THE FREE WATER OF THE SOIL. 217 
In the first place it is an unfailing and sufficient source of 
its elements,—hydrogen and oxygen,—and undoubtedly 
enters directly or indirectly into chemical combination 
with the carbon taken up from carbonic acid, to form sug- 
ar, starch, cellulose, and other carbohydrates. In the 
second place it performs important physical offices; is the 
vehicle or medium of all the circulation of matters in the 
plant; is directly concerned, it would appear, in imbibing 
gaseous food in the foliage and solid nutriment through 
the roots; and by the force with which it is absorbed, di- 
rectly influences the enlargement of the cells, and, per- 
haps, also the direction of their expansion,—an effect shown 
by the facts just adduced relative to the clover crops ex- 
amined by Ritthausen. 
Indirectly, also, water performs the most important ser- 
vice of continually solving and making accessible to crops 
the solid matters in the vicinity of their roots, as has 
been indicated in the chapter on the Origin of Soils. 
Combined Waiter of the Soil.—As already stated, there 
may exist in the soil compounds of which water is a chemi- 
cal component. True clay (kaolinite) and the zeolites, as 
well as the oxides of iron that result from weathering, con- 
tain chemically combined water. Hence a soil which has 
been totally deprived of its hygroscopic water by drying 
at 212°, may, and, unless consisting of pure sand, does, 
yield a further small amount of water by exposure to a 
higher heat. This combined water has no direct influence 
on the life of the plant or on the character of the soil, ex- 
cept so far as it is related to the properties of the com- 
pounds of which it is an ingredient. 
§ 2. 
THE AIR OF THE SOIL. 
As to the free Oxygen and Nitrogen which exist in the 
interstices or adhere to the particles of the soil, there is 
10 
