FOOD OF PLANTS, OF ANIMALS. 1013 



varying from 4 to 6 10,000ths. of its volume. Small as this 

 quantity appears it is shown to exceed considerably in amount 

 the whole carbon existing both in living vegetables and in the 

 fossil state as mineral coal. The variation in the proportion of 

 carbonic acid by night and by day, in winter and in summer 

 is rightly judged by M. Dumas to be a simple meteorological 

 phenomenon, depending upon this gas being brought down in 

 rain, and absorbed and retained in largest proportion by water 

 in the cold season. The gas is directly absorbed from the at- 

 mosphere by the leaves, and also from the humid soil by the 

 roots of plants. Boussingault observed vine leaves in a glass 

 vessel to absorb completely the carbonic acid from the air as 

 fast as it was carried to them, however rapid the current 

 through the vessel. M. Boucherie has also observed enormous 

 quantities of carbonic acid to escape from the trunk of a tree 

 cut when in full sap, evidently aspired from the soil by the 

 roots. Under the deoxidating influence of light, plants 

 decompose carbonic acid retaining its carbon for their own 

 use, and returning its oxygen to the atmosphere. Their green 

 leaves absorb the chemical rays of the sun so completely, as 

 to give no image in the Daguerreotype. Plants thus 

 possess energetic means of reduction which cannot be imi- 

 tated, for chemists are ignorant of any method of decom- 

 posing carbonic acid in the cold. Plants, however, also 

 exhale carbonic acid, particularly in the absence of light, and 

 this has been supposed analogous to the expiration of carbonic 

 acid by animals, and depending upon the respiration of plants. 

 M. Liebig, however, looks upon this exhalation as entirely 

 physical, as the escape by diffusion into air of the carbonic acid 

 dissolved in the fluids of the plant, in the absence of the re- 

 ducing light ; the carbonic acid being derived by the roots from 

 decomposing humus, and this exhalation most considerable from 

 plants growing in a rich soil. Thus, at night, plants allow 

 the carbonic acid to pass through them, without absorbing it. 

 Ammomia also finds its way into the atmosphere, being a 

 product of the putrefactive decomposition of all azotised bodies, 

 and is evolved from them principally in the condition of the vola- 

 tile carbonate. The existence, however, of this substance in 

 the air must be transient, as from its solubility in water, it will 

 be brought down to the earth by every shower. It thus enters 



