164 ON VEGETABLE RESPIRATION. 



remaining air contained 24 J^ per cent, of oxygen, in the place of the 

 21 per cent, which it contained at first. In the second, the quantity 

 of oxygen was not increased. The periwinkles of the first receiver 

 were submitted to analysis ; they contained eleven and-a-half centi- 

 grammes of carbon more than those which had been analysed at the 

 commencement of the experiment. The quantity of carbon was 

 not increased in the plants of the second receiver in which the air 

 had been deprived of every trace of carbonic acid. 



This remarkable experiment of Saussure has demonstrated the 

 great object of vegetable respiration, namely, the decomposition of 

 carbonic acid, exhalation of its oxygen, and fixation of its carbon. 

 The essential constituent of all plants is carbon, and it is the gieat 

 eflfort of vegetable vitality to fix this substance in the composition 

 of the plant. The surrounding atmosphere is therefore vivifying to 

 vegetation, just in proportion as it is deleterious to animal life by 

 the quantity of carbonic acid which it contains. 



It is not from the atmosphere alone that plants extract the carbon 

 so necessary to their existence ; there are two other sources which 

 aiTord a never-failing supply. By means of their roots they extract 

 carbonic acid from the soil which they afterwards decompose. In 

 order to demonstrate this fact, Senebier having procured two nearly 

 similar branches, placed the stem of one in carbonic acid, while the 

 other was exposed to common air. The first retained its freshness 

 and verdure when the other was entirely withered. In fact, vegetables 

 when forming carbonic acid, combine the oxygen absorbed during 

 the night with the carbon of the plant itself. Thus, we may say, 

 that during the night, the plant prepares the materials for the more 

 important labours of the day ; it absorbs the oxygen and exhales 

 the carbonic acid, which are to be decomposed, for its own advantage, 

 under the salutary influence of the solar rays. M. Dumas is of 

 opinion, that the organs of the plant are entirely passive during the 

 night ; merely allowing the carbonic, acid borrowed in the day, to 

 filter spontaneously through its tissues, and difl^use itself around ; and 

 that it is only active under the stimulus of the solar light. 



The green parts of vegetables which possess this admirable 

 faculty of decomposition, are also endowed with another and not less 

 mysterious power, which is, tlie retention of the chemical rays pro- 

 ceeding from the sun. From this we may perceive the weakness 



