ULTRA VIOLET RADIATIONS 29 



mentable by yeast. The authors suggested that the chlorophyll 

 in plants acts as a means of absorbing ultra-voilet rays, a sug- 

 gestion which has since been found to be true. Bertholet and 

 Gaudechon * found that formaldehyde is produced by the action 

 of ultra-violet rays on carbon dioxide in the presence of a re- 

 ducing agent, and, with regard to the reverse process, that 

 carbohydrates are decomposed by sunlight and by ultra-violet 

 light from a mercury lamp. The products of decomposition 

 are carbon monoxide, carbon dioxide, methane and hydrogen ; 

 aldehydic sugars differ from ketonic sugars both in the readiness 

 with which they are decomposed and in the composition of the 

 gaseous mixtures produced. 



Usher and Priestley f found that ultra-violet light can bring 

 about the decomposition of aqueous carbon dioxide without 

 the intervention of an optical or chemical sensitizer, a result 

 contrary to Stoklasa and Zdobnicky who found that formaldehyde 

 was not produced by the action of ultra-violet light on carbon 

 dioxide and water. In view of these contrary results, Spoehr j 

 tried the effect of ultra-violet radiations on carbonic acid and its 

 salts ; in no experiment was a sufficiency of formaldehyde pro- 

 duced to give a positive reaction with the reagents employed. 

 In all experiments Spoehr found that formic acid was the only 

 reduction product. Attempts to reduce formic acid to formalde- 

 hyde by sun or by ultra-violet light failed, but after ten to fifty 

 hours exposure there remained on evaporation a non-volatile 

 yellow syrup, of a composition not yet determined, which re- 

 duced Fehling solution. 



The subject has received renewed attention from Baly, 

 Heilbron, and Barker who find that an aqueous solution of 

 carbon dioxide yields formaldehyde when exposed to light of 

 very short wave length, 200 jifi. Under the influence of light 

 of wave length 290 /*,//,, however, formaldehyde in water is 

 polymerized to reducing sugars, but if substances, sodium 

 phenoxide for example, which absorb this wave length and 

 which are ineffective in the chemical actions involved, are 



Bertholet and Gaudechon: " Compt. rend.," 1910,150, 1690, 151, 395; 

 !9i2, 155, 401, 831. 



f Usher and Priestley: " Proc. Roy. Soc.," Lond., B. 1911, 84, 101. 



Spoehr : " Plant World," 1916, 19, i. 



Baly, Heilbron, and Barker: ' Joum. Chem. Soc.," Lond., 1921, 119, 

 1025. 



