158 THE CARBOHYDRATES 



mercury-vapour lamp, so that it appears that " ultra-violet 

 light can effect a measurable decomposition of aqueous carbon 

 dioxide without the intervention of an optical or chemical 

 sensitizer, whilst under normal conditions some such agent is 

 required ; moreover, the results furnish very strong support for 

 the belief that both formaldehyde and hydrogen peroxide are 

 formed in a green leaf". 



The following experiments, also by Usher and Priestly, 

 point to the same conclusion : — 



A glass plate was covered with a layer of gelatine made up 

 with a solution of catalase. When set, the film was painted 

 over with a film of chlorophyll, and the plate sealed up in a 

 glass tube which contained air and some caustic potash. The 

 tube was set up at 5 p.m., and at noon the next day the 

 chlorophyll was quite green and distorted with bubbles. In 

 a control experiment, lacking the catalase, the chlorophyll 

 showed signs of bleaching after one hour, and at the end of the 

 experiment was much bleached. 



Two other experiments were then set up (11.30 p.m.); 

 one exactly as the first, and the other having a solution of 

 carbon dioxide in place of the potash. At ii a.m. the 

 next day the chlorophyll of the last was considerably bleached 

 and showed the presence of relatively much formaldehyde, 

 whilst the chlorophyll of the former was quite green, and only 

 slight indications of aldehyde were obtained. 



As a result of such experiments Usher and Priestly have 

 no doubt that "the bleaching of chlorophyll in sunlight, 

 whether carbon dioxide is present or not, is due to the for- 

 mation of hydrogen peroxide. ... As regards the production 

 of formaldehyde, the experiments are equally conclusive in 

 showing that it is only detected by Schiff's reagent when 

 carbon dioxide is present." 



Using the living plant, the results were of the same nature, 

 but were not so consistent, owing to the difiiculty of controlling 

 the experiments. 



With regard to the evolution of oxygen, they confirm the 

 work of Molisch on the evolution of oxygen on the killed 

 green leaves of Laniium. Usher and Priestly used Elodea 

 which had been anaesthetized for fifteen hours, and found that 

 in the presence of carbon dioxide, oxygen is evolved in the 



