SECT. V. ELABORATION OF CARBONIC ACID. 147 



gas, therefore, is essential to vegetation in the 

 sun. 



But plants which were made to vegetate at the 

 same time in receivers filled with common air with- 

 out lime had effected no change in their atmosphere 

 either in purity or volume ; this seems to contradict 

 the necessity of the elaboration of carbonic acid. 

 But the process was here imperceptible only because 

 the plant again inhaled the gas in the same pro- 

 portion in which it had previously evolved it, or 

 formed it with the surrounding oxygene. 



It should be added, however, that the action of 

 lime water did not produce the same destructive 

 effect upon the leaves of succulent plants, such as 

 the Cactus, when treated as above ; which, owing 

 to their very thick parenchyma and less porous 

 epidermis, thus seem to retain more obstinately the 

 carbonic acid which they form. 



But the result of such experiments as were con- 

 ducted in the shade was very different ; for so far 

 were plants thus exposed from showing any symp- 

 toms of langour or decay when placed under re- 

 ceivers containing lime and lime water, that their 

 growth was even more vigorous than that of others 

 which were placed under receivers containing only 

 common air. The mean augmentation of each 

 plant in the receiver containing lime was seven 

 grains in the space of six days ; and the proportion 

 of carbonic acid remaining after the experi- 

 ment T-J-yj while the mean augmentation in the 



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