224 The l-Jltctrical Method of Soil Moisture Detenu i nation 



of colloid present, while in the natural calcareous sand (coarse) a 

 mechanical analysis showed 0-45 per cent, clay and the other sands and 

 soils used contained considerably more than this. Now in the case 

 where we have no colloid present we should expect little or no reduction 

 of resistance until the 5()/x/x film thickness of Quincke (7) is passed since 

 Terzaghi (8) has shown that there is no brownian movement and therefore 

 presumablv but little if any ionic movement in this layer. Owing to 

 evaporation it was found impossible to obtain any results in this region. 



Pure sand w. high resistance water 

 „ w. low „ 



>. + 1 % marble w. high reeiBlance water 

 „ „ w. low ,. 



+ ij % marble 



17 18 19 00 01 02 03 04 



Log. ot % water over hygroscopic 

 Fig. 4. 



Beyond this the addition of water might be expected to cause a more 

 rapid fall than in the third stage where it vnW be sensibly inversely pro- 

 portional to the moisture content up to the point at which surface 

 tension begins to overcome the force of adhesion ^ When surface tension 



^ For in this case the surfaoe of electrode wetted will be directly proportion.il to the 

 thickness of the moisture layer, thus if H is the radius of a particle, supposed spherical, 

 and IR the thickness of the water film upon it and if ti is the number of contacts between 



