226 The Electrical M<t/i(><l of Soil Moisture DetermiiKitiini 



iu the case of sodium oleute the gel stage has under similar conditions 

 of concentration and temperature the same conductivity as the sol. The 

 similarity of nature between this and the silica gel, with which we are 

 most probably dealing, suggests that a like phenomenon would be ob- 

 served here. Fortunately Laing and McBain's work itself suggests an 

 alternative explanation. They found the conductivity of the soap-curd 

 was much lower than that of the tran.sparent gel or sol and distinguisli 



6 0- 

 58 

 5 6 

 5 4 



5 2 



6 

 ■18 



"®- 



MiAM*CI 



_L 



I '®.l 



16 17 18 19 00 01 02 03 04 05 06 07 08 09 lO '1 12 1 '3. 1' 



Log. of % water over hygroscopic 



Fig. 6. 



sharply between coagulation and gelation. Thus in the tirst stage we 

 may be merely increasing the degree of hydration of the fibres of the 

 silica coagulum, in the second stage where the curve descends more 

 steeply we have a passage from a coagulum to a gel in which t he resistance 

 falls not only on account of the water added but also because the specific 

 resistance of the gel is less than that of the coagulum. finally when the 

 coagulum is all converted into gel we have the third stage due to pro- 

 gressive dilution of the gel or sol, it being immaterial which is present. 



