94 PRELIMINARY ABSORPTION AND DEFINITE ACTION OBSERVED BY PLANTS. 



as the whole universe is concerned, the amount of force never varies, there are peri- 

 odical variations in its distribution. The solar rays which year after year are impin- 

 ging on our earth, are so much taken from him, and so much given to us ; and, judging 

 from these things, we should infer that the amount of force exhibited on the surface of 

 our globe should steadily be on the increase. Are not these also the ideas which we 

 conceive from geological investigations 1 And is it not this which, from a silent and 

 tenantless waste, has made that surface the abode of myriads of living things 1 that 

 produces all over it locomotion and activity, and that makes all the difference between 

 what our earth was at the beginning of the secondary epoch, and what she is now? 



377. It was an observation of the older botanists, that the green parts of plants only 

 possess the quality of reducing carbon from the air. In other portions, and under cer- 

 tain circumstances, a reverse action takes place, and carbon, probably under the form 

 of sugar, is oxydized. This never takes place in the presence of chloropbyl. The 

 analogies which we have traced between the mode of action of light, heat, and the 

 tithonic rays, would serve to indicate, that in this remarkable decomposition the lumi- 

 nous rays undergo the same changes, and, indeed, act in a similar way to radiant heat 

 when it gives rise to decompositions, or the tithonic rays when they produce surface 

 alterations. From these analogies, we should judge that the active rays in this instance 

 undergo a true absorption, wliich is, perhaps, divided into two periods, as we have seen 

 is the case when chlorine and hydrogen unite (317), or iodide of silver is decomposed. 

 That the preliminary absorption is observed, numerous facts seem to indicate. When 

 leaves, immersed in water containing carbonic acid in solution, are set in the sunshine, 

 they do not all at once commence evolving gas, but a certain period of time elapses be- 

 fore the bubbles pass off with any rapidity, and that period once over, they seem to fol- 

 low the fluctuation of light with considerable accuracy. A cloud passing before the 

 sun restrains the speed of reduction, and an increased brilliancy of light is followed 

 by an increasing rate of decomposition. From the mode in which these experiments 

 are necessarily made, there is not a perfectly clear proof of the preliminary absorption, 

 for it is not impossible that the hesitation in evolving gas which is observed comes from 

 disturbing causes. It may require a certain time for the carbonated water to find its 

 way through the tissues of the leaf, and to reach the seat of action. It may also re- 

 quire a similar period of time for the evolved gas to percolate out. But still, allowing 

 for these disturbing actions as much time as might be reasonably supposed sufficient, 

 there can be little doubt that, when leaves are brought out of the dark and set in the 

 sunshine, a certain period elapses before vigorous action sets in a period which seems 

 to correspond to that of the preliminary absorption observed in other cases. 



378. If any doubt should remain on that point, there can be none on the circum- 

 stance that light observes the law of definite action. Not only is this apparent from 

 the phenomenon taking place with a rapidity corresponding to great increases and dim- 

 inutions of light, but also that minor differences are rigidly observed. Even in the 

 spectrum, we have seen that the rate of decomposition follows very closely the order 

 of illuminating power, and here we have rays of various refrangibility and of different 

 colours in action. If, under such circumstances, where variations of colour intervene, 



