00 Dr Gardner on the Action of Light on Vegetables. 



been conjectured, that the greening of plants, and decomposi- 

 tion of carbonic acid, are produced by the same agent — which 

 is also the active unponderable in producing vision — a pheno- 

 menon in no way similar, as suggested by M. Moser, to the 

 change of Daguerre's plate, which is a tithonic action. The 

 dependence of the depth of green colour in foliage upon bril- 

 liant light, is also shewn. The statements of travellers, in re- 

 spect to tropical vegetation, confirm this conclusion. 



(38.) Chlorophyl, the body generated in the yellow leaflets 

 of plants, raised in darkness by the action of light, is a hydro- 

 carbon, of the nature of wax. Whether it be produced by de- 

 composition of carbonic acid, or be the yellow matter, or some 

 other substance, as dextrine, already present in the leaf, which 

 has suffered deoxidation, is altogether unknown. The latter 

 view, applied to the formation of oils and fats in animals, by 

 Liebig, is probably correct ; 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 conclusive. In the formation of oils in seeds, 

 it is known that the deoxidation of sugar occurs ; for we have 

 the liberation of carbonic acid from the petals, &c., and a de- 

 struction of the organic matter. 



Subsequently to the production of chlorophyl, carbonic acid 

 is decomposed by light, and this function, directly or indirectly, 

 is sufficient to generate all organic matter. Hence the exist- 

 ence 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 blue and red. Sir J. F. W. Herschel 

 (Phil. Mag., Feb. 1843; found that the expressed juices of 

 leaves are acted upon by the spectrum with much uniformity. 

 In the case of elder leaves (fig. 8), there was a strong maxi- 

 mum, producing a nearly insulated solar image at — 11.5 of 

 his scale,* or nearly at the end of the red rays, — the action 



* By proceeding as in art. 13, a spectrum is obtained which has only 

 the width of the focal picture of the sun, and is of considerable length ; 

 these elements differ, however, with the focal distance of the lens. Upon 

 examining such a spectrum through cobalt-glass, a perfectly circular 

 image of the sun is seen at the extreme red end, another in the centre 



