EXPERIMENT STATION BULLETINS. 



507 



As in the case of the preceding experiments, this study shows that 

 the gravel in the air dry condition conducts the heat the best and ia 

 followed by sand, clay, loam, and peat respectively. 



A most interesting fact that was brought out very strikingly in 

 this heat conductivity study of the different soils both in the air dry 

 and moist state, was the difl'erence in time in which the temperature 

 of the column of soil began to rise at the different distances from the 

 source of heat. The table below shows typical examples of this fact. 

 The figures represent the time required for the heat to travel from one 

 inch to another. 



TABLE 10.— RATE OF RISE OF TEMPERATURE AT DIFFERENT DISTANCES FROM THE 



SOURCE OF HEAT. 



It will be seen that the rate of flow of heat decreases until a certain 

 distance is reached and then it increases. 



A possible explanation for this phenomenon may be found in the 

 rapid circulation of the soil atmosphere as eddy currents within the 

 minute spaces between the soil grains. These eddy currents are prob- 

 ably set up as hot soil atmosphere on one side of a soil cavity which 

 expands and moves up and is replaced by the colder gases. This circula- 

 tion of air or convectional currents, undoubtedly greatly decrease the 

 transfer resistance and as a consequence the heat conductivity values 

 obtained do not represent the true and real heat conducting pow^r of 

 the material. To obtain this it is necessary to eliminate entirely these 

 convectional currents. They can be eliminated to a very large extent 

 by heating the soils from the top as, is done under field conditions by 

 sun insolation, but the rate of diffusion would be increased in this case. 



As a result of the part that these convectional currents play in heat 

 conductivity, it is believed that the greater thermal transmitting power 

 of the sand over the other soils is partly due to its capacity to allow 

 a greater and more rapid circulation of air. 



It will be interesting now to compare the heat conducting power of 

 the different soils in these various conditions, both as to order and 

 magnitude. Attention must be called here, however, to the fact that the 

 magnitude of the results of the air dry soils cannot be compared with 

 those of the moist natural soils because of the fact that the size of the 

 vessels in the two cases was not the same, and consequently the volume 

 of the materials was not the same. Therefore the comparison must be 

 made among the different soils within the same experiment. Bearing 

 this in mind, it will be found then (1) that the order of the thermal 

 conductivity of the different types of soil in the air dry condition is ex- 

 actly the same as that of the same soils measured in the natural state 

 in the laboratoi-y and under field conditions; and (2) that the differ- 



