228 The Electrical Metliotl of Soil Jfoisfiin J)efenninafioit 



under exceptional circuinstiURes of izetting a nietastable condition of 

 some kind; possibly due, if the theory adumbrated is true, to something 

 of the nature of a supersaturation of the coagulum — or, more probably, 

 a time factor comes in, Lainj; and McBain having found that the con- 

 ductivity of a newly formed curd falls oil for a long time after its forma- 

 tion, thus we might expect a similarly retarded recovery if water is 

 added too rapidly. The probability of this is increased by the fact that 

 these workers found that the best way to obtain the gel was to warm 

 the curd very slowly. This error would in this case be unlikely to appear 

 in soil work. 



The agreement of the last portion of the curve as to slope in all 

 cases seems to show that in general at moisture contents exceeding 

 10 per cent, or thereabouts the American formula holds good approxi- 

 matelv and we mav therefore use it in the form given by Gardner, viz. : 



'^-y^' 



where W = per cent, moisture at time of standardisation, 

 R = the resistance corresponding to W, 

 i?j = the resistance observed, 

 and Wi = the per cent, moisture corresponding to the resistance R^, 

 provided always that we do not lose sight of the sources of error noted 

 above and of the limitations of the method dealt with earlier in this paper. 

 There remains the question of the effect of movement of soil salts 

 in the soil which, if considerable, may either vitiate the method alto- 

 gether, or render it too cumbrous for use by necessitating a too frequent 

 standardisation of the electrode. Experiments are proceeding on tliis 

 matter, but from the extreme slowness of the method adopted for this 

 purpose, the full results are not likely to be available for some time. It 

 may be stated however that the material so far to hand seems to indicate 

 that where a soil is protected from rain these salt movements do not 

 affect the moisture determination by as much as one half per cent. 



I take this o])])oi'tunit v of tiianking all those who by their unfailing 

 interest, help and encouragement have enabled me to bring this in- 

 vestigation to its present position. Among these I owe a special debt 

 of gratitude to Mr J. W. Capstiek. O.B.E., M.A., D.Sc. for continuous 

 advice and sympathetic criticism throughout the whole time that the 

 experiments were in progress. 



