Some Photochemical Experiments with Pure Chlorophyll. 359 



vessels by the zinc chlorophyll formed from a magnesium-free derivative of 

 chlorophyll. It suggests that Ewart's experiment on the re-formation of 

 chlorophyll from a mixture of xanthophyll and bleached chlorophyll (in one 

 case indeed zinc dust was added) should be interpreted as the taking up of a 

 metal by a yellowish or colourless magnesium-free derivative, by which a 

 complex, chlorophyll-green in colour, is produced. 



Conclusion. 



The result of this investigation is tp show how unsatisfactory is the 

 experimental evidence on which the various theories of the function of 

 chlorophyll are based. 



Summary. 



1. Certain photochemical reactions of chlorophyll, on which previous 

 workers have based theories of carbon assimilation, have been examined. 

 Earlier investigators have used crude chlorophyll containing, besides the two 

 chlorophyll pigments, yellow pigments and other substances. In the present 

 experiments pure chlorophyll (" a " + " b "), extracted by the method of 

 Willstatter and Stoll, and satisfying the criteria of purity given by them, 

 has been used. 



2. The chlorophyll was used as a sol, with water as dispersion medium. 

 This sol, in contact with various gases in closed vessels, was exposed to 

 light. The following- are the results obtained : — ■ 



(a) Nitrogen.- — -In nitrogen no change in the chromogen complex of the 

 chlorophyll molecule takes place. 



(b) Carbon Dioxide. — In carbon dioxide the magnesium-free derivative 

 phseophytin is produced. In this action carbon dioxide behaves in solution 

 like any other weak acid. No further change in the chromogen complex 

 takes place. Formaldehyde is not formed. 



(c) Oxygen. — In oxygen the first change observed is the yellowing and 

 ultimate bleaching of the chlorophyll. The yellowing is due to the presence of 

 phseophytin. If alkali is added this yellow stage is omitted and the bleaching 

 accelerated. In the first stages, while bleaching is in progress, formaldehyde 

 is produced only in very small quantities, but, after bleaching is complete , 

 the quantity of formaldehyde increases, rapidly reaching a maximum and 

 then diminishing. The acidity of the system, on the other hand, increases 

 throughout. It is suggested that the formaldehyde arises mainly from the 

 phytol, which is probably split off from the chlorophyll molecule under the 

 action of light and oxygen. 



3. The hypotheses put forward by Usher and Priestley, H. Wager, and 



VOL. LXXXIX. — B. 2 G 



