38 HOW CROPS FEED. 
The result of these investigations is, that while, perhaps, 
wilted foliage in a heavy rain may take up a small quan- 
tity of water, and while foliage and roots may absorb 
some vapor, yet in general and for the most part the at- 
mospheric water is not directly taken up to any great ex- 
tent by plants, and does not therefore contribute immedi- 
ately to their nourishment. 
Atmospheric Water Enters Crops through the Soil.— 
{t is only after the water of the atmosphere has become in- 
corporated with tlie soil, that it enters freely into agricul- 
tural plants. The relations of this substance to proper 
vegetable nutrition may then be most appropriately dis- 
cussed in detail when we come to consider the soil. (See 
p. 199.) 
It is probable that certain air plants (epiphytes) native to the tropics, 
which have no connection with the soil, and are not rooted in a medium 
capable of yielding water, condense vapor from the air in considerable 
quantity. So also it is proved that the mosses and lichens absorb water 
largely from moist air, and it is well known that they become dry and 
brittle in hot weather, recovering their freshness and flexibility when the 
air is damp. 
§ 5. 
RELATIONS OF CARBONIC ACID GAS TO VEGETABLE 
NUTRITION. 
Composition and Properties of Carbonic Acid, — 
When 12 grains of pure carbon are heated to redness 
in 32 grains of pure oxygen gas, the two bodies unite to- 
gether, themselves completely disappearing, and 44 grains 
of a gas are produced which has the same bulk as the 
oxygen had at the beginning of the experiment. The new 
gas is nearly one-half heavier than oxygen, and differs in 
most of its properties from both of its ingredients. It is 
carbonic acid. This substance is the product of the burn- 
ing of charcoal in oxygen gas, (H. C. G., p. 35, Exp. 6.) 
It is, in fact, produced whenever any organic body is 
