DECOMPOSITION IS DUE TO LIGHT. 



the yellow, yet that region of the spectrum is far from being devoid of heat and tithon- 

 icity. 



791. By considering, however, the prismatic analysis and the absorptive analysis to- 

 gether, the following facts appear : 1st, the place of maximum action in the spectrum 

 corresponds to the maximum of illumination ; 2d, at the place of the maximum of 

 heat (which in the prism here used is beyond the extreme red) no decomposition what- 

 ever takes effect ; this appears, therefore, to exclude calorific influence ; 3d, the point 

 of maximum action of the tithonic rays, which escape absorption by the bichromate of 

 potash, being towards the green, does not correspond with the place of maximum de- 

 composition, which is the yellow ; this seems to exclude the tithonic rays ; 4th, the de- 

 composition taking place almost as energetically under the bichromate of potash as in 

 the. unobstructed beams of the sun, and that salt absorbing all but a mere trace of the 

 tithonic rays, if the effect was due to them it ought to be retarded to an extent cor- 

 responding to their loss by absorption, which is far from being the case ; the retardation 

 which is observed appearing to be attributable rather to the loss of light by reflexion 

 from the faces of the trough, and the partial turbidity (want of translucence) of its 

 glasses and solutions. 



792. For these reasons, I conclude that the decomposition of carbonic acid by the 

 leaves of plants is brought about by the rays of LIGHT; and that the calorific and 

 tithonic rays do not participate in the phenomenon. As was stated before, therefore, 

 the rays of light are just as much entitled to the appellation of chemical rays as those 

 which have heretofore passed under that name. 



793. I might observe, in passing, that there is a degree of precision attached to 

 results of the decomposition of carbonic acid which is wholly wanting in most similar 

 experiments. In the stains on Daguerreotype plates, or on photographic papers, though 

 there is no difficulty in ascertaining the place of maximum effect, yet nothing in the 

 shape of absolute measures of quantities can be obtained. When, however, gas can 

 be collected and its volume determined, as in the voltameter and in the experiments 

 just described, the results possess a degree of exactness which enables us to draw from 

 them definite conclusions. 



794. Let us now proceed to determine the constitution of the gaseous mixture given 

 off during these decompositions. It is not pure oxygen, as has often been supposed and 

 often disproved, but a mixture of oxygen, nitrogen, and carbonic acid. It is mainly to 

 the ratio of the two former that attention has to be directed ; the amount of the latter is 

 always variable in different trials. Before proceeding to this, there are certain obser- 

 vations to be premised, the results of which, though familiar to chemists accustomed to 

 gaseous analysis, deserve a place here, for thev seem to be wholly overlooked in many 

 of the experiments connected with the so-called respiration, but, rather, digestion of 

 plants recorded in the books of botany. 



795. When gas of any kind is confined over water in the pneumatic trough, its con- 

 stitution is undergoing incessant change. A portion of it dissolves more or less slowly 

 in the water, and in exchange it receives from the water gas, which is always dissolved 

 therein. If two jars, filled with different gases, stand side by side on the shelf, each 



