THE PHYSICAL PROPERTIES OF THE SOIL. 953 



variable, depending on the physical structure of the soil, its covering, 

 and on the meteorological conditions. In the case of varying quanti- 

 ties of organic matter, its influence on the quantity of carbonic acid 

 may be diminished or entirely eliminated if the more important factors 

 of decomposition (water and temperature) act in the reverse direction. 

 Further than those already given, general laws to account for the very 

 Avide variations mentioned can not be framed. In individual instances 

 the local influences alone decide the result. 



In conclusion we may note that carbonic acid can pass out from those 

 strata where it is most copiously produced into layers above and below. 

 Its penetration into lower layers and its escape into the atmosphere 

 probably account for the fact that the proportion of carbonic acid is 

 generally greater in the lower layers of the soil. The sinking of car- 

 bonic acid into lower layers and its escape into the atmosphere depend 

 essentially on the physical structure of the soil as a whole and of its 

 separate strata, in that the accumulation of the gas in the pores of the 

 soil is promoted by fineness of grain structure and an increase of 

 humidity, and vice versa. The influence of the quantity of organic 

 matter and of the external factors of decomposition on the proportion 

 of carbonic acid in the soil air may, therefore, under certain circum- 

 stances, be marked by variations in permeability. Thus a coarse- 

 grained sandy soil, rich in humus, may contain less carbonic acid than 

 a fine-grained soil poor in humus, because in the former case the gas will 

 be given off to the atmosphere very much more quickly than in the latter. 

 Such facts, in conjunction with those already adduced, justify the con- 

 clusion that the quantity of free carbonic acid in the soil does not show 

 the intensity of the organic processes nor the quantity of organic mat- 

 ter in the soil.? 



V. THE BEHAVIOR OF THE SOIL TOWARD HEAT. 



As the sources of heat for the soil we must consider the heat of the 

 sun, the heat due to chemical action and absorption of gases, and the 

 internal heat of the earth. The heat of the sun is of primary impor- 

 tance, while the other factors have very slight influence. 



The heat resulting from the oxidation of organic matter (decay, slow 

 combustion) is, as a rule, insufiicieut to cause any considerable rise in 



' Boussingault and L^vy, Ann. China, et Phys., ser. 3, 37 (1853), p. 1. J. von 

 Todor, Hygienisclie Untersuchiingeu iiber Luft, Bodeu und Wasser, 1892, 2, p. 106; 

 Vierteljaliresschrift fiir oifentliclie Gesundheitspflege, 7, p. 205. M. von Pettenkofer, 

 Ztschr. Biol., 7, 9, 12. G. Wolffliiegel, Ibid., 15. P. Smolenski, Ibid., 13, p. 383. 

 H. Fleck, Jahresbericlit der cbemischeu Centralstelle fUr oftentlicbe Gesundheits- 

 pflege in Dresden, 2, p. 15; 3, p. 3. Lewis and Cunningham, Ann. Rpt. Sanitary 

 Commission, Government of India, 1874. R. Nichols, Ann. Rpt. Mass. State Bd. 

 Health, 1875, and Rpt. Sewerage Commission, Boston, 1876. E. Ebermayer, Forsch. 

 Geb. agr. Phys., 1, p. 158 ; Ibid., 13, p. 15. F. Moller, Mittheiluug aus dem forstlichen 

 Versuchssvesen Oesterreichs, 1, p. 121; Forsch. Geb. agr. Phys., 2, p. 329. E. Wollny, 

 Vierteljahresschrift fiir oflentliche Gesundheitspflege, 1883; Landw. Vers. Stat., 25 

 and 36; Jour. Landw., 34 (1886), p. 213; Forsch. Geb. agr. Phys., 3, p. 1; 4, p. 1; 5, 

 p. 299; and 9, p. 165. 



