500 Plant Case for growing Plants 



perceive its production by the plants which vegetate without 

 lime in common air, is, because they then decompose it in pro- 

 portion as they form it with the surrounding oxygen. (Re- 

 ckerches Chim., p. 35, 36.) 



This inference, respecting the simultaneous formation and 

 decomposition of carbonic acid, derived from experiments made 

 with common air, is supported by others, in which an artificial 

 atmosphere, containing about 7 per cent of carbonic acid, 

 was employed. Plants of the same species as those before men- 

 tioned were made use of, the same periods of alternate exposure 

 in the shade and in sunshine were observed, and the same times 

 allotted for the duration of the experiments. The total volume 

 of air, at the end of the experiments, had undergone little vari- 

 ation, but its composition was greatly changed. The carbonic 

 acid gas which was added to the atmosphere had more or less 

 completely disappeared, and its place was supplied by an increase 

 of oxygen gas, so as to raise its proportion from 21 to 24 or 26 

 per cent. In these experiments, therefore, not only was the car- 

 bonic acid, naturally formed by the vegetation of the plants, 

 decomposed, but the excess of that gas which was added to the 

 atmosphere underwent the same change ; and the proportion of 

 oxygen gas was consequently increased by 5 or 6 per cent be- 

 yond that which occurred in the experiments with common air. 



From the results of these experiments, we learn that plants, 

 like seeds, require the presence of oxygen gas in the atmosphere 

 in which they grow, and like them, also, convert a portion of it 

 into an equal volume of carbonic acid gas. This conversion is 

 alike effected by their growth in the shade and sunshine. In 

 the former case, however, the presence of this acid gas may be 

 readily detected in the residual air by the usual tests ; but, in the 

 latter, it escapes detection, because it is then decomposed, as soon 

 as formed, by the joint agency of the plants and solar light. 

 Under a bright sunshine, therefore, the two processes, by which 

 carbonic acid is alternately formed and decomposed, go on 

 simultaneously; and their necessary operation, in as far as 

 regards the condition of the air, is that of counteracting each 

 other. Hence, though both may be continually exercised in 

 favourable circumstances, the effects of neither on the atmo- 

 sphere can be ascertained by ordinary means ; and, consequently, 

 though, in the experiments of De Saussure with common air, 

 the production and decomposition of carbonic acid by plants in 

 sunshine must have been continually going on, yet, in all the 

 analyses which he made, the air was found unchanged, either in 

 purity or in volume ; in other words, the processes of formation 

 and decomposition of this acid gas exactly counterbalanced each 

 other. 



Of the two processes which have been now described, each 



