26 MANURING THE SOIL. [chap. ii. 
and yet continually watered; and from their 
possessors loading them with manure., in the 
hope of rendering them fertile. 
As it is known to chemists that it is only 
the humic acid, and carbonic acid gas, con¬ 
tained in manure, which make that substance 
nourishing to plants; and as these acids 
must be dissolved in water before the roots 
can take them up, it may seem strange that 
any solution of them in water, however 
strong it may be, should be injurious to 
vegetation. The fact is, however, that it is 
the great quantity of food contained in the 
water that renders it unwholesome. When 
the roots of a plant and their little sponge¬ 
like terminations, are examined in a power¬ 
ful microscope, it will be clearly seen that 
no thick substance can pass through them. 
Thus water loaded with gross coarse matter, 
as it is when saturated with humic acid, 
must be more than the poor spongioles can 
swallow; and yet, as they are truly sponge¬ 
like, their nature prompts them, whenever 
they find moisture, to attempt to take it up, 
without having the power of discriminating 
between what is good for them, and what 
