SOURCES OF HYDROGEN. 155 



1. Water, which is composed of eight parts of oxygen and 

 one of hydrogen. Water pervades the atmosphere in the 

 form of vapor, is deposited in dew or rain, and absorbed by 

 the leaves of plants. It is also taken in by the roots, and 

 forms a large portion of the sap. When we call to mind the 

 fact of its remarkable tendency to pass through transforma- 

 tions, there can be no doubt, but that it furnishes the largest 

 portion of the hydrogen of plants. 



2. Ammonia is another substance which contains hydrogen. 

 It is always present in fermenting manures, and in the atmos- 

 phere. It must, like water, be absorbed by the leaves of 

 plants, and by their roots ; and as it is similar to water in the 

 facility with which it is decomposed, it must furnish hydro- 

 gen to the vegetable products. Hence one reason for its 

 powerful effects upon vegetation. 



3. Light carbureted hydrogen is found in the atmosphere 

 and in the soil, and may be the source of a part of the hydro- 

 gen of plants ; although it is doubtful, whether they draw 

 upon this source, when there are at hand, more abundant and 

 far better sources. It may, however, be decomposed in 

 the air by electric discharges, and resolved into carbonic 

 acid and water. 



4. Geine or humus of soils contains hydrogen, in the form 

 of humic, crenic and apocrenic acids. These substances en- 

 ter the organs of plants, and may yield a large portion of the 

 hydrogen. Liebig derives the hydrogen wholly from water. 



III. Source of the Oxygen of Plants. Oxygen may be de- 

 rived from several sources. 



1. The atmosphere contains twenty-one parts of oxygen in 

 one hundred, and as the leaves of plants are known to absorb 

 it (p. 75), they obtain a part of their oxygen from this inex- 

 haustible source. 



2. Water contains eight parts in nine of oxygen. This is 

 absorbed both by the leaves and roots of plants in large 

 quantities, and is doubtless the principal source of the oxygen, 

 as well as the hydrogen of vegetable bodies. 



