_Carbonic dioxid and ammonia gases_, both normal constituents of the atmosphere, and of high importance to plant nutrition, 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 pressure, 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 volume 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”) multiplied 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. _Absorption of gases by dry soils._—Perfectly dry soils are powerful absorbers of ammonia, and their absorption of this gas, as well as of carbonic gas, can readily be shown by the arrangement shown on the page opposite. The two tubes shown to the left are filled with carbonic gas, those to the right with ammonia gas. After being immersed in a mercurial trough, there are introduced into each tube through the mercury small cylinders (conveniently one cubic centimeter in volume) consisting respectively of a very sandy soil or loose hardpan, a gray plastic clay, a gray clay soil or adobe, a very black “adobe” clay, and a highly ferruginous and humous soil (from Hawaii), which gives the highest absorption of all; next brown peat, and pine charcoal. The latter, and the ferruginous soil, were also exposed for the absorption of carbonic gas. All the absorbing cylinders are first heated for an hour to 110°C. (218°F) for the purpose of expelling from them moisture, air, and other absorbed gases. They are then quickly introduced into the tubes through the mercury and allowed to absorb the gases enclosed until the mercury columns cease to show any farther rise; in which condition they are shown in the figure.