THE PHYSICAL PROPERTIES OF THE SOIL. 951 



same way as gases are absorbed by b'qiiids, partly on account of tlie 

 bigli pressure to wbicb tbey are subjected and partly on account of the 

 very tine capillaries. This process takes place more slowly than sur- 

 face attraction, and ceases when tbe resistance opposed to the penetra- 

 tion of the gas molecules can no longer be overcome. 



Absorption. — Owing to surface attraction, an exceedingly tliin layer 

 of liquid is very soon formed over the surface of the solid body, and 

 this layer now absorbs gases. Assisted by the high jnessure to which 

 this layer is subjected (Bunsen estimates the pressure at hundreds of 

 atmospheres) its capacity for absorption of gases rises to a degree far 

 exceeding our ideas of such processes. This process may also continue 

 for years before reaching its conclusion (Bunsen). 



Chemical reactions. — Leaving out of account simple chemical attrac- 

 tions, which do not belong in the realm of gas absori)tion, there are 

 widely varying, and under ordinary circumstances for tbe most part 

 unknown, chemical activities between the molecules of the solid body 

 and those of the penetrating gas. The high capillary pressure together 

 with the disengagement of considerable quantities of heat during the 

 initial condensation bring about and keep in action processes which 

 chenucal affinity alone would not cause. 



The numerous investigations on this subject have given the following 

 results: All the soil constituents have a not inconsiderable absorptive 

 capacity, which is greatest in the case of humus (and ferric oxid), next 

 highest in clay, and least in quartz. Mixed soils behave as their indi- 

 vidual constituents. The absorptive capacity increases in any case 

 with the fineness of the particles, and decreases with arise of temper- 

 ature. In the absorption of water vapor from air saturated with 

 moisture the latter law is modified only to the extent that the absolute 

 quantity of water increases with the temperature; in which case the 

 influence of the rise in temperature is almost eliminated by the increase 

 in humidity. If the absolute humidity of the air is constant while the 

 temperature rises the influence is the same as that observed in the 

 case of gas absorption. At a constant temperature the hygros(oi)icity 

 increases with the relative humidity of the air. If the soil is more than 

 hygroscopically moist the gas absorption we have so far considered is 

 rei»Iaced by the absorption of gases in the soil water. Kising temper- 

 atures accelerate the condensation ijrocess. Gases are in general more 

 easily absorbed the more readily they alter their molecular arrange- 

 ment and the more easily they decomjiose.^ 



The significance of the absorptive capacity in relation to the fertility 

 of the soil is often overestimated. Gases capable of increasing the 

 sup])ly of fertilizing material occur only in very minute quantity in the 

 atmosphere or in the soil air. The absorption of oxygen is of most 

 importance, because it is through this means that the foil is capable of 



' G. Aminon, Forsch. Geb. agr. Pliys., 2, p. 1. A. von Dobencck, Ibid., p. 163. Both 

 these articles contain a comjilete general review of the copious literature on this 

 subject. 



