of isocrotonic acid (liquid) into crotonic acid (solid) (Wis- 

 licenus). In all these cases the modification of lower melting- 

 point and greater solubility is transformed by light into the 

 less soluble form melting at a higher temperature. When light 

 is excluded the changes referred to do not take place even in 

 presence of halogen. Thus, for example, when 2 grms. of maleic 

 acid is dissolved in water (5 c.c.) and a little bromine water is 

 added, crystals of fumaric acid separate within a minute when 

 the mixture is exposed to light but in darkness no separation 

 occurs after several hours. Iodine acts far less rapidly than 

 bromine in accelerating the photo-chemical change. The above 

 changes can be expressed by the equations : 



H . C . C0 2 H H . C . C0 2 H 



II -> II 



H . C . C0 2 H C0 2 H . C . H 



Maleic acid. Fumaric acid. 



CH 3 . C . H H . C . CH3 



II -> li 



CH, C.C0 2 H CH 3 .C.C(XH 



Angelic acid. Tiglic acid. 



Similar results were obtained by Liebermann l and may be 

 summarised as follows : 



I. a/Zo-Furfuracrylic acid -> Furfuracrylic acid 



C 4 H 3 . C . H C 4 HsO . CH 



II -> II 



HO.CO.C.H H.C.CO.OH 



mp. 103 . nip. 141 . 



Change takes place slowly in benzene on exposure to sunlight 

 in absence of iodine. When iodine is present 90 per cent, is 

 converted in sunlight in fifteen minutes ; in darkness no action 

 occurs, even when iodine is present. 



II. a/Zo-Cinnamic acid -> Cinnamic acid. 



C K H 5 .CH H.C.QH 



6*-l5 



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



mp. 66°. mp. 133 . 



III. The most striking of all is the case of alio - cinnamy- 

 lideneacetic acid, CHPh :CH . CH : CH . C0 2 H. When dis- 

 solved in benzene the addition of 3 per cent, of its weight of 

 iodine causes the solution, on exposure to light, to set to a 

 crystalline mass of cinnamylideneacetic acid within three 



1 Ber. 1895, 28, 1443. 



