274 SOILS. 



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 re- 

 spectively 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 ferru- 

 ginous and humous soil (from Hawaii), which gives the highest absorp- 

 tion 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 i ioC. (2 r8F) 

 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. 



It will be seen that this absorption is a different one, not 

 only for each of the different substances used, but is also 

 differently proportioned for the two gases. For it will be 

 noted that while the clay soil has absorbed a very much larger 

 amount of ammonia than the charcoal, and the sandy soil has 

 remained far behind both : yet the charcoal has absorbed a 

 considerably larger proportion of carbonic gas than either the 

 clay or the sandy soil, proving that charcoal has a strong 

 specific absorptive power for carbonic gas, independently of the 

 relative size of clay and charcoal particles respectively. The 

 sandy soil shows, by its low absorption even of ammonia gas, 

 the coarseness of its particles and the scarcity of clay in its 

 composition. The highest absorption of all is shown by the 

 ferruginous soil from Hawaii, containing nearly 40 % of 

 ferric oxid together with 3 1-3% of humus. The moisture- 

 absorption of this soil at the ordinary temperature is 19.7 per 

 cent. The difference in the absorbing power of the (non- 

 humous) gray clay and gray adobe soil indicates the strong in- 

 fluence of humus upon the absorption; which is still farther 

 emphasized by the difference between the gray and black adobe, 



