Dr. Gardner on the Action of Light upon Vegetables. 15 



to the supply of hydrogen, in plants ; for the evidence, that wa- 

 ter is decomposed in their structures, is by no means conclusive. 

 In the formacion 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 destruction 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 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 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 maximum, pro- 

 ducing a nearly insulated solar image at —11.5, of his scale,* 

 or nearly at the end of the red rays — the action thence was fee- 

 ble, with two minima at -5.0 and +6.8, with a slight interme- 

 diate maximum at (0.0) the centre of the yellow, and beyond 

 these, or about the termination of the green, the action again 

 increases, reaches another maximum at +20.0, which corres- 

 ponds to the centre of Fraunhofer's indigo, after which it de- 

 clines to a point beyond the violet +45.0. 



I have been thus precise in giving his result, because my ex- 

 periments made with ethereal 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 ray, and 

 it attains a maximum incomparably greater at that point than 

 elsewhere — the next place affected is in the indigo, and accom- 



* 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 of the yellow, and the termination of the 

 violet is sharp and distinct. Sir John Herschel takes the centre of the yellow 

 thus insulated as his zero point, and using a scale of thirtieths of an inch divides 

 the distance between it and the red end into negative parts, and in the direction 

 of the violet positively. The spectrum he used contained 13.30 negative, and 

 40.62 positive parts, and was therefore -%^- inches long. My spectrum corres- 

 ponded with this very closely. 



