INFLUENCE OF LIGHT ON PLANTS. 65 



be presently shown) require a less amount of this agent than those of a 

 higher kind. Thus in the lake Solfatara in Italy, an unusual supply 

 of carbonic acid is afforded by the constant escape of that gas from 

 fissures in the bed of the lake, with a violence that gives to the water 

 an appearance of ebullition ; and on its surface there are numerous 

 floating islands, which consist almost entirely of Confervas and other 

 simple cellular plants, growing most luxuriantly on this rich pabulum. 

 And it has been remarked, that the vegetation around the springs in 

 the valley of Gottingen, which abound in carbonic acid, is very rich and 

 luxuriant ; appearing several weeks earlier in the spring, and continuing 

 much later in the autumn, than at other spots in the same district. 

 Many circumstances lead to the belief, that at former epochs in the 

 Earth's history, the atmosphere was much more highly charged with 

 carbonic acid than at present ; and that to this circumstance, in con- 

 junction with a more intense and constant influence of light and heat, we 

 are to attribute that extraordinary luxuriance of the vegetation of those 

 periods, of which we have most abundant evidence, in the vast beds of 

 disintegrated vegetable matter Coal that are of such value to Man, 

 and in the remains which have been more perfectly preserved to us, and 

 which indicate that not only the general forest mass, but many of the 

 individual forms, attained a degree of development, which cannot now 

 be paralleled even between the Tropics. 



86. Various experiments have been recently made, with the view of 

 determining more precisely the conditions under which Light acts, in 

 producing the chemical changes that have been now discussed. These 

 experiments for the most part agree in the very interesting result, that 

 the amount of carbonic acid decomposed by plants subjected to the dif- 

 ferently-coloured rays of the solar spectrum, but otherwise placed in 

 similar circumstances, varies with the illuminating power of the rays, 

 and not with their heating or their chemical power. The method adopted 

 by Prof. Draper, which seems altogether the most satisfactory, consisted 

 in exposing leaves of grass, in tubes filled with water which had been 

 saturated with carbonic acid (after the expulsion of the previously dis- 

 solved air by boiling), to the influence of the different rays of the solar 

 spectrum, dispersed by a prism ; these were kept motionless upon the 

 tubes for a sufficient length of time to produce an active decomposition 

 of the gas in the tubes which were most favourably influenced by the 

 solar beams ; and the relative quantities of the oxygen set free were 

 then measured. It was then evident that the action had been almost 

 entirely confined to two of the tubes, one of them being placed in the 

 red and orange part of the spectrum, and the other in the yellow and 

 green. The quantity of carbonic acid decomposed by the plant in the 

 latter of these, was to that decomposed in the former, in the ratio of 

 nine to five ; the quantity found in the tube that had been placed in the 

 green and blue portion of the spectrum, would not amount, in the same 

 proportion, to one; and in the other tubes, it was either absolutely 

 nothing, or extremely minute. Hence it is obvious that the yellow ray, 

 verging into orange on one side, and into green on the other, is the 

 situation of the greatest exciting power possessed by light on this most 

 important function of plants ; and as this coincides with the seat of the 



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