AGRICULTURE AND RURAL ECONOMY. 531 



few years particularly, busied itself more and more with the 

 building-up of this department of the science of the soil. By 

 such studies, in connection with a fuller understanding of its 

 chemical composition, must the true conception of the soil as 

 the home of plants be formed, and thus a fruitful source of 

 information be gained to clear up the still unexplained phe- 

 nomena of plant-life." 



Anions the most important factors for the fertility of the 

 soil are its relations to water and to heat. These, with the 

 texture, doubtless have as much and often a good deal more 

 to do in deciding how much a soil will yield than the amount 

 of plant- food it contains. As we follow the investigations 

 by which these regulators of fertility are being found, we 

 cannot help being impressed by their simple obedience to 

 the lately established laws of physical science. 



RELATIONS OF THE SOIL TO HEAT. 



Yon Liebenbero; has called renewed attention to the need 

 of more accurate study of the relations of the soil to heat. 

 The sources of heat in the soil are the sun, the heat of the 

 interior of the earth, and chemical processes .going on within 

 it. To understand the relations of the soil to heat it is nec- 

 essary to study the factors ; namely, its thermal capacity, 

 specific heat, and its conducting, absorbing, and emitting 

 powers. According to general physical laws, a soil will grow 

 warm the faster in proportion as its specific heat is smaller 

 and its conducting and absorbing powers larger. It will cool 

 the more slowly in proportion as its specific heat is greater 

 and its emitting and conducting powers less. A series of ex- 

 periments are detailed, from which perhaps the most impor- 

 tant conclusion is that the main factors of the capacity of the 

 soil for heat (by weight) are its contents of humus and of wa- 

 ter (Forschungen aufdem Gebiete der Agrikultiirphysik,\.,$). 



The Thermal Capacity of the Constituents of Soils 



has been studied by Lange in a very thorough and philo- 

 sophical manner. He first discusses the question from the 

 standpoint of the mechanical theory of heat, and then de- 

 scribes an elaborate series of experiments, by which the con- 

 clusions are substantiated and new facts brought out. The 



