54 



but if, at the same time, it be as constantly absorb- 

 ed by the leaves, how can its presence be mani- 

 fested in such quantity, and in such progression, as 

 experiment evinces that it is ? If plants do die in a 

 given bulk of air, and that, too, more or less rapid- 

 ly as carbonic acid is produced (30. ), with what 

 propriety can we hold that substance to be essential 

 to their life ? If they do live and flourish in air de- 

 prived of carbonic acid (30.), and this acid be a 

 consequence of their growth, on what principle can 

 we at the same time assign it as a cause ; or how can 

 we consider that substance to be essential to the ex- 

 istence of this process, when it did not itself exist 

 until produced by the very process to which its ex- 

 istence was essential. 



43. But it is said, that M. Saussure, by putting a 

 quantity of lime into the glass-vessel in which plants 

 were vegetating, found that they no longer conti- 

 nued to grow, and that the leaves in a few days fell 

 off *. Admitting the correctness of this experiment, 

 is it any proof that the process ceased from the want 

 of carbonic acid? Would not the lime equally abstract 

 the moisture of the plant, and would not this be 

 more likely to occasion the fall of the leaf? The 

 leaf, indeed, might die from the absence of any elas- 

 tic fluid essential to its life ; but, if it did, it would 

 still adhere to the stem. The following experiment 

 is offered in support of this opinion. Two equally 

 sized pots of earth, containing four growing peas 

 each, were placed under two equal jars of atmosphe- 



* Thomson's Chemistry, vol. iv. p. 283. 



