THE CHEMICAL ACTION OF LIGHT ON 

 ORGANIC COMPOUNDS 



By W. A. DAVIS, B.Sc. 



Without question the most important chemical change induced 

 by light is the transformation of carbon dioxide into sugars 

 and starch under the influence of the chlorophyll of green 

 leaves, involving as it does the absorption of much energy. 

 In most cases studied, light brings about a change involving 

 a loss of energy; in the minor number of cases in which energy 

 is undoubtedly absorbed and its amount can be approximately 

 calculated, the absorption, expressed in thermal units, is ex- 

 ceedingly small : thus in the polymerisation of anthracene to 

 dianthracene, which has been studied by Luther and Weigert, 1 

 the amount of energy absorbed, though greater than in most 

 other cases, is yet only about forty calories per gramme of 

 anthracene transformed. In the formation in the leaf of each 

 gramme of starch from carbon dioxide and water, an amount 

 of energy represented by 4,230 calories must be supplied ; this 

 is more than 100 times as great as that absorbed in any 

 other known photochemical change. The rapidity also of the 

 synthetic action effected in plant foliage is far greater than 

 that observed in the majority of other photochemical changes, 

 especially in comparison with those in which energy is ab- 

 sorbed. The assimilation of carbon in the plant is, in fact, 

 an unique phenomenon. The object aimed at in the present 

 article is to give an account of the experimental work which 

 has been carried out during the past few years, especially by 

 Professors Ciamician and Silber at Bologna, to obtain direct 

 information as to the general character of the changes brought 

 about in organic compounds by the action of light. 2 



1 Zeit. Phys. Chem. 1905, 53, 416. 



3 Three monographs on the chemical action of light have been published 

 recently : (1) Die Chemische Wirkungen des Lichts, by Fritz Weigert (Ahrens' 

 Sammlung Chemischer unci Chemisch-technischer Vortrage, 191 1, vol. 17, pp. 

 183-296) ; this deals with the question mainly from the physical side. (2) Les 

 Actions chimiques de la Lumiere, an address delivered by Prof. Ciamician before 

 the Chem. Society of Paris (Bull. Soc. C/iim. t 1908), in which his experiments up 

 to that date are discussed. (3) Photochemie, by Joh. Plotnikow (Knapp, Halle- 

 a-Sa., 1910), a general treatise. 



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