1)50 EXPERIMENT STATION RECORD. 



it is present in the largest quantities. Tlie carbonic acid is due prin- 

 cipally to the decomposition of organic substances. The amount 

 present depends upon the richness of the soil in humus and the 

 lapidity of decomposition, but it is almost always much more abun- 

 dant in the soil than in the atmosphere. Under these circumstances it 

 is of interest to note that there is a constant movement in the soil 

 atmosphere not due to external influences, but to diftusion, i. e., the 

 tendency for the external and internal gases to come into equilibrium. 

 Tlie diffusion of the carbonic acid (as well as ammonia and other 

 gases) from the soil at a constant temperature depends principally on 

 the sum of the pores of the cross section and not on their size. Hence 

 the absolute quantity of diffused gases is larger the greater the total 

 volume of the pores, and vice versa. Every decrease in the volume 

 of the pores, whether due to pressure or to an increase of moisture 

 in the soil, is accompanied by a decrease in the quantity of gas diffused. 

 The escape of carbonic acid into the atmosphere by diffusion decreases 

 with the fineness of the grains of the soil, the compactness of their 

 arrangement, and the greater the water capacity of the soil, and vice 

 versa. The quantity of carbonic acid diffused diminishes as the thick- 

 ness of the ?oil increases, but to a less extent. In soils which become 

 superficially saturated, and into which water penetrates slowly, the 

 diffusion of carbonic acid is considerably diminished or brought to a 

 standstill.' 



THE ABSORPTIVE CAPACITY (OR CONDENSATION CAPACITY) OF THE SOIL FOR GASES. 



Absorption, in the widest sense of the word, is a molecular phenom- 

 enon and includes all those causes which bring about the retention of 

 gas molecules on and in solid bodies. It is only a matter of custom to 

 distinguish between hygroscopicity and gas absorption (absorption in 

 the narrow sense), which are esseutially identical phenomena. The 

 retention of the gas molecules is caused by a series of separate proc- 

 esses. These are discussed below. 



SSurfacc attraction. — There are on the surface of all solid bodies molec- 

 ular forces related to adhesion, which attract the nearest gas molecules 

 to the surface of the solid body, and there force them together in a 

 dense, partly liquid or perhaps solid envelope, until an equilibrium is 

 reached which remains undisturbed until the constitution of the sur- 

 rouiuling medium changes. This process takes place almost instanta- 

 neously and with great energy, so that in the beginning, at least, the 

 whole process of absorption may be attributed to it. It also causes ini- 

 tially a rapid rise of the absorption curve and produces a considerable 

 amount of heat, which gives the impetus to a series of further phe- 

 nomena. 



Molecular penetration. — The attracted gas i)articles penetrate to a cer- 

 tain extent into the intramolecular si^aces of the solid body in much the 



•F. Hanneiij Forsch. Geb. agr. Phys., 15, p. 1. 



