PLANTS AND THE ATMOSPHERE. 423 



repose. Physicists and naturalists seemed to regard the question as exhausted 

 and withdrew their special attention to subjects which they considered more 

 fertile. However, the more recent labors of Messrs. Daubeny, Draper, Cloes 

 and Gratiolet, and above all of M. Boussingault have successively been undertaken 

 to remove disputes undecided to the present day ; but I desire to omit all that 

 is not specially connected with the general theory ; I shall treat neither of 

 nitrogen, which plants seem always to give forth at the same time as oxygen, nor 

 of certain deleterious gases such as carbonic oxide and cai'buretted hydrogen 

 which M. Boussingault has just found among the products of their exhalations, nor, 

 finally, of the experiments made without much success in order to estimate the 

 special influence of different solar rays. What I desire to shew is, that, after the 

 early studies just alluded to, we find ourselves face to face with a second series 

 much more vast and complicated, upon which we must now enter. We must 

 endeavor to find what becomes of the carbon that remains in plants after the 

 decomposition of carbonic acid. 



While the atmosphere furnishes leaves with carbon, the branches supply them 

 with water, which has been drawn up from the soil, and it is natural to think that 

 these two bodies meeting one another, would mutually combine ; they do, in fact, 

 combine, but in very variable proportions; let us cite a few examples. If 12 

 atoms of carbon unite with 10 atoms ot water, they may give birth, either to cel- 

 lulose, which at the same time constitutes the cells and the whole skeleton of the 

 plant, or to starch, with which everybody is acquainted, or, finally, to dextrin, 

 which is valuable, and of which syrups are sometimes made ; but, according to 

 circumstances and organs, the proportion of two bodies may change, and with that 

 the chemical products to which they give rise. Thus, 12 atoms of carbon com- 

 bined with 12 atoms of water constitute glucose or grape sugar, which is found in 

 ripe grapes, and if, from this glucose, two atoms of water be taken away, the re- 

 sult will be cane or beet sugar. In a word, by processes unknown to us, water 

 and carbon meeting in leaves, form a chemical union, and produce an infinite 

 variety of compounds, according to the position, organs, nature, age and exterior 

 conditions of the plant. 



Besides the substances just mentioned, and which are composed of carbon and 

 water, plants create another class characterised by an excess of hydrogen. These 

 are fats, oils, resins, balsams, volatile oils, &c. Whence comes this hydrogen ! 

 They also form compounds in which appears a fourth element, nitrogen ; does it 

 come from the atmosphere ? Is it imbibed from the soil? These are questions 

 directly affecting agriculture, and upon which it is obliged to consult chemistry. — 

 The first and the best treatment of these questions is due to M. Boussingault, who 

 found himself most happily situated for the purpose, being at once placed at the 

 head of a great agricultural commission, and habituated to the most delicate ope- 

 rations of chemical analysis. The method employed by him is peculiar to himself ; 

 it is so general and fruitful as to apply to the requirements of all individual cases. 

 Here is what it consists in. In soil analysed beforehand is sown a small num- 

 ber of seeds, of which the chemical composition has been determined, and these 



