222 HOW CROPS FEED. 
rent of air. Through a mass of this earth 20 inches deep 
and 3 inches in diameter, pure distilled water (free 
from carbonic acid) was allowed to filter. J¢ ran through 
without taking up any of the gas. Again, water contain- 
ing its own volume of carbonic acid was filtered through 
a similar body of the same earth. This water gave up all 
its carbonic acid while in contact with the soil. After a 
certain amount had run off, however, the subsequent por- 
tions contained it. In other words, the soils experiment- 
ed with were able to absorb carbonic acid from its aqueous 
solution, even when their interstices contained the gas in 
the free state. These extraordinary phenomena deserve 
further study. 
§ 3. 
NON-NITROGENOUS ORGANIC MATTERS OF THE SOIL.— 
CARBOHYDRATES. VEGETABLE ACIDS. VOLATILE 
ORGANIC ACIDS. HUMUS. 
Carbohydrates, or Bodies of the Cellulose Group.— 
The steps by which organic matters become incorporated 
with the soil have been recounted on p. 135. When plants 
perish, their proximate principles become mixed with the 
soil, These organic matters shortly begin to decay or to 
pass into humus. In most circumstances, however, the 
soil must contain, temporarily or periodically, unalter- 
ed carbohydrates. Cellulose, especially, may be often 
found in an unaltered state in the form of fragments of 
straw, etc. 
' De Saussure (Recherches, p. 174) found that water dis- 
solved from a rich garden soil that had been highly ma- 
nured for a long time, several thousandths of organic 
matter, giving an extract, which, when concentrated, had 
an almost syrupy consistence and a sweet taste, was 
neither acid nor alkaline in reaction, and comported itself 
not unlike an impure mixture of glucose and dextrin, 
