and of Indigo Light on Plants. 13 



decomposition of carbonic acid, or be the deoxidized yellow 

 matter, or some other substance, as dextrine, already present 

 in the leaf, is unknown. The latter view, applied to the for- 

 mation of oils and fats in animals by Liebig, is probably cor- 

 rect; by adopting it we are relieved from all difficulty in regard 

 to the supply of hydrogen in plants, for the evidence, that 

 water is decomposed in their structures, is by no means con- 

 clusive. In the formation of oils in seeds it is clear that the 

 deoxidation of sugar occurs, for we have a liberation of car- 

 bonic acid from the petals and a destruction of the organic 

 matter. 



Subsequently to the production of chlorophyl carbonic 

 acid is decomposed by light, and this function, directly or in- 

 directly, is sufficient to generate all organic matter. Hence 

 the existence of all organic matter is due to the light of the 

 sun. 



39. On the destruction of chlorophyl by light. — The pro- 

 duction of green matter by the yellow rays leads us to infer 

 its destruction by the red and blue. Sir J. F. Herschel 

 (Phil. Mag., Feb. 1843) found that the juices pressed from 

 the leaves of plants are acted upon by the spectrum with much 

 uniformity. In the case of elder leaves (fig. 8) there was a 

 strong maximum, producing a nearly insulated solar image at 

 — 11*5 of his scale, or nearly at the end of the red rays; the 

 action thence was feeble, with two minima at — 5*0, + 6*8, 

 with a slight intermediate maximum at (0*0) the yellow ; and 

 beyond these, or about the termination of the green, the ac- 

 tion again increases, reaches another maximum at + 20*0, 

 which corresponds to the centre of Fraunhofer's indigo, after 

 which it declines to a point beyond the violet + 45*0. I have 

 been thus precise in giving his result, because my experi- 

 ments made with sethereal solution of chlorophyl, from grass 

 leaves spread upon paper, gave similar spectra. There are 

 two points however which it is necessary to discuss. 



The first action of light is perceived in the mean red rays, 

 and it attains a maximum incomparably greater at that point 

 than elsewhere; the next point affected is in the indigo, and 

 accompanying it there is an action from + 10*5 to + 36*0 

 (of the same scale) beginning abruptly in Fraunhofer's blue. 

 So striking is this whole result, that some of my earlier spectra 

 contained a perfectly neutral space from — 5 to + 10*5, in 

 which the chlorophyl was in no way changed, whilst the solar 

 picture in the red was sharp and of a dazzling white, and the 

 maximum of the indigo was also bleached, producing a linear 

 spectrum, as follows : — , in which the orange, 



yellow and green rays are neutral ; these it will be remembered 



