182 VEGETATION AND THE ATMOSPHERE. 
all these phenomena was necessary ; it was Thomas de Saussure who under- 
took to supply this, and who, without adding any capital fact to the cluster of 
previous observations, succeeded in giving them an experimental confirmation 
which has not since been contested. After these celebrated experiments, there 
was a long period of rest. Physicists and naturalists seem to have considered 
the question as exhausted, and transferred their inquiries to subjects which 
they thought more fertile. Nevertheless, the more recent labors of MM. Dau- 
beny, Draper, Cloés, and Gratiolet, and above all of M. Boussingault, have suc- 
cessively intervened to raise difficulties which to this day remain in suspense ; 
but I prefer to leave out of view whatever does not possess the interest of the 
general theory, and shall speak neither of the azote which vegetables seem al- 
ways to disengage at the same time with oxygen, nor of certain deleterious 
gases, such as the oxide of carbon and carbonated hydrogen, which M. Boussin- 
galt has recently detected among the products of their exhalations ; nor, lastly, 
of the attempts made without much success to appreciate the special influence 
of the different solar rays. What I wish to show is, that after the first investi- 
gations above recounted, we find ourselves confronted by a second class, far 
more extensive and complex, with which it is now necessary to deal. It is 
necessary to inquire what becomes of the carbon which remains in vegetables 
after the decomposition of carbonic acid. 
TE: 
While the atmosphere furnishes carbon to the leaves, the boughs bring them 
water, which has been drawn from the soil, and it is natural to think that these 
two bodies, in meeting, pass mutually into combination; they, in effect, do 
combine and in very variable proportions ; we will cite some examples: if 12 
molecules of carbon unite with 20 moletules of water they are enabled to form 
either cellulose, which constitutes at once the vessels and entire skeleton of 
the plant, or fecula, which is known by everybody, or lastly dextrine, which 
is soluble and of which sirups are sometimes made; but, according jo cireum- 
stances and the organs, the proportion of the two bodies may change, and with 
it the chemical products which take place. ‘Thus 12 molecules of carbon com- 
bined with 14 molecules of water constitute glucose, or the sugar contained in 
ripe grapes; and if from this glucose we retrench two molecules of water, it 
is the sugar of the cane or the beet which would be formed. In fine, by pro- 
cesses which are unknown to us, water and carbon meeting in the leaves com- 
bine chemically, and produce an infinite number of compounds, differing ac- 
cording to the place, the organs, the nature, the age and the external conditions 
of the vegetable. 
Besides the substances just spoken of, and which are compounds of carbon 
and water, plants give rise to still another class of substances which are char- 
acterized by an excess of hydrogen. ‘These are the gums, oils, wax, balsams, 
essences, &c. Whence comes this hydrogen? ‘They form also substances 
in which a fourth element, azote, makes its appearance. Does this come from 
the atmosphere? is it derived from the humus! These are questions which 
directly concern agriculture and for which it must consult chemistry. M. 
Boussingault is he who has treated of them first and best, and he has been 
placed in the most favorable circumstances for doing so, being at the same time 
at the head of a great agricultural enterprise, and habituated to the most deli- 
cate procedures of chemical analysis. ‘lhe method he employs is proper to 
himself; it is sufficiently general and flexible to adapt itself to the demands of 
all special cases. It consists as follows: In a soil previously analyzed is sown 
a small number of grains whose chemical composition has been determined, 
and pure water is applied. This last disappears almost wholly by evaporation, 
and a small portion only becomes fixed. ‘lhe plant grows, gains in weight, . 
