69 EVOLTTTION AND GROWTH. 



Nevertheless, the presence of hydrogen in excess in certain im- 

 mediate vegetable principles is no decisive proof of the disjunction 

 of the elements of water ; and if no definitive conclusion lias been 

 come to on the point, up to the present moment, it is because these 

 hydrogenized principles are produced in plants which live under the 

 influence of certain organic substances that are met with in the soil, 

 where they act as manures, their composition being always complex, 

 and often highly hydrogenized. 



The experiments of M. de Saussure do not lead us to suspect 

 the decomposition of water ; inasmuch as by keeping plants for a 

 whole month, under receivers filled with atmospheric air freed from 

 carbonic acid, no apparent evolution of oxygen was observed. 

 Operating in the same manner with air containing a certain propor- 

 tion of carbonic acid, the quantity of oxygen disengaged was always 

 less than that which entered into the constitution of the acid de- 

 composed. 



This is the place to observe, and in connection with these very 

 experiments of M. de Saussure, how little satisfactory this partial 

 decomposition of carbonic acid, which corresponds to no definite 

 proportion, appears. We already feel the difficulty of conceiving 

 that this acid should be completely reduced by a living plant ; that 

 is to say, that the whole of its carbon should become assimilated. 

 The entire separation of a body so greedy of oxygen as carbon from 

 its most highly oxygenated compound, must needs excite the greatest 

 astonishment. 



The readiest conception suggested by the facts is this ; that by 

 the agency of the solar light, and under the influence of the green 

 matter, carbonic acid is turned into carbonic oxide by losing a por- 

 tion of its oxygen. This modification appears more in conformity 

 with the ascertained principles of chemical and physiological 

 science. Still it must be allowed, that facts agree as little with 

 this mode of viewing the question as with that which assumes the 

 entire decomposition of the carbonic acid. On the first assumption, 

 the proportion of oxygen set at liberty is too small ; in the second, 

 it is too great. 



The negative results of M. de Saussure, in relation to the separa- 

 tion of the elements of water during vegetation, were obtained in 

 the absence of carbonic acid, whilst the experiments which estab- 

 lished the decomposition of this latter body, were necessarily made 

 under the influence of moisture. It is possible, therefore, that the 

 water and the carbonic acid underwent simultaneous decomposition ; 

 and it becomes interesting, taking this view, to inquire whether the 

 hypothesis according to which carbonic acid undergoes transforma- 

 tion into carbonic oxide does not acquire a certain degree of proba- 

 bility by calling in the effect of the decomposition of water in the 

 phenomena observed. 



One volume of the gaseous oxide of carbon takes half a volume 

 of oxygen gas to form one volume of carbonic acid. Reciprocally, 

 ©ne volume of carbonic acid gas, in undergoing transformation inta 



