468 



Dr. Dana is of opinion that geine or humus is an original creation, co- 

 eval with the creation of hydrogen and oxygen and carbon. The con- 

 jecture is sufficiently plausible, but it would be idle to advance any opin- 

 ion on the subject. The only fact which can be said to favor one opin- 

 ion above the other is, that the plants found in the earliest coal forma- 

 tion are plants with small roots and expanded foliage, implying that they 

 drew their chief nourishment from the air. 



The inquiry which next arises is, if plants do not derive their carbon, 

 or but a very small portion of it from the soil, whence is it obtained ? This 

 interesting question Liebig discusses at large, and certainly with much 

 ability. The seed itself contains the first supply of nourishment for the 

 roots of the infant germ of the plant. Before it appears above the sur- 

 face, the humus in the soil quickens and invigorates its growth by the 

 supply of carbonic acid. This supply of carbonic acid is furnished by 

 the accession of atmospheric air from the loosening of the soil, the car- 

 bon of the humus combining with the oxygen of the air to produce 

 nourishment for the young plant. When it rises above the surface, and 

 its external organs of nutrition, its stem and its leaves, are fully devel- 

 oped, it ceases to draw nourishment from the earth, and obtains all its 

 carbon from the air. It is not a new doctrine that plants absorb car- 

 bonic acid from the atmosphere. This fact has been long established ; 

 but it is new that this is the principal source ; and the inquiry naturally 

 arises whether the atmosphere, containing, as it does, only a thousandth 

 part of carbonic acid, can furnish in this way a supply of all the car- 

 bon which is required by the plant. To this inquiry Liebig replies as 

 before, by making it matter of exact calculation. 



" It can be shown, that the atmosphere contains 3,000 billion Hessian 

 lbs. of carbon ; a quantity which amounts to more than the weight of 

 all the plants, and of all the strata of mineral and brown coal, which 

 exist upon the earth. This carbon is therefore more than adequate to 

 all the purposes for which it is required." — p. 74. 



The absorption of carbonic acid from the air, in his opinion, is a purely 

 chemical process. Many others have chosen to regard it as a vital op- 

 eration ; and have considered the leaves as respiratory organs, resemb- 

 ling the lungs of animals. He does not admit the analogy, and thinks 

 that the cause of science is injured hy the supposition of a resemblance, 

 where no similitude exists. The absorption of carbonic acid from the 

 air, the assimilation of its carbon, and the return of its oxygen to the 

 air, are chemical processes, effected under the operation of light and 



