16 Professor Macaire on the 



tion of beds of coal by the interment of the forests of the old 

 world. "When we are acquainted with the fact, that in nine 

 years an existing forest would deprive the atmosphere which 

 covers it of all the carbonic acid it contains, and yet would only 

 furnish a very inconsiderable bed of coal, we can scarcely com- 

 prehend the vegetable origin of mineral coal, however evident 

 it may be in other respects. But when it is proved that a 

 greater proportion of carbonic acid in the air, a fact which 

 everything else indicates as having been one of the characters 

 of the atmosphere in geological times, must have enormously 

 increased the vigour and dimensions of the vegetables then 

 living, we have data enabling us to conceive the possibility of 

 such enormous deposits of carbonaceous matters, and the cause 

 of the gradual purification of the air, and, as a consequence, 

 of the weakening of vegetation. 



After having examined the influence of carbonic acid gas on 

 vegetation, Theodore de Saussure arrived at that of oxygen. 

 He found that, during the night, this gas is absorbed by the 

 green leaves of plants, and, in part, replaced in the air by 

 carbonic acid, unless the leaves be very thick, like those of 

 the cactus, in which case the oxygen is absorbed without any 

 disengagement of carbonic acid. The leaves do not absorb 

 any gas in media deprived of oxygen gas, so that it is here ob- 

 viously a vital function, and not a mechanical action. These 

 inspirations in the night, and expirations in the day, of 

 oxygen, either pure or mingled with carbonic acid, serve 

 essentially to form this latter gas, and to present to plants 

 under this modification, such elements as they can assimilate. 

 After verifying these results on fifty-seven diff^erent species of 

 leaves, Theodore de Saussure satisfied himself by experiment, 

 that the contact of oxygen gas with the roots is useful to 

 vegetation ; and that chestnut trees torn up from the earth, 

 and. whose roots were in contact with azote, hydrogen, or 

 carbonic acid, died much sooner than when these roots were 

 placed in common air. The roots, wood, petals — all the 

 parts not green — absorb much oxygen, and convert it into 

 carbonic acid ; but this gas ascends with the sap, in order to 

 be decomposed by the leaves, and does not directly fix itself 



