ABSORPTION BY SOILS. 273 



is condensed by either pressure or cooling-. Hence the very 

 large amount of water-gas or vapor which may be absorbed by 

 soils, as shown in a preceding chapter. But excepting perhaps 

 the case of ammonia, moist soils are less absorbent of gases 

 than dry ones. 



Oxygen and nitrogen, the main constituents of the atmos- 

 phere, being difficultly condensable by either pressure or cold, 

 are absorbed by soils only to a relatively small, yet by no 

 means unimportant extent. The condensation of oxygen with- 

 in the soil-mass is doubtless of considerable importance in the 

 processes of oxidation, as is shown by its partial replacement 

 by carbonic gas in the free air of the soil (see chap. 2, p. 17). 

 The intensifying of oxidizing action caused by surface conden- 

 sation is well illustrated in the case of finely divided platinum, 

 in which hydrogen is brought to rapid combustion when mixed 

 with oxygen ; as well as by the effect of bedding tainted meat 

 in charcoal powder, when all odors of decay disappear, both 

 by absorption and oxidation, ammonia and carbonic gas alone 

 ultimately escaping through the powder. 



Carbonic dio.vid and ammonia gases, both normal consti- 

 tuents of the atmosphere, and of high importance to plant nu- 

 trition, are more readily condensable than either oxygen or 

 nitrogen, and consequently may be taken up by the soil in 

 larger relative proportions. Especially is this the case with 

 ammonia gas, which is not only readily condensed by pres- 

 sure, but is also extremely soluble in water; so much so that 

 it rushes into a tube filled with this gas almost as quickly as 

 though it were a vacuum. Water will absorb at the ordinary 

 temperature, under normal pressure, about 700 times its vol- 

 ume of ammonia gas; but inasmuch as the proportion of the 

 latter in the atmosphere amounts to only a few millionths, the 

 actual amount taken up can only (as in the case of all gases) 

 be proportional to its proportion (or " partial pressure ") mul- 

 tiplied into its coefficient of absorption. Consequently, water 

 exposed to the ordinary air can absorb at best only a small 

 fraction of a per cent of ammonia. Its presence in soils can 

 be readily demonstrated by passing through the warmed soil 

 a current of purified air. which is made to bubble through 

 Nessler's reagent (potassio-mercuric iodid) solution. 

 18 



