1903.] The Cosmical Function of the Green Plant. 449 



Absorption of Solar Energy by the Chlorophyll of Leaves. 

 Direct Sunlight. 



Maple, 1 leaf (mean of 32 expts.) 27 per cent. 



3 leaves ( 4 ) 31 



Lime, 1 leaf (mean of 6 expts.) 29 



3 leaves ( 3 ) 35 



Oak, 1 leaf (mean of 4 expts.) 23'5 



Plantain, 1 leaf 33-4 



I'otamogrton I wens, 1 leaf (mean of 8 expts.) ... 20 



Yellow Light. 

 Maple, 1 leaf (mean of 8 expts.) 13'8 



Keel Light. 

 Maple, 1 leaf 10 



Before applying these values to a full discussion of the question of 

 the economic coefficient and maximum chemical efficiency, the second 

 of the problems we have just stated the influence of the intensity of 

 light must be studied. It is too often admitted that the photo- 

 chemical effect stands in direct proportion to the light intensity, 

 though the contrary has been put beyond any doubt in some cases 

 where most exact methods have been applied. I have but to men- 

 tion the masterly researches of Sir William Abney on photographic 

 plates. 



The influence of the light-intensity on the reduction of carbonic 

 dioxide has been studied by Wolkoff, Van Tieghem, Kreussler, Famin- 

 tzin, and Reinke, but the results being very discordant and difficult to 

 reconcile, I undertook a somewhat detailed study of the question. The 

 methods adopted were the following (fig. 9). In a dark room a pencil 

 of light was obtained by means of a powerful Foucault heliostat (the 

 mirror measuring 18 cm. by 40 cm.) and a lens 25 cm. in diameter. 

 Parallel to the axis of this cone of light was placed an optical bench 

 2 mm. in length with a scale divided in centimetres, and furnished 

 with five movable and adjustable oblong stands placed across the 

 bench. On each of these stands was placed one of those apparatus 

 with pieces of leaves already described. The distances on the bench 

 were so chosen that the relative intensities of light corresponded to 

 the numbers 1, , J, J, T V> the unit being direct sunlight 3 or 4 hours 

 on each side of noon, when the variation of intensity (as may be seen 

 in the adjoining diagram, borrowed from Crova) is not very consider- 

 able. Any experiment, thanks to the gasometric method already 

 described, did not last more than 20 minutes, so that one could be sure 

 of disposing of continual sunshine. 



