224 PHYSIOGRAPHY. [chap. 



out of the comparatively simple raw materials supplied 

 to it. 



In the case supposed, the fluid with which the pea is 

 supplied contains only hydrogen, oxygen, nitrogen, phos- 

 phorus, sulphur, and certain metallic bases; but another 

 element, carbon, enters, largely, into every one of the manu 

 factured articles which are to be found in the full-grown 

 plant. The presence of this carbon, and its great relative 

 amount, may be made manifest enough if the plant is strongly 

 heated in a closed vessel, when the carbon remains, as a 

 conspicuous mass of charcoal. Whence is this carbon de- 

 rived ? Under the conditions defined, the only possible 

 source of supply is the carbonic acid diffused through the 

 atmosphere ; which, though it forms so small a percentage of 

 tlie air, yet amounts to an enormous absolute quantity/ In 

 fact, it is known that, under the influence of sunlight, a 

 green plant decomposes carbonic acid into its elements ; 

 and, setting free the oxygen, builds up the carbon, together 

 with the nitrogen, hydrogen, and oxygen, and mineral matters 

 derived from other sources, into the complex compounds of 

 its own living substance. 



Thus the green plant transmutes the fluid and gaseous 

 matters, which it draws from the soil and from the atmos- 

 phere, into the solid materials of its own body ; and thus, to 

 a certain extent, reclaims the solids lost by aqueous solution 

 and igneous decomposition. Under ordifiary circumstances, 

 the restoration of solid matter to the earth, thus effected by 

 plant-life, is only temporary. Even during life, the activity 

 of the green plant, like all vital activity, is accompanied by 

 the slow oxidation and destruction of its protoplasmic matter; 



^ There are about 2,150 tons of carbonic acid in the air covering an 

 acre of ground, which is nearly the weight of the year's rainfall over 

 the same area in London (see p. 45). 



