228 HOW CROPS FEED. 
Carbon, 56.47 
Hydrogen, 2.75 
Oxygen, 40.78 
100.00 
Crenate of copper is lastly precipitated as a grass-green 
substance by adding acetate of copper to the liquid from 
which the apocrenate of copper was separated, and then 
neutralizing the free acid with ammonia. From this com- 
pound crenic acid may be prepared as a white, solid body 
of sour taste, to which Mulder ascribes the formula C,, 
H,, O,, + 3H,O, and in 100 parts the following compo- 
sition * 
Carbon, 45.70 
Hydrogen, 4.80 
Oxygen, 49.50 
100.00 
Mutual Conversion of Apocrenic and Crenic Acids, 
—When, on the one hand, apocrenic acid is placed in 
contact with zine and dilute sulphuric acid, the hydrogen 
evolved from the latter converts the brown apocrenic acid 
(by uniting with a portion of its oxygen) into colorless 
crenic acid. On the other hand, the solution of crenic 
acid exposed to the air shortly becomes brown by absorp- 
tion of oxygen and formation of apocrenic acid. These 
changes may be repeated many times with the same por- 
tion of these substances, 
Mulder remarks (Chemie der Ackerkrume, p. 350): 
“Tn every fertile soil these acids always occur together in 
not inconsiderable quantities. When the earth is turned 
over by the plow, two essentially different processes fol- 
low each other: oxidation, where the air has free access; 
reduction, where its access is more or less limited by the 
adhesion of the particles and especially by moisture. In 
the loose, dry earth apocrenic acid is formed; in the firm, 
