Internal Surface of Soils. 121 



Here it is seen that the* computed surface, 4,949, is very 

 far indeed from either of the true values given under A, 

 B and C. 



142. Effective Diameter of Soil Grains. While it is not 

 possible to determine either the mean diameter of the 

 grains in an ordinary soil or the amount of surface a given 

 weight of soil may possess with even approximate accu- 

 racy, it is possible for the simple sands, at least, to deter- 

 mine the diameter of a grain which, if substituted for the 

 actual ones, would permit, under like conditions, the same 

 amount of air or of water to flow through. 



The method is based upon the laws of flow of fluids 

 through capillary tubes and aims to compute from the ob- 

 served rate of flow of air through a given column of soil 

 the effective diameter of the capillary pores and from this 

 the size of spherical grains which would be required to 

 form such capillary tubes as those computed. The theory 

 of the method is fully set forth in Prof. C. S. Slichter's 

 paper. 1 



143. Description of the Method. The apparatus used to 

 determine the effective size of soil grains is represented in 

 Fig. 39, and consists of a cylinder in which a sample 

 of soil is carefully . packed and weighed to determine 

 the per cent, of pore space. When this has been done 

 the tube is connected with the aspirator and the rate at 

 which air will flow through it under a measured tempera- 

 ture and pressure found. When these data have been ob- 

 tained, then the formula below, used with the table given, 

 enables the effective diameter to be computed when the 

 flow has been measured at the temperature of 20 C. 



Nineteenth Annual Eeport of the U. S. Geol. Survey, Part II. 



