2 2 • PHYSIOLOGY OF NUTRITION 



energy maximum to lie between lines B and C; 1 and, according to the latest 

 researches, the position of this maximum is not constant but varies from the 

 region of the red to that of the yellow-green, according to the hour of the day. 

 Finally, chemically active or "actinic" rays, with a maximum in the violet 

 region, are frequently differentiated. The term actinic rays really refers to 

 the power of light to decompose silver salts, which is most pronounced in the 

 blue-violet region of the solar spectrum. Many other compounds are decom- 

 posed by light, however, frequently in other regions than the blue-violet, and 

 the wave-lengths producing such decomposition are those that are absorbed 

 by the substances decomposed: thus, chlorophyll is most rapidly decomposed 

 by rays between B and C, exactly the ones most completely absorbed by 

 chlorophyll. Therefore, the curve of chemical intensity, as usually given, has 

 no importance excepting with reference to silver salts: there are no specific 

 "chemical" rays. 



Fig. 12. — Graphs of the prismatic solar spectrum. PA, infra-red; AH, visible; HS, 

 ultra-violet rays; PTS, temperature curve; ALH, curve of light intensity; DKS, curve of effect 

 of light upon the decomposition of silver salts. 



Researches upon the influence of light on the decomposition of carbon di- 

 oxide and water by plants fall into two groups. One group includes studies 

 dealing with the qualitative side of the question, as to which rays or wave- 

 lengths are most effective in the process. The other includes quantitative in- 

 vestigations, as to how much energy is needed for this decomposition. The 

 first qualitative work was done by Daubeny™ and Draper" the former using 



i Langley, [S. P.], Observations du spectre solaire. Compt. rend. Paris 95: 482-487. 1882. Idem, 

 Energy and vision. Phil. mag. V, 27 : 1-23. 1889. [Sunlight as it reaches plants is so variable in both 

 quality and intensity that each quantitative experiment on photosynthesis, etc., in natural illumination, 

 should be carried out with very careful measurements of solar radiation. Nutting states that the sun's 

 total radiation varies over a range of 8 per cent, of the mean, while the earth's atmosphere, even with a clear 

 sky, absorbs from 20 to 50 per cent., and this varies from minute to minute and from hour to hour of the 

 day. Nutting gives a table (p. 202) of mean solar energy quantities reaching the surface of the earth at 

 Washington at noon, for 26 different wave-lengths, from 38s to 428^. (See Nutting, P. G., Outlines of 

 applied optics. Philadelphia, 191 2.) The wave-length showing the maximum energy value also varies 

 markedly in natural sunlight. For further information see: Abbot, C. G., and Fowle, F. E., Jr., Primary 

 standard pyrheliometer. Ann. Astrophys. Observ. Smithsonian Inst. 2: 39-47- 1908. Idem, The 

 value of the solar constant of radiation. Astrophys. jour. 33: 191-196. 19". Also see Pulling, H. E., Sun- 

 light and its measurement. Plant World 22: 151-171, 187-209. 1919— Ed 



••' Daubeny, Charles, On the action of light upon plants, and of plants upon the atmosphere. 

 Phil, trans. Roy. Soc. London 126: 149-175. 1836. — Ed. 



" Draper, John W., On the decomposition of carbonic acid gas and the alkaline carbonates 

 by the light of the sun. Phil. mag. Ill, 23: 161-175- l8 43- Idem, Scientific memoires. 

 473 p. New York, 1878. P. 184-185.— Ed. 



