THE AIR OF THE SOIL. , 221 
much of it as may be expressed by pressure, is not nearly 
saturated with this gas. 
De Saussure (Recherches Chimiques sur la Végétation, 
p. 168) filled large vessels with soils rich in organite mat- 
ters, poured on as much water as the earth could imbibe, 
allowing the excess to drain off and the vessels to stand 
five days. Then the soils were subjected to powerful 
pressure, and the water thus extracted was examined for 
carbonic acid. It contained but 2°|, of its volume of the 
gas. 
Since at ‘a medium temperature (60° F.) water is capa- 
ble of dissolving 100°|, (its own bulk) of carbonic acid, it 
would appear on first thought inexplicable that the soil- 
water should hold but 2 percent. Henry and Dalton long 
ago demonstrated that the relative proportion in which 
the ingredients of a gaseous.mixture are absorbed by wa- 
ter depends not only on the relative solubility of each gas 
by itself, but also on the proportions in which they exist 
in the mixture. The large quantities of oxygen, and 
especially of nitrogen, associated with carbonic acid in the 
pores of the soil, thus act to prevent the last-named gas 
being taken up in greater amount; for, while carbonic 
acid is about fifty times more soluble than the atmos- 
pheric mixture of oxygen and nitrogen, the latter is pres- 
ent in fifty times (more or less) the quantity of the former. 
Absorption of Carbonic Acid by the Soil._— According to 
Van den Broek, (Ann. der Chemie u. Ph., 115, p. 87) certain 
wells in the vicinity of Utrecht, Holland, which are exca- 
vated only a few feet deep in the soil of gardens, contain 
water which is destitute of carbonic acid (gives no precipi- 
tate with lime-water), while those which penetrate into the . 
underlying sand contain large quantities of carbonate of 
lime in solution in carbonic acid. 
Van den Broek made the following experiments with 
garden-soil newly manured, and containing free carbonic 
acid in its interstices, which could be displaced by a cur- 
