498 STATE BOARD OF AGRICULTURE. 



transmission are independent of and essentially different from each 

 other. In the process of conduction the heat is transferred from one 

 body to another, or from one part of the body to another part by con- 

 tact, or by means of the molecules; while in radiation the heat is 

 propagated as a free-wave motion in the ether without permanently 

 affecting the intervening space between the radiating body and the body 

 receiving the radiant energy. The process of convection, on the other 

 hand, takes place chiefly in liquids and gases, where heat is conveyed 

 or transmitted by the interchange of the unequally heated currents of 

 these liquids or gases. 



Heat transference in soils has already received considerable atten- 

 tion from investigators. Especially important is the work on the sub- 

 ject of 'Habei-laud, v. ^Littrow, "Pott, ^nVagner and "Patten. The 

 method of study of all these investigators consisted in determining the 

 heat conductivity of powders of soils of different sized particles in dry, 

 moist, and wet condition, and with different degrees of compactness. 

 The materials were placed in a well-insulated vessel, one side of which 

 consisted of some metal which was brought in contact with a constant 

 source of heat. The results obtained by Haberland, v. Littrow, Pott and 

 Wagner are only relative or qualitative, while those of Patten are 

 quantitative. 



While the results of all these investigations are valuable and interest- 

 ing, they have only a theoretical and suggestive value, because they 

 Avere all obtained under artificial conditions. It is a well-known fact 

 that it is practically impossible to imitate in the laboratory the abso- 

 lute condition which the soil possesses in the field. The heat conduc- 

 tivity is noticeably influenced by many factors, especially by texture, 

 structure, and moisture content. When one of these factors is changed 

 the others also are changed. Under field conditions, the soil, from long 

 standing, has given these factors an opportunity to come to equilibrium 

 and, therefore, it will possess a certain rate of heat transmission at an}' 

 gi\'en time. In the laboratory, however, these factors cannot be brought 

 into equilibrium, and hence the heat conductivity measurements under 

 this condition have mostly a suggestive and theoretical value, however, 

 qualitative or quantitative they may be. 



In order that data on the heat transmission in soils may be of prac- 

 tical value, they must be obtained under natural field conditions. There 

 are many dithculties that arise or are involved in such operations which 

 tend to minimize the importance of the results. In the first place, the 

 different soils possess different shades of colors and would tend to ab- 

 sorb heat unequally and, therefore, would be heated unequally. In the 

 second place, the different soils contain different amounts of moisture 

 and different powers of capillary rise of water, hence there will be 

 unequal evaporation of water on the surface, and consequently a differ- 

 ent rate of heating. 



(7) Wissenschaftl. praktische Untersuchungen auf dem Gebiete des Pflanzenbaiies. Wein 1875. 



(8) Sitzungsber. d. k. Akademie d. Wissenschaften 2, Abteil, Wien 1875. Januarlieft. 



(9) Landw. Vereuchs-Stationen Bd. XX: 273 u. f. 



(10) Forsch. a. d. Geb. d. Agrikullurplyisik Bd. 6:1 u. f. 



(11) Bulletin 59, Bureau of Soils, IT. S. DejMirtment of Agriculture. 



