64 DECOMPOSITION OK CARBONIC ACID IN THE PRISMATIC SPECTRUM. 



over the sky. This ray (a b,fig. 122) is to be intercepted by a glass prism, c, which 

 disperses it into its different coloured beams. And now, having provided a set of glass 

 tubes filled with spring water, or, rather, water holding carbonic acid gas in solution, 

 and in each placed the same number of leaves of grass or of some other plant, so that 

 each tube may be as nearly like all the others as may be, these tubes, inserted in a small 

 pneumatic trough, c d, are to be set in the spectrum of light, in its different coloured 

 spaces, one in the red, one in the orange, one in the yellow, &c. Care also should be 

 taken to exclude all extraneous light, so that that which causes any action among the 

 leaves may be derived from the ray which comes in through the shutter only. Very 

 soon, if the sky is clear and the sun brilliant, the phenomenon begins. In the tube 

 which is in the most luminous part of the yellow ray small bubbles are evolved, and 

 these rising to the top of that tube, there collect, so that after the lapse of a few hours 

 a sufficient quantity may be gathered for measurement and analysis. The tubes that 

 are in the orange and green lights simultaneously go into action, and when the sky 

 possesses an intense brilliancy, they will even approach in the rapidity of their action 

 to the maximum yellow. A few bubbles also make their appearance in the blue, but 

 after an exposure of many hours, if scrupulous care is taken to shut out all extraneous 

 light, no action whatever is perceptible in the extreme violet, where SENNEBIER supposed 

 the decomposing force to be situated. From these things we therefore gather, that it 

 is in the yellow light that the power controlling the function of digestion of plants is 

 to be found, the other coloured beams, orange, green, red, blue, &c., following in the 

 order of their illuminating power. 



232. This prismatic experiment is one of the most beautiful objects which organic 

 chemistry can offer, carried on in a chamber which would be totally dark were it 

 not for the intensely coloured curves which are cast upon the walls by reflexion from 

 the tubes, curves which often are many yards in length, indicating by their gaudy tints 

 and brilliancy the intensity of the sun's light. The tubes and the vegetable leaves glow 

 with the colours in which they are immersed. Meantime, the most interesting phe- 

 nomenon whichvcau be witnessed is silently going forward ; dead and inanimate matter 

 is, under the influence of the plastic beam, putting on the form of organization and 

 life. Oxygen and nitrogen gases are exhaling, chlorophyl, and gum, and sugar, fib- 

 rine, and albumen are coming into existence. These are compounds which, under 

 ordinary circumstances, are destined to be used as the food, and form part of the bodies 

 of animals. For it is from atmospheric air, as organic chemistry shows, that plants 

 spring, condensed out of it, as it were, by the agency of the solar beam ; to the same 

 source also it is that gradually during life, and totally after death, the parts of animals 

 hasten to return. 



233. A little reflection shows the great advantages which this mode of experiment- 

 ing possesses. In the white light of the solar beam there is a fixed proportion of each 

 of the component colours, and when such a beam is dispersed by prismatic action, and 

 is simultaneously received upon vegetable leaves, we, in effect, measure out to them 

 similar quantities of the different coloured rays, and observe the resulting action. When 

 pieces of glass are used, as in the former experiment (226), a great deal depends on their 



